The CODE is a three-part visual exploration of how the mathematics that are the “blueprint” for our universe are concealed in nature. My reaction: Why isn’t mathematics introduced to children using this concrete visual method? For those of us who are “math as abstract language” impaired, videos like this are essential to grasping the importance of mathematics.
There are many other videos by Sautoy available on youtube, notably the history of the development of math languages.
Part 2 introduces geometry essential to geologic processes and structures; very familiar to anyone interested in mineralogy AND so incredibly beautiful and simple. If you don’t view anything else, at least check out the “bubble” segment starting at 10:53.
Disappointing video quality: looking for something better!
Despite persistent belief, both inside and outside the supposed “science / religion” boundary, that humans are “a special supernatural creation,” and therefore require magical and murky socio-supernatural explanations for our behavior, we are animals. Thanks to the work of “animal scientists” we do have access to REAL information about Homo sapiens: mammal, primate, ape. Via papers such as this, we can understand how physical parameters (not manmade social constructs) drive physiology and behavior in Homo sapiens, just as in any other mammal.
Calculating Climate Effects on Birds and Mammals: Impacts on Biodiversity, Conservation, Population Parameters, and Global Community Structure
Ever since the era of Charles Darwin biologists have been intrigued by how and why animals live where they do and what is it about their properties that makes them appear where they do, and appear in the species associations that they form. Hutchinson (1959) defined the concept of the niche. MacArthur et al. (1966), Roughgarden (1974) and many others explored aspects of how size and habitat may influence community structure. Norris (1967) and Bartlett and Gates (1967) were the first to calculate explicitly how climate affects animal heat and mass balance and the consequences for body temperature in outdoor environments. The climate space concept emerged from steady state heat and mass balance calculations and was used to explore how climates might constrain animal survival outdoors (Porter and Gates, 1969).
Those early animal models of the 1960s were limited by the lack of models for distributed heat generation internally, distributed evaporative water loss internally, and a first principles model of gut function. Batch reactor, plug flow and other models were already in existence in the chemical engineering literature (Bird et al., 1960) and it would take time for the biological community to rediscover them. Also missing were a first principles model of porous insulation for fur or feathers, an appendage model, and a general microclimate model that could use local macroclimate data to calculate the range of local microenvironments above and below ground. It became possible to estimate convection heat transfer properties knowing only the volume of an animal (Mitchell, 1976). Another useful development was the appearance of a countercurrent heat exchange model for appendages (Mitchell and Myers, 1968) and the measurement of heat transfer characteristics from animal appendage shapes (Wathen et al., 1971, 1974). It also became possible to deal with outdoor turbulence effects on convective heat transport (Kowalski and Mitchell, 1976). A general-purpose microclimate model emerged in the early 1970s (Beckman et al., 1971; Porter et al., 1973; Mitchell et al., 1975) that calculated above and below ground microclimates. The ability to deal with local environmental heterogeneity and calculate percent of thermally available habitat came later (Grant and Porter, 1992). Over time general-purpose conduction–radiation porous media models for fur appeared in the biological literature (Kowalski, 1978) and it became possible to refine and test them in a variety of habitats and on many species (Porter et al., 1994). The extension of the models to radial instead of Cartesian coordinates and the implementation of first principles fluid mechanics in the porous media (Stewart et al., 1993; Budaraju et al., 1994, 1997) added important new dimensions to the models, which could now calculate temperature and velocity profiles and therefore heat and mass transfer within the fur from basic principles. A test of the ectotherm and microclimate models to estimate a species’ survivorship, growth and reproduction at a continental scale appeared in the mid 1990s (Adolph and Porter, 1993, 1996).
Thanks to these developments and the ones reported in this paper, such as the temperature dependent behavior linked to the new thermoregulatory model, it is now possible to ask: “How does climate affect individual animals’ temperature dependent behavior and physiology and what role(s) does it play in population dynamics and community structure?” This paper attempts to address some of these questions.
We approach the problem from the perspective of a combination of heat and mass transfer engineering and specific aspects of morphology, physiology and temperature dependent behavior of individuals. We show how this interactive combination is essential to calculate preferred activity time that minimizes size specific heat/water stress.
Preferred activity time is a key link between individual energetics and population level variables of survivorship, growth and reproduction, since it impacts all three population variables. Both individual and population level effects may place constraints on community structure. At the individual level, climate at any given time and food type and quality affect the optimal body size that maximizes discretionary mass and energy, the resources needed for growth and reproduction. Climate also affects community structure by affecting individual survivorship directly (heat balance/metabolic costs) and indirectly (activity time overlap of predator and prey). Climate affects seasonal food availability, distribution of food in space and time, and the cost of foraging for that food at different times during a day. Survivorship is affected by temperature dependent behavior changes that allow animals to move to less costly microenvironments at any time. For small mammals, underground burrows or under snow tunnels provide temperatures that never stay below 0°C due to local heating effects of the animal’s metabolic heat production.
At the population level,climate plays a very important role in population numbers. Each species interacts in its own way with climate, affecting its abundance, and community structure. As Ives et al. (1999 p. 546) have pointed out
Our main result is that interspecific competition and species number have little influence on community-level variances; the variance in total community biomass depends only on how species respond to environmental fluctuations. This contrasts with arguments (Tilman and Downing, 1994; Lawton and Brown, 1993) that interspecific competition may decrease community-level variances by driving negative covariances between species abundances. We show that negative covariances are counteracted by increased species-level variances created by interspecific competition.
Consequently, assessing the effect of biodiversity on community variability should emphasize species-environment interactions and differences in species’ sensitivities to environmental fluctuations (for example, drought-tolerant species and phosphorus-limited species) (McNaughton, 1977, 1985; Frost et al., 1994). Competitive interactions are relatively unimportant except through their effects on mean abundances. We have focused on competitive communities, because much current experimental work has addressed competition among plants. Nonetheless, the same results can be shown to hold for more complex models with multiple trophic levels.
Exactly how climate variation, vegetation differences, animal morphology, and foraging behavior all interact to constrain multiple functional types’ existence as a community is still largely unknown. Very little is known about temperature dependent foraging in mammals, although this has been well studied in reptiles and insects. Quantitative consequences of functional morphology on encounter probability and food handling time also are relatively unexplored as yet in mammals.
Temporal climate variation in a locality creates the opportunity for multiple optimal body sizes over annual cycles.The spatial local variation in topography and vegetation creates multiple local climates. Thus temporal and spatial variation in climate creates opportunities for multiple functional types (sizes) to coexist as communities, because as we shall see below, different body sizes interact differently with climate. Qualitatively, this idea is not new. However with likely major shifts in global climates and the rapid global changes in land use, there is urgent need to move these qualitative ideas to a quantitative framework for protection of biodiversity, conservation biology, and a number of other applications. We focus in this paper on applications to mammals and birds.
An overview of this paper
The structure of the paper begins with an overview of how macroclimate drives microclimates, which in turn impact individual animal properties. We then show how key individual properties determine population level parameters that can be used to calculate population dynamics variables. We then illustrate how individual properties also impact on community structure, that in turn feed back to temperature dependent animal properties of individuals.
The initial overview provides a context for an analysis of the model components and their interactions in hierarchical contexts. We start with the model components from the core to the skin, then from the skin through the insulation to the environment. We demonstrate how these components collectively can define the metabolic cost to mammals ranging in size from mice to elephants. We show how the empirical mouse-to-elephant metabolic regression line for animals of different sizes changes depending upon the animal’s climate and posture.
Then we explore how changing mammal body size affects discretionary energy across all climates. Once the mammal model is explored, we repeat the process for the bird model. We demonstrate how we can estimate metabolic cost across bird sizes ranging from hummingbirds to ostriches. We show how postural changes and air temperature can alter metabolic cost estimates for birds.
Once sensitivity analyses are completed, we explore how temporal and spatial variation in global climate impact body size dependent discretionary energy assuming no food limitation and thereby place constraints on the potential combinations of body sizes (community structure) of mammals at the global scale.
Finally, we show how these models can be applied to estimate for the first time from basic principles the metabolic costs and food requirements of an endangered species of bird, the Orange-bellied Parrot of Tasmania and Australia. We show these results for body sizes ranging from hatchling to fully mature adult for a wide range of environmental conditions.
MATERIALS AND METHODS
(go to original paper for text and figures; topics and some sample text follow)
Survivorship (mortality) probability/hour
Growth and reproduction potential
Different sizes of animals
Model cross section
Inside the body
Heat generation models
Temperature regulation model
Temperature dependent feeding
Fur vs. feathers
Finite elements and flow through the fur
Modeling an individual
Internal body temperature profiles
Flow at very low wind
Scaling across mammal body sizes
Mouse to elephant metabolic rate
Mouse to elephant discretionary energy uptake
Diet effects on optimal body size
These results are reminiscent of Bergmann’s rule, an empirical observation that as climates get colder, animal sizes tend to get larger. Body size increases with decreasing temperature provide the greatest advantage at small size (Steudel et al., 1994). At larger body sizes, changes in fur insulation confer a greater advantage Steudel et al., 1994). Experimental data from different types of fur on a flat plate (Scholander et al., 1950) suggested this, but animals of larger size also have thicker boundary layers. A thicker boundary layer reduces convective heat loss and simultaneously enhances radiation temperature effects (Porter and Gates, 1969). Larger animals are taller, which means exposure to greater wind speeds higher above the ground. Higher wind speed reduces boundary layer thickness and may engender greater wind penetration of the fur. A first principles fur model can separate boundary layer effects due to size and wind from fur properties effects and provide better estimates of combined effects.
Assessment of consequences of Bergmann’s rule have pointed out that larger animals have the advantage of longer fasting ability under conditions of climate or food availability stress (Morrison, 1960). However, smaller animals have the advantage of lowering body temperature and seeking much more favorable microclimates, especially underground habits in severe cold. Careful transient modeling analyses of these two strategies in the animals’ microclimates would yield a testable hypothesis of the relative benefits of these different solutions to the same problem of dealing with cold.
Of course, survival in extreme temperature events is also important in affecting community structure. However, extreme temperature survival may be overrated in terms of its effects on community structure, at least for mammals. Temperature dependent behavior and selection of microhabitats by both small and large animals can greatly reduce cold or heat stress. For example, moving under or into trees and modifying the solar and infrared radiation and wind protection they provide can change equivalent local microenvironment temperatures by 20°C or more. Underground burrows or tunneling beneath the snow can provide habitats that typically do not drop below 0°C in winter when an animal is present, due to local heat from metabolism. Photoperiod-induced temperature dependent physiology, such as hibernation or estivation is another way that mammals can persist in habitats during periods of extreme heat or cold stress and thereby maintain community structure. Birds typically opt to migrate from extremely cold habitats in winter that they occupy in the summer. By exercising temperature dependent behavioral selection of microclimates through migration, the scale of their selection movements is simply larger due to the short time and lower costs of long distance bird transport.
Scaling across bird body size
Hummingbird to ostrich metabolic rates—Air temperature effect
Global communities-climatic constraints
Figure 16 shows temporal and spatial variation in optimal body size based on discretionary mass/energy for mammals for the months of January and July on a global scale. In January (winter) in the Northern Hemisphere, the optimal sizes are larger as one moves north. Large topographic features, such as the Rocky Mountains, are also predicted to have larger animals with their optima. In the Southern Hemisphere, where it is summer, topographic features do not stand out as strongly.
In July (winter) in the Southern Hemisphere there is somewhat of a “mirror image” effect on optimal body size. However, different topographic and latitudinal features create somewhat different patterns. In general, though, the model suggests that larger animals have the advantage. In the Northern Hemisphere at the same time smaller animals should have the advantage. Large topographic features like the Tibetan plateau with its cool weather in summer still show up fairly clearly as affecting optimal body size. For clarity, variation in vegetation type and food quality were not included in these graphs.
The criteria for optimization were maximum discretionary energy uptake for a given temperature at all possible body sizes. This figure was generated from the endotherm model driven by global weather data at half-degree intervals in latitude and longitude.
The map of optimal body size is different at different seasons of the year. This suggests that climate places important constraints on what functional types can coexist in a locality. Because the environment is constantly changing, it creates a constantly changing optimal body size in any locality. Changing environments create the opportunity for multiple functional types to coexist in the same area.
What is unknown at present is over what time intervals does natural selection integrate time and environmental conditions to “choose” body size? Figure 16 represents the beginnings of the effort to understand climatic constraints on community structure from basic principles. The vegetation on the landscape is certainly a very important variable that will modify the current version of the model. The spatial and temporal distribution of available food places important additional constraints on optimal body size. These constraints include encounter probabilities, handling time, food energy value and metabolic cost to get to the food. Three of these variables are related to body size and the “packaging” and “distribution” of food on the landscape. It is clear that this construct can also be applied to species of birds to study migratory patterns and other aspects of bird ecology.
It is important to note, as one reviewer did, that “evolution may select less for optima under average daily climate cycles and more for adaptations that increase survivorship during winnowing events. At any given time a population may consist of individuals with below or above optimal body sizes, should recent history include high mortality linked to extreme climate, with availability, or predation.” These important considerations have not been added to these models yet.
Conservation application: The Orange-bellied Parrot, Neophema chrysogaster
Ontogeny of metabolic costs
Surrogates for size in modeling metabolism
Body weight is a surrogate for body radius. Posture is a surrogate for body geometry. Empirical metabolism data collected since the time of Benedict in the 1930s have related metabolic heat production to body mass. However, mass is only one of the variables that drive metabolic heat production. A key variable is the radius of the trunk of the animal, which is in turn a function of the posture. Most of the analyses of metabolic scaling in the literature that we know ignore this important aspect. Furthermore, the role of a variety of environmental variables and different types of porous insulation in modifying metabolic demand have not been predictable because of the lack of reliable quantitative models.
Body size affects discretionary mass and energy intake. Growth and reproduction potential affects fitness. As Figures 11 through 15 demonstrate, body size has important impacts through geometric form and radial dimensions on energy expenditure and intake. The surrogate for these primary variables is body weight (mass). We have pointed out here how air and radiant temperature and posture can make important modifications in energy cost in different environments. These energy costs are not linear with body size. Heat transfer mechanisms are not all linear with body size and neither are temperature regulation responses. Scaling of the gut is not linear with body size, either (Calder, 1984). The combinations of these nonlinear functions result in calculations that suggest discontinuous optimal body size with temperature. This is consistent with empirical data (Brown et al., 1993; Brown and Maurer, 1987; Brown and Nicoletto, 1991; Holling, 1992; Maurer et al., 1992; Peterson et al., 1998). However, there is an important reanalysis questioning these empirical results (Siemann and Brown, 1999). Our results of climate/body size/gut modeling suggest that whether or not animal sizes are clumped in nature may depend on the digestive efficiencies of foods consumed and the locations of those foods. High quality foods suggest greater clumping, low quality foods suggest very little in the way of body size clumping (Fig. 13a–d).
Body size effects on cost of foraging: temperature dependent foraging/activity time
The work presented here explains that changes in boundary conditions, such as environmental constraints on heat and mass exchange, alter fluxes and therefore alter internal scaling requirements that must adapt to changing needs. Thus, we suggest that temperature dependent behavior may be an important response to environmental change that tends to keep the organism as close as possible to optimal function as dictated by its internal and external anatomy, thereby maximizing fitness.
Body size determines whether a species can be fossorial or not, which affects diurnal microclimates and heat and mass balances. Body size affects likelihood of predation, which can be cast as a cost of foraging (Brown et al., 1994). Body size affects competition, which alters temperature-dependent activity time, which also affects cost of foraging.
Body size effects on total annual activity time
Body size effects on total annual activity time are mediated through heat and mass exchange with the environment. The onset of heat or cold stress appears to be an important constraint in limiting activity. That is, temperatures that force skin temperatures below 3°C or conditions where evaporative water loss must be elevated to protect organism integrity are bounds on activity time that impact animal fitness.
The boundary layer thickness in the air next to the animal surface constrains mass and heat transfer from an animal. Boundary layer thickness is a function of the friction between the animal surface and the air. The amount of friction depends on the dimension of the animal, fluid and animal speed relative to each other, and fluid properties of density, viscosity and thermal conductivity. On the one hand small animals have thin boundary layers and are more responsive to convective environments than to radiant heat exchange (Porter and Gates, 1969). On the other hand, large animals have thicker boundary layers and are more sensitive to the diurnal changes in infrared radiation and solar radiation fluxes in the environment. For large animals, absorption of radiant energy is a much greater challenge, since cooling by convective heat transfer is diminished because of the thicker insulating boundary layer around the larger animal.
Body size affects competitive success, hence temperature-dependent behavior including habitat utilization, which impacts on total annual activity time.
Vegetation/body size effects on biodiversity
Vegetation modifies microclimate conditions available to animals in predictable ways. Animal body size determines where animals spend their time in the wind patterns near the ground. Figure 16 is based on empirical climate data. Those empirical data reflect how vegetation may modify local microclimates. Vegetation also affects animal energetics either by direct shading of the animals or by providing cool surfaces that radiate back to animals. Thus, by directly and indirectly affecting the animal heat fluxes, vegetation impacts optimal body size and constrains functional types that might coexist in a community.
The distribution and quality of food in space and time changes in an annual cycle. Animal food encounter probabilities, and food handling time are consequences of vegetation structure and type. The calculations used in Figure 16 do not yet incorporate various possible distributions of food of various types in the environment. Diverse food distributions have not yet been explored using our models. Food encounter probabilities and handling times, which are a key part of food intake, are only beginning to be explored. The different food types, sizes and spacing also place important constraints on the range of body sizes of animals, which can efficiently utilize them.
Body size, cost of locomotion, and home range size are also interconnected. Home range size must be a function of body size, cost of locomotion, and the foraging thermal and vegetative environment. The minimum time and cost to forage for a particular type, distribution and size of food should be calculable for a broad range of body sizes and environments.
Feathers and plumage
When we watch the development of feathers through the ontogeny of a bird, it is apparent that the down structure is very much like the extremely dense fur of some mammals. Both types of fibers emerge from single openings in the skin as multiple fibers and then “fan out” in three dimensions as multiple fibers as they grow. In so doing they extend the layer of still air above the skin (and in the insulation) substantially. The second stage of plumage development with the eruption of feathers that tend to seal off air flow even further from the skin is unique in its efficiency of cross linking elements to hold complex units together and seal out air flow. The only fur that seems even closely comparable is that of the snowshoe hare that has fur tips that are flattened like tiny shovels (Porter, unpublished data). These structures probably assist in minimizing air and snow penetration into the coat.
The restriction of feather tracts to portions of a bird’s skin provide for flexibility in opening up skin areas to much more rapid heat transfer is also unique to birds. Some mammals like polar bears have inguinal regions that are highly vascularized and lightly furred. Polar bears sometimes apply them to the snow to dissipate heat, but mammals, unlike birds, have not evolved the ability to open large areas of nearly bare skin to dissipate or absorb heat.
1. Temporal and spatial variation in physical environments impose important constraints on functional types of animals that can coexist in biological communities. These constraints are further refined locally by food diversity representing different digestive qualities.
2. Morphology, physiology, and temperature-dependent activity in animals link individual energetics to population dynamics and community structure by specifying total annual activity time and mass/energy available for growth and reproduction.
3. Porous insulation in birds at rest can be modeled with current state-of-the-art fur models. Resting birds have feather positions that tend to seal off convective transport. This creates a conduction–radiation heat transfer environment. This is simpler to calculate than an environment where three heat transfer mechanisms are all important.
4. Posture plays an important role in metabolic heat loss. This is true mainly because posture affects the radial dimension of the animal, which is a key variable in the equation governing an animal’s total heat generation requirements. Posture is typically ignored in metabolic chamber metabolism studies. The model presented here allows the calculation of the upper and lower limits of metabolic expenditure for a wide variety of climatic conditions.
5. Animal geometry and posture, insulation properties, and environmental conditions influence “thermal conductance.” Thermal conductance is a term implying a passive transport of heat through a non-heat-generating medium. Thus, it is inappropriate for describing fluxes through flesh, where heat generation is occurring. It is also inappropriate in porous media that “act alive” by absorbing solar radiation in the insulation. Thermal conductance is affected by properties and boundary conditions that can have nonlinear effects on heat transport through the medium in question. It can be useful as a descriptive concept for heat source-free systems if all of the relevant boundary conditions and properties are specified.
6. The novel thermoregulatory model in conjunction with user specifications for diurnal/nocturnal/crepuscular activity allows for estimates of activity time that are in good agreement with published data.
7. Climate/body size/gut model calculations for different food types suggest that optimal body size (maximizing discretionary mass/energy) changes with different food types and their associated digestive efficiencies and the temperature. This suggests that vegetation diversity in a locality allows for specific multiple body sizes to coexist at the same point in time. As food quality declines from high digestive efficiencies of flesh/seeds to lower digestive efficiencies of grasses/leaves, optimal body size increases, lowest survival temperature rises, and the degree of clumping predicted for species in nature declines. Land use changes that tend toward monocultures would appear to dictate that fewer species would survive as vegetation diversity declines. Global warming trends would lead to smaller optimal body sizes with no change in vegetation. However vegetation changes associated with climate warming would specify larger or smaller body sizes depending on whether vegetation digestive qualities decrease or increase respectively.
8. Application of the microclimate and endotherm models to rare or endangered species requires relatively few, easily measured data to estimate food and water requirements, potential for activity time, growth, and reproduction for a wide variety of habits. This information will be useful as an aid for identification of potential reserves/transplantation sites and modification/management of existing habitats.
From the Symposium Evolutionary Origin of Feathers presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado.
As an Asperger (?) or visual thinker, my attention to time is highly variable; when concentrating on a “visual” object or scene, time does not seem to exist. Time “markers” (these really are social in origin) such as calendars, schedules, appointments, fixed places and dates in time, are irritating interruptions to this highly pleasant lack of “feeling” for time. When these social “markers” are inevitable, as many are, I don’t feel well; anxiety may accompany the commitment to “be there” “show up” “put in an appearance.” This “regulation of time” by social entities feels alien.
My experience of the natural environment is fluid; determined by sensory “cues” – light, the motion of the atmosphere, color changes, sounds that merge and pass smoothly. The “human environment” is by contrast, incoherent; abrupt interruptions of sound, artificial light, space confined by walls and obstacles, jagged stop and go movement, no “time” to “enjoy” the senses. No peace.
In short, when in a natural environment I am within the ‘time sense’ of that environment; sensory embedded-ness, might be a description. In a human environment (except those few highly aesthetically conscious spaces), the sensory input is simply “all wrong”.
K. Michaelian, Instituto de Física, Universidad Nacional Autónoma de México Cto. de la Investigación Científica Cuidad Universitaria, Mexico D.F., C.P. 04510
Equations that govern physical reality.
Darwin suggested that life was at the mercy of the forces of Nature and would necessarily adapt through natural selection to the demands of the external environmental. However, it has since become apparent that life plays a pivotal role in altering its physical environment (Lovelock, 1988) and what once appeared to be biotic evolution in response to abiotic pressure is now seen as coevolution of the biotic together with the abiotic to greater levels of complexity, stability, and entropy production (Ulanowicz and Hannon, 1987). Such an understanding, difficult to reconcile within traditional Darwinian theory, fits perfectly well within the framework of non-equilibrium thermodynamics in which dissipative processes spontaneously arise and coevolve in such a manner so as to increase the entropy production of the system plus its environment (Prigogine, 1972, Ulanowicz and Hannon, 1987, Swenson, 1989, Kleidon and Lorenz, 2005, Michaelian, 2005, Michaelian, 2009a).
Life is found everywhere on Earth. On the surface, the components of greatest biomass are the archea, prokaryote, and eukaryote life based on photosynthesis. In the sea, photosynthetic phytoplankton (archea, diatoms, cyanobacteria, and dinoflagallates) can be found in great density (up to 109/ml at the surface) in the euphotic zone which extends to a depth of 50 meters. Almost all photosynthesis ends at the bottom of the Epipelagic zone at about 200 m. Approaching these depths, special pigments are needed to utilize the only faint blue light that can penetrate. On land, diatoms, cyanobacteria, and plants, which evolved from ocean cyanobacteria some 470 million years ago (Wellman and Gray, 2000; Raven and Edwards, 2001), cover almost every available area, becoming sparse only where conditions are extremely harsh, particularly where liquid water is scarce. Photosynthesizing cyanobacteria have been found thriving in hotsprings at over 70 °C (Whitton and Potts, 2000) and on mountain glaciers and Antarctic ice (Parker et al., 1982) where absorption of solar radiation and its dissipation into heat by organic and lithogenic material produces the vital liquid water, even deep within the ice (Priscu et al., 2005). The thermodynamic driving force for the process of photosynthesis that sustains surface life derives from the low entropy of sunlight and the second law of thermodynamics. Only twenty seven years after Darwin’s publication of the theory of evolution through natural selection, Boltzmann (1886) wrote: “The general struggle for existence of animate beings is therefore not a struggle for raw materials – nor for energy which exists in plenty in any body in the form of heat — but a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth”.
In photosynthesis, high-energy photons in the visible region of the Sun’s spectrum are converted by the chloroplasts into low energy photonsin the infrared region.Part of the free energy made available in the process is utilized to maintain and propagate life.In this manner, photosynthetic life obtains its sustenance through the conversion of the low entropy of sunlight into the higher entropy of heat and thereby contributes to the positive entropy production of the Earth as a whole.
However, the proportion of the Sun’s light spectrum utilized in photosynthesis is small and thus the entropy producing potential of photosynthesis is small. Gates (1980) has estimated that the percentage of available (free) energy in solar radiation that shows up in the net primary production of the biosphere is less than 0.1%. Respiration consumes a similarly small quantity (Gates, 1980). Of all the irreversible processes performed by living organisms, the process generating by far the greatest amount of entropy (consuming the greatest amount of free energy) is the absorption of sunlight by organic molecules in the presence of water leading to evapotranspiration. Great quantities of water are absorbed by the root systems of plants and brought upwards to the leaves and then evaporated into the atmosphere. More than 90% of the free energy available in the sunlight captured by the leaves of plants is used in transpiration. In the oceans, phytoplankton within the euphotic zone absorb sunlight and transform it into heat that can be efficiently absorbed by the water. The temperature of the ocean surface is thereby raised by phytoplankton (Kahru et al., 1993) leading to increased evaporation, thereby promoting the water cycle. There appears to be no important physiological need for the vast amount of transpiration carried out by land plants. It is known that only 3% of the water transpired by plants is used in photosynthesis and metabolism. In fact, most plants can grow normally under laboratory conditions of 100% humidity, at which the vapor pressure in the stoma of the leaves must be less than or equal to that of the atmosphere, and therefore transpiration is necessarily zero (Hernández Candia, 2009). Transpiration has often been considered as an unfortunate by-product of the process of photosynthesis in which water is unavoidably given off through the stoma of plants which are open in order to exchange CO2 and O2 with the atmosphere (Gates, 1980). Plants consist of up to 90% water by mass and thus appear to expose themselves to great risk of drying by transpiring so much water. Others have argued
that transpiration is useful to plants in that it helps to cool its leaves to a temperature optimal for photosynthesis. Such an explanation, however, is not convincing since Nature has produced examples of efficient photosynthesis at temperatures of up to 70 °C (Whitton and Potts, 2000). In any case, there exists other simpler and less free energy demanding strategies to reduce leaf temperature such as smaller or less photo-absorbent leaves. On the contrary, the evolutionary record indicates that plants and phytoplankton have evolved new pigments to absorb ever more completely the Sun’s spectrum. Dense pine forests appear black in the midday sun. Most plants appear green, not so much for lack of absorption at these wavelengths, as for the fact that the spectral response of human eyes peaks precisely at these wavelengths (Chang, 2000).
x Transpiration is in fact extremely free energy intensive and, according to Darwinian Theory, such a process, with little direct utility to the plant, should have been eliminated or suppressed through natural selection. Plants which are able to take in CO2 while reducing water loss, by either opening their stoma only at night (CAM photosynthesis), or by reducing photorespiration (C4 photosynthesis, see below), indeed have evolved 32 and 9 million years ago respectively (Osborne and Freckleton, 2009). However, the water conserving photosynthesis has not displaced the older, heavily transpiring C3 photosynthesis which is still relevant for 95% of the biomass of Earth.Instead, new ecological niches in water scarce areas have opened up for the CAM and C4 plants, as, for example, the cacti of deserts.
x All irreversible processes, including living systems, arise and persist to produce entropy.This is not incidental, but rather a fundamental principle of Nature. Excessive transpiration has not been eliminated from plants, despite the extraordinary free energy costs, precisely because the basic thermodynamic function of a plantis to increase the global entropy production of the Earth and this is achieved by dissipating high energy photons in the presence of water and thereby augmenting the global water cycle.
The Water Cycle Absorption of sunlight in the leaves of plants may increase their temperature by as much as 20°C over that of the ambient air (Gates, 1980). This leads to an increase of the H2O vapor pressure inside the cavities of the leaf with respect to that of the colder surrounding air. H2O vapor diffuses across this gradient of chemical potential from the wet mesophyll cell walls (containing the chloroplasts), through the intercellular cavities, and finally through the stoma and into the external atmosphere. There is also a parallel, but less efficient, circuit for diffusion of H2O vapor in leaves through the cuticle, providing up to 10% more transpiration (Gates, 1980). The H2O chemical potential of the air at the leaf surface itself depends on the ambient relative humidity and temperature, and thus on such factors as the local wind speed and insolation. Diffusion of H2O vapor into the atmosphere causes a drop in the water potential inside the leaf which provides the force to draw up new water from the root system of the plants.
Evaporation from moist turf (dense cut grass) can reach 80% of that of a natural water surface such as a lake (Gates, 1980), while that of a tropical forest can often surpass by 200% that of such a water surface (Michaelian, 2009b). Single trees in the Amazon rain forest have been measured to evaporate as much as 1180 liters/day (Wullschleger et al., 1998). This is principally due to the much larger surface area for evaporation that a tree offers with all of its leaves. Natural water surfaces, in turn, evaporate approximately 130% of distilled water surfaces due to the increased UV and visible photon absorption at the surface as a result of phytoplankton and other suspended organic materials, including a large component (up to 109/ml at the surface) of viral and dissolved DNA resulting from viral lysing of bacteria (Wommack and Colwell, 2000).
The water vapor transpired by the leaves, or evaporated by the phytoplankton, rises in the atmosphere, because water vapor at 0.804 g/l is less dense than dry air at 1.27 g/l, to a height corresponding to a temperature of about 259 K (-14 °C) (Newell et al., 1974) at which it condenses around suspended microscopic particles forming clouds. Over oceans, an important constituent of these microscopic particles acting as seeds of condensation are the sulfate aerosols produced by the oxidation of dimethylsulfide released by the phytoplankton themselves (Charlson et al., 1987). Condensation of the water releases an amount of latent heat of condensation ( 6 10427.2 × J /kg) into the upper atmosphere, much of which is then radiated into outer space at infrared wavelengths. In this manner, the Earth maintains its energy balance with space; the total energy incident on the biosphere in the form of sunlight is approximately equal to the total energy radiated by the biosphere into space at infrared wavelengths. Energy is conserved while the entropy of the Universe is augmented in the process.
The formation of clouds may at first consideration seem to have a detrimental effect on the water cycle since cloud cover on Earth reflects approximately 20% of light in the visible region of the Sun’s spectrum (Pidwirny and Budicova, 2008), thereby reducing the potential for evaporation. However, evapotranspiration is a strong function of the local relative humidity of the air around the leaves of plants or above the surface of the oceans. By producing regions of local cooling during the day on the Earth’s surface, clouds are able to maintain the average wind speed at the Earth’s surface within dense vegetation (see for example, Speck (2003)) at values above the threshold of 0.25 m/s required to make the boundary-layer resistance to water loss almost negligible in a plant leaf, thus procuring maximal transpiration (Gates, 1980).
Sublimation and ablation of ice over the polar regions, promoted in part by photon absorption of cyanobacteria within the ice, is also important to the water cycle, evaporating up to 30 cm of ice per year (Priscu et al., 2005).
Production of Entropy
The driving force of all irreversible processes, including the water cycle, is the production of entropy. The basic entropy producing process occurring on Earth is the absorption and dissipation of high energy photons to low energy photons, facilitated in part by the plants and cyanobacteria in the presence of water.
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The Importance of Life to the Water Cycle
The very existence of liquid water on Earth can be attributed to the existence of life.Through mechanisms related to the regulation of atmospheric carbon dioxide first espoused in the Gaia hypothesis (Lovelock, 1988), life is able to maintain the temperature of the Earth within the narrow region required for liquid water, even though the amount of radiation from the Sun has increased by about 25% since the beginnings of life (Newman and Rood, 1977, Gough, 1981). Physical mechanisms exist that disassociate water into its hydrogen and oxygen components, for example through photo-dissociation of water by ultraviolet light (Chang, 2000). Photo-dissociation of methane has been suggested as a more important path to loosing the hydrogen necessary for water (Catling et al., 2001). Free hydrogen, being very light, can escape Earth’s gravity and drift into space, being dragged along by the solar wind. This loss of hydrogen would have lead to a gradual depletion of the Earth’s water(Lovelock, 2005). However, photosynthetic life sequesters oxygen from carbon dioxide thereby providing the potentiality for its recombination with the free hydrogen to produce water. For example, hydrogen sulfide is oxidized by aerobic chemoautotrophic bacteria, giving water as a waste product (Lovelock, 1988). Oxygen released by photosynthetic life also forms ozone in the upper atmosphere which protects water vapor and methane in the lower atmosphere from ultraviolet photo-dissociation. In this manner, the amount of water on Earth has been kept relatively constant since the beginnings of life.
It has been estimated that about 496,000 km3 of water is evaporated yearly, with 425,000 km3 (86%) of this from the ocean surface and the remaining 71,000 km3 (14%) from the land (Hubbart and Pidwirny, 2007). Evaporation rates depend on numerous physical factors such as insolation, absorption properties of air and water, temperature, relative humidity, and local wind speed. Most of these factors are non-linearly coupled. For example, local variations in sea surface temperature due to differential photon absorption rates caused by clouds or local phytoplankton blooms, leads to local wind currents. Global winds are driven by latitude variation of the solar irradiance and absorption, and the rotation of the Earth. Relative humidity is a function of temperature but also a function of the quantity of microscopic particles available for seeds of condensation (a significant amount of which are supplied by biology (Lovelock, 1988)).
The couplings of the different factors affecting the water cycle imply that quantifying the effect of biology on the cycle is difficult. However, simulations using climate models taking into account the important physical factors have been used to estimate the importance of vegetation on land to evapotranspiration. Kleidon (2008) has shown that without plants, average evaporation rates on land would decrease from their actual average values of 2.4 mm/d to 1.4 mm/d, suggesting that plants may be responsible for as much as 42% of the actual evaporation over land. There appears to be little recognition in the literature of the importance of cyanobacteria and other organic matter floating at the ocean surface to evaporation rates. Irrespective of other factors such as wind speed and humidity, evaporation rates should be at least related to the energy deposited in the sea surface layer. A calculation can therefore be made of the effect of biology on the evaporation rates over oceans and lakes.
Before attempting such a calculation, it is relevant to review the biological nature of the air / sea surface interface, and energy transfer within this layer, based on knowledge that has emerged over the last decade. This skin surface layer of roughly 1 mm thickness has its particular ecosystem of high density in organic material (up to 104 the density in water slightly below (Grammatika and Zimmerman, 2001)). This is due to the scavenging action of rising air bubbles due to breaking waves, surface tension, and natural buoyancy (Grammatika and Zimmerman, 2001). The organic material consists of cyanobacteria, diatoms, viruses, free floating RNA/DNA, and other living and non-living organic material such as chlorophyll and other pigments. Most of the heat exchange between the ocean and atmosphere of today occurs from within this upper 1 mm of ocean water. For example, most of the radiated infrared radiation from the sea comes from the upper 100 µ m (Schlussel, 1999). About 52% of the heat transfer from this ocean layer to atmosphere is in the form of latent heat (evaporation), radiated longwave radiation accounts for 33%, and sensible heat through direct conduction accounts for the remaining 15%.
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Some theories have the origin of life dissipating other sources of free energy, such as chemical energy released from hydrothermal vents at deep ocean trenches. Whether life originated to dissipate the free energy in sunlight or the free energy in made available through chemical transformations, the quantity of life at hydrothermal vents today corresponds to a very minute portion of all life on Earth implying that its contribution to the actual entropy production of the Earth can be considered negligible. The rich ecosystems existing at these vents are, in fact, not completely autonomous, but dependent on the dissolved oxygen and nutrients of photosynthetic life living closer to the surface. An Earth without photosynthetic life would thus correspond to one in a wholly different class of thermodynamic stationary states, one probably with little involvement of a water cycle.
Evidence for Evolutionary Increases in the Water Cycle
Plants, far from eliminating transpiration as a wasteful use of free energy, have in fact evolved over time ever more efficient water transport and transpiration systems(Sperry, 2003). There is a general trend in evolution, and in ecosystem succession over shorter times, to ever increasing transpiration rates. For example, conifer forests are more efficient at transpiration than deciduous forests principally because of the greater surface area offered for evaporation by the needles as compared to the leaves. Conifers appeared later in the fossil record (late carboniferous) and appear in the late successional stage of ecosystems. Root systems are also much more extended in late evolutionary and successional species, allowing them to access water at ever greater depths (Raven and Edwards, 2001). New pigments besides chlorophyll have appeared in the evolutionary history of plants and cyanobacteria, covering an ever greater portion of the intense region of the solar spectrum, even though they have little or no effect on photosynthesis, for example, the carotenoids in plants, or the MAA’s found in phytoplankton which absorb across the UVB and UVA regions (310-400 nm) (Whitehead and Hedges, 2002). This is particularly notable in red algae, for example, where its total absorption spectrum has little correspondence with its photosynthetic activation spectrum (Berkaloff et al., 1971).
x There exist complex mechanisms in plants to dissipate photons directly into heat, bypassing completely photosynthesis.These mechanisms involve inducing particular electronic de-excitations using dedicated enzymes and proteins and come in a number of distinct classes. Constitutive mechanisms, allow for intersystem crossing of the excited chlorophyll molecule into triplet, long-lived, states which are subsequently quenched by energy transfer to the carotenoids. Inducible mechanisms can be regulated by the plant itself, for example, changing lumen pH causes the production of special enzymes that permit the non-photochemical de-excitation of chlorophyll. Sustained mechanisms are similar to inducible mechanisms but have been adapted to long term environmental stress. For example, over-wintering evergreen leaves produce little photosynthesis due to the extreme cold but continue transpiring by absorbing photons and degrading these to heat through non-photochemical de-excitation of chlorophyll. Hitherto, these mechanisms were considered as “safety valves” for photosynthesis, protecting the photosynthetic apparatus against light-induced damage (Niyogi, 2000). However, their existence and evolution can better be understood in a thermodynamic context as augmenting the entropy production potential of the plant through increased transpiration.
The recent findings of microsporine-like amino acids (MAAs) produced by plants and phytoplankton having strong absorption properties in the UVB and UVA regions follows their discovery in fungi (Leach, 1965). They are small (< 400 Da), water-soluble compounds composed of aminocyclohexenone or aminocycloheximine rings with nitrogen or imino alcohol substituents (Carreto et al., 1990) which display strong UV absorption maximum between 310 and 360 nm and high molar extinction (Whitehead and Hedges, 2002). These molecules have been assigned a UV photoprotective role in these organisms, but this appears dubious since more than 20 MAAs have been found in the same organism, each with different but overlapping absorption spectrum, determined by the particular molecular side chain (Whitehead and Hedges, 2002). If their principle function were photoprotective, there existence would be confined to those UV wavelengths that cause damage to the organism, and not to the whole UV broadband spectrum.
Plants also perform a free energy intensive process known as photorespiration in which O2 instead of CO2 is captured by the binding enzyme RuBisCo, the main enzyme of the light iindependent part of photosynthesis. This capture of O2 instead of CO2 (occurring about 25% of the time) is detrimental to the plant for a number of reasons, including the production of toxins that must be removed (Govindjee, 2005) and does not lead to ATP production. There is no apparent utility to the plant in performing photorespiration and in fact it reduces the efficiency of photosynthesis. It has often been considered as an “evolutionary relic” (Niyogi, 2000), still existing from the days when O2 was less prevalent in the atmosphere than today and CO2 more so (0.78% CO2 by volume at the rise of land plants during the Ordovician (ca. 470 Ma) compared with only 0.038% today). However, such an explanation is not in accord with the known efficacy of natural selection to eliminate useless or wasteful processes.Another theory has photorespiration as a way to dissipate excess photons and electrons and thus protect the plants photosynthesizing system from excess light-induced damage (Niyogi, 2000). Since photorespiration is common to all C3 plants, independent of their insolation environments, it is more plausible that photorespiration, being completely analogous to photosynthesis with respect to the dissipation of light into heat in the presence of water (by quenching of excited chlorophylls) and subsequent transpiration of water, is retained for its complimentary role in evapotranspiration and thus entropy production.
Plants not only evaporate water during sunlight hours, but also at night (Snyder et al., 2003). Common house plants evaporate up to 1/3 of the daily transpired water at night (Hernández Candía, 2009). Not all the stoma in C3 and C4 photosynthetic plants are closed at night and some water vapor also diffuses through the cuticle at night. The physiological reason, in benefit of the plant, for night transpiration, if one exists, remains unclear. It, of course, can have no relevance to cooling leaves for optimal photosynthetic rates. Explanations range from improving nutrient acquisition, recovery of water conductance from stressful daytime xylem cavitation events, and preventing excess leaf turgor when water potentials become large during the day (Snyder et al., 2003). However, night transpiration is less of an enigma if considered as a complement to the thermodynamic function of life to augment the entropy production of Earth through the water cycle. In this context, it is also relevant that chlorophyll has an anomalous absorption peak in the infrared at between about 4,000 and 10,000 nm (Gates, 1980), close to the wavelength at which the blackbody radiation of the Earth’s surface at 14 °C peaks.
Cyanobacteria have been found to be living within Antarctic ice at depths of up to 2 m. These bacteria and other lithogenic material absorb solar radiation which causes the formation of liquid water within the ice even though the outside air temperatures may be well below freezing. This heating from below causes excess ablation and sublimation of the overlying ice at rates as high as 30 cm per year (Priscu et al., 2005).
Finally, by analyzing latent heat fluxes (evaporation) and the CO2 flux for plants from various published data sets, Wang et al. (2007) have found vanishing derivatives of transpiration rates with respect to leaf temperature and CO2 flux, suggesting a maximum transpiration rate for plants, i.e. that the particular partition of latent and sensible heat fluxes is such that it leads to a leaf temperature and leaf water potential giving maximal transpiration rates, and thus maximal production of entropy (Wang et al., 2007).
x The Function of Animals
If the primary thermodynamic function of the plants and cyanobacteria is to augment the entropy production of the Earth by absorbing light in the presence of liquid water, it may then be asked: What is the function of higher mobile animal life?Because of their intricate root system which allows the plants to draw up water for evaporation from great depths, plants are not mobile and depend on insects and other animals for their supply of nutrients, cross fertilization, and seed dissemination and dispersal into new environments. The mobility and the short life span of insects and animals mean that through excrement and eventual death, they provide a reliable mechanism for dispersal of nutrients and seeds.
Crustaceans and animal marine life in water perform a similar function as insect and animal life on land. These higher forms of life distribute nutrients throughout the ocean surface through excrement and dying. It is noteworthy that dead fish and mammals do not sink rapidly to the bottom of the sea, but remain floating for considerable time on the surface where, as on land, bacteria break down the organism into its components, allowing photon dissipating phytoplankton to reuse the nutrients, particularly nitrogen. It is interesting that many algae blooms produce a neurotoxin with apparently no other end than to kill higher marine life. There is also a continual cycling of nutrients from the depths of the ocean to the surface as deep diving mammals preying on bottom feeders release nutrients at the surface through excrement and death. Because of this cycling and mobility of animals, a much larger portion of the ocean surface is rendered suitable for phytoplankton growth, offering a much larger area for efficient surface absorption of sunlight and evaporation of waterthan would otherwise be the case.
From this thermodynamic viewpoint, animal life provides a specialized gardening service to the plants and cyanobacteria, which in turn catalyze the absorption and dissipation of sunlight in the presence of water, promoting entropy production through the water cycle. There is strong empirical evidence suggesting that ecosystem complexity, in terms of species diversity, is correlated with potential evapotranspiration (Gaston, 2000). The traditional ecological pyramid should thus be turned on its pinnacle. Instead of plants and phytoplankton being considered as the base that sustains animal life, animals are in fact the unwitting but content servants of plant and phytoplankton life, obtaining thermodynamic relevance only in how they increase the plant and phytoplankton potential for evaporation of water.
We have argued that the basic thermodynamic function of life (and organic material in general) is to absorb and dissipate high energy photons such that the heat can be absorbed by liquid water and eventually transferred to space through the water cycle. Photosynthesis, although relevant to cyanobacteria and plant growth, has only minor relevance to the thermodynamic function of life. Augmenting the water cycle through increased photon absorption and radiation-less relaxation, life augments the entropy production of the Earth in its interaction with its solar environment. We have presented empirical evidence indicating that the evolutionary history of Earth’s biosphere is one of increased photon absorption and dissipation over time, whether on shorter successional, or longer evolutionary, time scales.
x This thermodynamic perspective on life views it as a catalyst for entropy production through the water cycle, and ocean and wind currents.It ties biotic processes to abiotic processes with the universal goal of increasing Earth’s global entropy production and thus provides a framework within which coevolution of the biotic with the abiotic can be accommodated.In important distinction to the hypothesis of Gaia, that mixed biotic-abiotic mechanisms have evolved to maintain the conditions on Earth suitable to life, it is here suggested instead that these biotic-abiotic mechanisms have evolved to augment the entropy production of Earth, principally, but not exclusively, through the facilitation of the water cycle. Life, as we know it, is an important, perhaps even inevitable, but certainly not indispensable, catalyst for the production of entropy on Earth.
Popular hypotheses credit a primordial soup, a bolt of lightning and a colossal stroke of luck. But if a provocative new theory is correct, luck may have little to do with it. Instead, according to the physicist proposing the idea, the origin and subsequent evolution of life follow from the fundamental laws of nature and “should be as unsurprising as rocks rolling downhill.”
From the standpoint of physics, there is one essential difference between living things and inanimate clumps of carbon atoms: The former tend to be much better at capturing energy from their environment and dissipating that energy as heat.Jeremy England, a 31-year-old assistant professor at the Massachusetts Institute of Technology, has derived a mathematical formula that he believes explains this capacity. The formula, based on established physics, indicates that when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life.
“You start with a random clump of atoms, and if you shine light on it for long enough, it should not be so surprising that you get a plant,” England said.
England’s theory is meant to underlie, rather than replace, Darwin’s theory of evolution by natural selection, which provides a powerful description of life at the level of genes and populations. “I am certainly not saying that Darwinian ideas are wrong,” he explained. “On the contrary, I am just saying that from the perspective of the physics, you might call Darwinian evolution a special case of a more general phenomenon.”
England’s theoretical results are generally considered valid. It is his interpretation — that his formula represents the driving force behind a class of phenomena in nature that includes life — that remains unproven. But already, there are ideas about how to test that interpretation in the lab.
“He’s trying something radically different,” said Mara Prentiss, a professor of physics at Harvard who is contemplating such an experiment after learning about England’s work. “As an organizing lens, I think he has a fabulous idea. Right or wrong, it’s going to be very much worth the investigation.”
At the heart of England’s idea is the second law of thermodynamics, also known as the law of increasing entropy or the “arrow of time.” Hot things cool down, gas diffuses through air, eggs scramble but never spontaneously unscramble; in short, energy tends to disperse or spread out as time progresses. Entropy is a measure of this tendency, quantifying how dispersed the energy is among the particles in a system, and how diffuse those particles are throughout space. It increases as a simple matter of probability: There are more ways for energy to be spread out than for it to be concentrated. Thus, as particles in a system move around and interact, they will, through sheer chance, tend to adopt configurations in which the energy is spread out. Eventually, the system arrives at a state of maximum entropy called “thermodynamic equilibrium,” in which energy is uniformly distributed. A cup of coffee and the room it sits in become the same temperature, for example. As long as the cup and the room are left alone, this process is irreversible. The coffee never spontaneously heats up again because the odds are overwhelmingly stacked against so much of the room’s energy randomly concentrating in its atoms.
Although entropy must increase over time in an isolated or “closed” system, an “open” system can keep its entropy low — that is, divide energy unevenly among its atoms — by greatly increasing the entropy of its surroundings. In his influential 1944 monograph “What Is Life?” the eminent quantum physicist Erwin Schrödinger argued that this is what living things must do. A plant, for example, absorbs extremely energetic sunlight, uses it to build sugars, and ejects infrared light, a much less concentrated form of energy. The overall entropy of the universe increases during photosynthesis as the sunlight dissipates, even as the plant prevents itself from decaying by maintaining an orderly internal structure.
Life does not violate the second law of thermodynamics, but until recently, physicists were unable to use thermodynamics to explain why it should arise in the first place. In Schrödinger’s day, they could solve the equations of thermodynamics only for closed systems in equilibrium. In the 1960s, the Belgian physicist Ilya Prigogine made progress on predicting the behavior of open systems weakly driven by external energy sources (for which he won the 1977 Nobel Prize in chemistry). But the behavior of systems that are far from equilibrium, which are connected to the outside environment and strongly driven by external sources of energy, could not be predicted.
This situation changed in the late 1990s, due primarily to the work of Chris Jarzynski, now at the University of Maryland, and Gavin Crooks, now at Lawrence Berkeley National Laboratory. Jarzynski and Crooks showed that the entropy produced by a thermodynamic process, such as the cooling of a cup of coffee, corresponds to a simple ratio: the probability that the atoms will undergo that process divided by their probability of undergoing the reverse process (that is, spontaneously interacting in such a way that the coffee warms up). As entropy production increases, so does this ratio: A system’s behavior becomes more and more “irreversible.” The simple yet rigorous formula could in principle be applied to any thermodynamic process, no matter how fast or far from equilibrium. “Our understanding of far-from-equilibrium statistical mechanics greatly improved,” Grosberg said. England, who is trained in both biochemistry and physics, started his own lab at MIT two years ago and decided to apply the new knowledge of statistical physics to biology.
Using Jarzynski and Crooks’ formulation, he derived a generalization of the second law of thermodynamics that holds for systems of particles with certain characteristics: The systems are strongly driven by an external energy source such as an electromagnetic wave, and they can dump heat into a surrounding bath. This class of systems includes all living things. England then determined how such systems tend to evolve over time as they increase their irreversibility. “We can show very simply from the formula that the more likely evolutionary outcomes are going to be the ones that absorbed and dissipated more energy from the environment’s external drives on the way to getting there,” he said. The finding makes intuitive sense: Particles tend to dissipate more energy when they resonate with a driving force, or move in the direction it is pushing them, and they are more likely to move in that direction than any other at any given moment.
“This means clumps of atoms surrounded by a bath at some temperature, like the atmosphere or the ocean, should tend over time to arrange themselves to resonate better and better with the sources of mechanical, electromagnetic or chemical work in their environments,” England explained.
Self-replication (or reproduction, in biological terms), the process that drives the evolution of life on Earth, is one such mechanism by which a system might dissipate an increasing amount of energy over time. As England put it, “A great way of dissipating more is to make more copies of yourself.” In a September paper in the Journal of Chemical Physics, he reported the theoretical minimum amount of dissipation that can occur during the self-replication of RNA molecules and bacterial cells, and showed that it is very close to the actual amounts these systems dissipate when replicating. He also showed that RNA, the nucleic acid that many scientists believe served as the precursor to DNA-based life, is a particularly cheap building material. Once RNA arose, he argues, its “Darwinian takeover” was perhaps not surprising.
The chemistry of the primordial soup, random mutations, geography, catastrophic events and countless other factors have contributed to the fine details of Earth’s diverse flora and fauna. But according to England’s theory, the underlying principle driving the whole process is dissipation-driven adaptation of matter.
This principle would apply to inanimate matter as well. “It is very tempting to speculate about what phenomena in nature we can now fit under this big tent of dissipation-driven adaptive organization,” England said. “Many examples could just be right under our nose, but because we haven’t been looking for them we haven’t noticed them.”
Scientists have already observed self-replication in nonliving systems. According to new research led by Philip Marcus of the University of California, Berkeley, and reported in Physical Review Letters in August, vortices in turbulent fluids spontaneously replicate themselves by drawing energy from shear in the surrounding fluid. And in a paper appearing online this week in Proceedings of the National Academy of Sciences, Michael Brenner, a professor of applied mathematics and physics at Harvard, and his collaborators present theoretical models and simulations of microstructures that self-replicate. These clusters of specially coated microspheres dissipate energy by roping nearby spheres into forming identical clusters. “This connects very much to what Jeremy is saying,” Brenner said.
Besides self-replication, greater structural organization is another means by which strongly driven systems ramp up their ability to dissipate energy. A plant, for example, is much better at capturing and routing solar energy through itself than an unstructured heap of carbon atoms. Thus, England argues that under certain conditions, matter will spontaneously self-organize. This tendency could account for the internal order of living things and of many inanimate structures as well. “Snowflakes, sand dunes and turbulent vortices all have in common that they are strikingly patterned structures that emerge in many-particle systems driven by some dissipative process,” he said. Condensation, wind and viscous drag are the relevant processes in these particular cases.
“He is making me think that the distinction between living and nonliving matter is not sharp,” said Carl Franck, a biological physicist at Cornell University, in an email. “I’m particularly impressed by this notion when one considers systems as small as chemical circuits involving a few biomolecules.”
Prentiss, who runs an experimental biophysics lab at Harvard, says England’s theory could be tested by comparing cells with different mutations and looking for a correlation between the amount of energy the cells dissipate and their replication rates. “One has to be careful because any mutation might do many things,” she said. “But if one kept doing many of these experiments on different systems and if [dissipation and replication success] are indeed correlated, that would suggest this is the correct organizing principle.”
Brenner said he hopes to connect England’s theory to his own microsphere constructions and determine whether the theory correctly predicts which self-replication and self-assembly processes can occur — “a fundamental question in science,” he said.
Having an overarching principle of life and evolution would give researchers a broader perspective on the emergence of structure and function in living things, many of the researchers said. “Natural selection doesn’t explain certain characteristics,” said Ard Louis, a biophysicist at Oxford University, in an email. These characteristics include a heritable change to gene expression called methylation, increases in complexity in the absence of natural selection, and certain molecular changes Louis has recently studied.
If England’s approach stands up to more testing, it could further liberate biologists from seeking a Darwinian explanation for every adaptation
and allow them to think more generally in terms of dissipation-driven organization. They might find, for example, that “the reason that an organism shows characteristic X rather than Y may not be because X is more fit than Y, but because physical constraints make it easier for X to evolve than for Y to evolve,” Louis said.
“People often get stuck in thinking about individual problems,” Prentiss said. Whether or not England’s ideas turn out to be exactly right, she said, “thinking more broadly is where many scientific breakthroughs are made.”
A new study analyzing over 21,000 participants found that differences in activation of brain regions in different psychological “disorders” may have been overestimated, and confirms that there is still no brain scan capable of diagnosing a mental health concern.
A new study, published in the journal Human Brain Mapping, questions previous findings that specific brain regions are implicated in particular mental health conditions. Instead, according to the researchers, biased study design and the difficulty of publishing negative findings may have led to inaccurate results. While the researchers did find some differences in brain activation between people with mental health conditions and people without mental health conditions, they were not able to discriminate between specific diagnoses. The current study suggests that there are few, if any, differences in brain regions activated by specific mental health conditions. That is, there is still no brain scan that can tell whether a person has depression, social anxiety, or schizophrenia, for example.
Researchers have theorized that the different symptom clusters that form mental health diagnoses are linked to specific regions of the brain. If confirmed, such a finding would suggest that mental health diagnoses have biological components that could be targeted medically. However, the finding of the current study undermines this theory. Instead, the results indicate that while there is a general tendency for large parts of the brain (such as the amygdala and the hypothalamus) to be activated in a number of mental health conditions (as well as when humans are under stress in a number of ways), there is little difference between the varying diagnoses—even for diagnoses as seemingly different as social anxiety, depression, and schizophrenia.
The researchers were led by Emma Sprooten (Department of Psychiatry, Icahn School of Medicine at Mount Sinai, New York City). They used statistical tests to combine the results from 547 studies, which enabled them to analyze the data from 21,692 participants. The studies compared the brain scans of healthy participants with participants who were diagnosed with major depressive disorder, bipolar disorder, schizophrenia, obsessive compulsive disorder (OCD), and anxiety disorders, including social anxiety disorder, generalized anxiety disorder, panic disorder, specific phobias, and post-traumatic stress disorder (PTSD).
The studies in question used functional magnetic resonance imaging (fMRI), a common type of brain scan which creates images based on blood oxygenation levels within the brain. Higher blood oxygenation levels are assumed to indicate areas involved in more activity. Thus, an fMRI result is theorized to indicate which areas of the brain are activated or deactivated for particular tasks or states of being.
Importantly, fMRI has endured its own questions of bias. A recent article, published in the Proceedings of the National Academy of Sciences, confirmed a previous finding that up to 70% of the results in fMRI studies may actually be “false positives”—that is, finding a result when there actually is none. Nikos K. Logothetis wrote, in a 2008 article in Nature, that the fMRI “is an excellent tool for formulating intelligent, data-based hypotheses, but only in certain special cases can it be really useful for unambiguously selecting one of them, or for explaining the detailed neural mechanisms underlying the studied cognitive capacities.” That is, fMRI results can inform the questions we ask, but they can rarely answer those questions. Unfortunately, the neuropsychiatric literature is riddled with fMRI studies that purport to do just that.
Another recent study attempted to showcase just how much fMRI results rely on subjective interpretation. The researcher, Joshua Carp of the University of Michigan, examined a single fMRI event and found that there were 34,560 different results that could be reached by following different analysis procedures. He argues that the choice of analysis procedure is a subjective one, and researchers may try numerous procedures in order to achieve a positive result. He suggests that in the future, researchers must clearly specify which procedure they will use in order to reduce this extraordinary bias.
Sprooten and her colleagues framed their results as addressing the common practice of “reverse inference,” which has been challenged by other researchers as well. In reverse inference, researchers pre-select which brain regions (ROIs) they are going to study in order to maximize potential results—rather than examine the whole brain to determine which areas are activated. Put simply, if you study a particular area, then you will never see if there is activation in other brain regions during your test. You will only find activation in your pre-selected area. This result is often taken to indicate that particular disorders are associated with activation in particular regions—but this conclusion rests on the assumption that researchers would not have found other areas had they examined the whole brain.
The strength of the current study was its ability to compare ROI studies (studies that focused on only specific regions of the brain) with the results from whole-brain studies. The ROI studies tended to find differences in which brain regions were activated by different mental health conditions. However, once the whole-brain studies were factored in, these findings disappeared. When all studies were included, there were no differences between the diagnoses.
Notably, the researchers only included studies that found significant results—that is, those that purported to find differences between those with mental health diagnoses and those without. Their results would likely be even more striking if they factored in the studies with negative results—studies that did not find differences.
“The pre-selection of ROIs, possibly in combination with the difficulty of publishing negative results, seems to bias the literature and may indirectly lead to oversimplification and over-localization of neurobiological models of behavior and symptoms.”
Choosing a brain region to examine, rather than examining the whole brain, appears to lead to biased, oversimplified results. Likewise, the conclusion that Logothetis reaches in his Nature article is that “the limitations of fMRI are not related to physics or poor engineering, and are unlikely to be resolved by increasing the sophistication and power of the scanners; they are instead due to the circuitry and functional organization of the brain, as well as to inappropriate experimental protocols that ignore this organization […]The magnitude of the fMRI signal cannot be quantified to reflect accurately differences between brain regions, or between tasks within the same region.”
The study conducted by Sprooten and her colleagues suggests that many fMRI studies misrepresent the abilities of brain scans. As Logothetis argues,
using fMRI results to confirm pre-existing theories of brain region activation in mental health diagnoses is in direct contradiction of the abilities of the fMRI technology.(It’s FRAUD!)
In short, brain scan research is of limited use in explaining the complex psychological states of human beings. If a neurological answer seems clear and easy, it may be being misrepresented and oversimplified.
Sprooten, E., Rasgon, A., Goodman, M., Carlin, A., Leibu, E., Lee, W. H., & Frangou, S. (2016). Addressing reverse inference in psychiatric neuroimaging: Meta-analyses of task-related brain activation in common mental disorders. Human Brain Mapping. doi:10.1002/hbm.23486 (Abstract)
The ability to communicate effectively with other individuals plays a critical role in the lives of all animals. Whether we are examining how moths attract a mate, ground squirrels convey information about nearby predators, or chimpanzees maintain positions in a dominance hierarchy, communication systems are involved. Here, I provide a primer about the types of communication signals used by animals and the variety of functions they serve.
Animal communication is classically defined as occurring when “…the action of or cue given by one organism [the sender] is perceived by and thus alters the probability pattern of behavior in another organism [the receiver] in a fashion adaptive to either one both of the participants” (Wilson 1975). While both a sender and receiver must be involved for communication to occur (Figure 1), in some cases only one player benefits from the interaction. For example, female Photuris fireflies manipulate smaller, male Photinus fireflies by mimicking the flash signals produced by Photinus females. When males investigate the signal, they are voraciously consumed by the larger firefly (Lloyd 1975; Figure 2). This is clearly a case where the sender benefits and the receiver does not. Alternatively, in the case of fringe-lipped bats, Trachops cirrhosus, and tungara frogs, Physalaemus pustulosus, the receiver is the only player that benefits from the interaction. Male tungara frogs produce advertisement calls to attract females to their location; while the signal is designed to be received by females, eavesdropping fringe-lipped bats also detect the calls, and use that information to locate and capture frogs (Ryan et al. 1982). Despite these examples, there are many cases in which both the sender and receiver benefit from exchanging information. Greater sage grouse nicely illustrate such “true communication”; during the mating season, males produce strutting displays that are energetically expensive, and females use this honest information about male quality to choose which individuals to mate with (Vehrencamp et al. 1989).
I think it is obvious that these situations pertain also to human communication; Asperger individuals may “trust” that true communication is at work: that “honest” communication is the objective. Where we come up short, is not being aware that much so-called “social communication” relies on, and intends, that either the sender or receiver is meant to benefit from the signals (benign outcome); in hypersocial human communication, an unequal outcome is expected.
Animals use a variety of sensory channels, or signal modalities, for communication. Visual signals are very effective for animals that are active during the day. Some visual signals are permanent advertisements; for example, the bright red epaulets of male red-winged blackbirds, Agelaius phoeniceus, which are always displayed, are important for territory defense. When researchers experimentally blackened epaulets, males were subject to much higher rates of intrusion by other males (Smith 1972). Alternatively, some visual signals are actively produced by an individual only under appropriate conditions. Male green anoles, Anolis carolinensis, bob their head and extend a brightly colored throat fan (dewlap) when signaling territory ownership.
Acoustic communication is also exceedingly abundant in nature, likely because sound can be adapted to a wide variety of environmental conditions and behavioral situations. Sounds can vary substantially in amplitude, duration, and frequency structure, all of which impact how far the sound will travel in the environment and how easily the receiver can localize the position of the sender. For example, many passerine birds emit pure-tone alarm calls that make localization difficult, while the same species produce more complex, broadband mate attraction songs that allow conspecifics to easily find the sender (Marler 1955). A particularly specialized form of acoustic communication is seen in microchiropteran bats and cetaceans that use high-frequency sounds to detect and localize prey. After sound emission, the returning echo is detected and processed, ultimately allowing the animal to build a picture of their surrounding environment and make very accurate assessments of prey location.
Compared to visual and acoustic modalities, chemical signals travel much more slowly through the environment since they must diffuse from the point source of production.Yet, these signals can be transmitted over long distances and fade slowly once produced. In many moth species, females produce chemical cues and males follow the trail to the female’s location. Researchers attempted to tease apart the role of visual and chemical signaling in silkmoths, Bombyx mori, by giving males the choice between a female in a transparent airtight box and a piece of filter paper soaked in chemicals produced by a sexually receptive female. Invariably, males were drawn to the source of the chemical signal and did not respond to the sight of the isolated female (Schneider 1974; Figure 3). Chemical communication also plays a critical role in the lives of other animals, some of which have a specialized vomeronasal organ that is used exclusively to detect chemical cues. For example, male Asian elephants, Elaphus maximus, use the vomeronasal organ to process chemical cues in female’s urine and detect if she is sexually receptive (Rasmussen et al. 1982).
Tactile signals, in which physical contact occurs between the sender and the receiver, can only be transmitted over very short distances. Tactile communication is often very important in building and maintaining relationship among social animals. For example, chimpanzees that regularly groom other individuals are rewarded with greater levels of cooperation and food sharing (de Waal 1989).
Tactile signals seem to be problematic in Asperger’s types. Why? To me, it seems that these are instinctively “suspect”, in that they are invasive.
For aquatic animals living in murky waters, electrical signaling is an ideal mode of communication. Several species of mormyrid fish produce species-specific electrical pulses, which are primarily used for locating prey via electrolocation, but also allow individuals searching for mates to distinguish conspecifics from heterospecifics. Foraging sharks have the ability to detect electrical signals using specialized electroreceptor cells in the head region, which are used for eavesdropping on the weak bioelectric fields of prey (von der Emde 1998).
Some of the most extravagant communication signals play important roles in sexual advertisement and mate attraction. Successful reproduction requires identifying a mate of the appropriate species and sex, as well as assessing indicators of mate quality. Male satin bowerbirds, Ptilonorhynchus violaceus, use visual signals to attract females by building elaborate bowers decorated with brightly colored objects. When a female approaches the bower, the male produces an elaborate dance, which may or may not end with the female allowing the male to copulate with her (Borgia 1985). Males that do not produce such visual signals have little chance of securing a mate. While females are generally the choosy sex due to greater reproductive investment, there are species in which sexual roles are reversed and females produce signals to attract males. For example, in the deep-snouted pipefish, Syngnathus typhle, females that produce a temporary striped pattern during the mating period are more attractive to males than unornamented females (Berglund et al. 1997).
Communication signals also play an important role in conflict resolution, including territory defense. When males are competing for access to females, the costs of engaging in physical combat can be very high; hence natural selection has favored the evolution of communication systems that allow males to honestly assess the fighting ability of their opponents without engaging in combat. Red deer, Cervus elaphus, exhibit such a complex signaling system. During the mating season, males strongly defend a group of females, yet fighting among males is relatively uncommon. Instead, males exchange signals indicative of fighting ability, including roaring and parallel walks. An altercation between two males most often escalates to a physical fight when individuals are closely matched in size, and the exchange of visual and acoustic signals is insufficient for determining which animal is most likely to win a fight (Clutton-Brock et al. 1979).
It would seem that “conflict resolution” signals are very important in humans, but observation yields a suspicion that “natural signals” are overridden by socio-cultural signals, causing conflict resolution to be difficult.
Communication signals are often critical for allowing animals to relocate and accurately identify their own young. In species that produce altricial young, adults regularly leave their offspring at refugia, such as a nest, to forage and gather resources. Upon returning, adults must identify their own offspring, which can be especially difficult in highly colonial species. Brazilian free-tailed bats, Tadarida brasiliensis, form cave colonies containing millions of bats; when females leave the cave each night to forage, they place their pup in a crèche that contains thousands of other young. When females return to the roost, they face the challenge of locating their own pups among thousands of others. Researchers originally thought that such a discriminatory task was impossible, and that females simply fed any pups that approached them, yet further work revealed that females find and nurse their own pup 83% of the time (McCracken 1984, Balcombe 1990). Females are able to make such fantastic discriminations using a combination of spatial memory, acoustic signaling, and chemical signaling. Specifically, pups produce individually-distinct “isolation calls”, which the mother can recognize and detect from a moderate distance. Upon closer inspection of a pup, females use scent to further confirm the pup’s identity.
How does this work in human females?
Many animals rely heavily on communication systems to convey information about the environment to conspecifics, especially close relatives. A fantastic illustration comes from vervet monkeys, Chlorocebus pygerythrus, in which adults give alarm calls to warn colony members about the presence of a specific type of predator. This is especially valuable as it conveys the information needed to take appropriate actions given the characteristics of the predator (Figure 4). For example, emitting a “cough” call indicates the presence of an aerial predator, such as an eagle; colony members respond by seeking cover amongst vegetation on the ground (Seyfarth & Cheney 1980). Such an evasive reaction would not be appropriate if a terrestrial predator, such as a leopard, were approaching.
Humans seem to have difficulty identifying “true predators” due to the ability of human predators to disguise themselves using socio-cultural signals that are accepted as non-threatening, familiar and “normal”.
One of the welcome trends in recent social psychology and neuroscience has been the increasing interest in the processes and mechanisms that underlie moral cognition. (Autistics have been assumed by many researchers to have no ability to make moral judgements) Less obviously welcome is the dominant role given, within this research, to moral dilemmas where one must decide whether to sacrifice one person to save a greater number (for a review, see Christensen & Gomila, 2012). These sacrificial dilemmas were inspired by the thought experiments of moral philosophers involving runaway trolleys (Foot, 1967; Thomson, 1985), but in other variants they also include out of control epidemics, desperate survivors on a lifeboat, swinging cranes, and the like.
NT nonsense:The control freak “creators” of these situations ignore other possibilities, for one, that one or more of the 6 people who are potentially at risk will fail to REACT to the oncoming trolley; they will all stand there like idiots and do nothing. Such “research” is trapped in a simplistic magical mental dimension without correlation to actual human behavior, which is particular to the individual.
Sorry, but the Asperger reaction to this “cartoon” version of a physics / engineering travesty presented as a “moral dilemma” is that it is simply ridiculous… Pushing a “FAT MAN” (this illus. is PC version) off a bridge to stop a SPECIFIC trolley, tram, train, truck, or large vehicle is ludicrous. Does the tram have an emergency braking system? How fast is it traveling; how much mass does it have; how far is it from the bridge and the people? Etc., Are you, as the person doing the ‘pushing off’ a 3’6″ child or a defenseman in the NHL? Only a “dumb” NT could imagine that this “might work” and save the other 5 people.
This research focus is rather puzzling.(Thank-you!)These hypothetical dilemmas are complex, far-fetched, and often convoluted.It would be strange to think that they offer the key to understanding moral judgment in general. If we wanted to identify the building blocks of moral judgment, it would presumably be more sensible to start by investigating simple instances of moral judgment such as the judgment that a malicious lie or bullying violence are wrong, and—giving special focus to developmental questions—work our way up from there. Eventually, we are likely to arrive at special cases where it can seem that lying and violence could nevertheless be permitted (whether because needed to prevent an even greater harm or for some other reason). Sacrificial dilemmas would thus be just a minor (if interesting) branch within a much broader inquiry, and their interpretation would depend on prior groundwork done on much simpler, more basic cases. 1
So why this odd focus? One simple explanation is that one of the first neuroimaging studies of moral cognition (Greene, Sommerville, Nystrom, Darley, & Cohen, 2001) used these dilemmas and was published in a major journal, receiving a vast amount of attention. That attention led other researchers to employ this paradigm in other studies. And once a body of research grows around a paradigm, it is easier to build on it than to come up with a new experimental design. Soon everyone is using this paradigm, just because everyone else is. Needless to say, this sociological point is not a good reason to focus so much research on this peculiar paradigm.
A somewhat better reason for this research focus is that sacrificial dilemmas are widely seen as a way to shed light on the fundamental ethical division between utilitarian and non-utilitarian (or “deontological”) approaches to ethics—it is often assumed that by employing such dilemmas, we can uncover the psychological processes and neural mechanisms underlying these opposing ways of thinking about morality (Greene, Nystrom, Engell, Darley, & Cohen, 2004), and perhaps even resolve this fundamental ethical conflict (Greene, 2008; Singer, 2005). Engaging in such grand questions certainly seems more exciting than studying pedestrian moral judgments about everyday harm or dishonesty. However, the relation between sacrificial dilemmas and these philosophical debates is often misunderstood in this literature.
Researchers in this area often seem to assume that philosophers originally introduced “classical” sacrificial dilemmas in order to highlight the division between utilitarianism and deontology, and that such dilemmas play a key role in the dispute between these views. 2 This however is a misunderstanding of the philosophical purpose of these thought experiments.(Yes, indeed! And typical of so many misunderstandings in psychological dogma) The debate between utilitarians and their opponents has indeed often appealed to elaborate thought experiments and fanciful examples, both to criticize utilitarianism and to support it—thought experiments involving, for instance, archbishops and chambermaids in a burning building (Godwin, 1793/1926), the moral integrity of a chemist (Williams, 1973), a child drowning in a pond (Singer, 1972), or a rich uncle drowning in a bathtub (Rachels, 1975). But dilemmas involving runaway trolleys do not figure very prominently in this debate. They were first introduced, and most heavily discussed, as a problem within a strand of deontological ethics (Foot, 1967; Kamm, 2007; Thomson, 1985). To the extent that the aim of this recent empirical research on moral dilemmas is to use the hypothetical cases that most sharply divide utilitarians and their opponents, then this research may be focusing on the wrong examples.
It might be thought that sacrificial dilemmas nevertheless do present a contrast between a utilitarian view (sacrifice one to save a greater number) and opposing deontological view (it is wrong to do so), and as such can still shed light on this ethical division, even if their original philosophical purpose was somewhat different. I will argue however that it is a mistake to interpret the moral judgments of ordinary folk in terms of these philosophical theories.Ordinary responses to sacrificial dilemmas tell us little about utilitarianism or about any grand philosophical dispute.
MORAL DILEMMAS, RIGHT OR WRONG
Some issues with the sacrificial dilemmas paradigm start at a basic level, and can already be traced to that first study, Greene et al. (2001). That study introduced a battery of “personal” and “impersonal” sacrificial dilemmas. Some of these dilemmas—such as the famous sidetrack and footbridge trolley cases—were directly based on philosophical thought experiments. But many were invented for the occasion,and, unfortunately, a significant proportion of these new dilemmas does not involve anything like a clear contrast between utilitarian and non-utilitarian options. For example, in one new “personal” dilemma subjects were asked whether it is morally appropriate to murder an annoying architect—an amoral action that neither utilitarianism nor its opponents would dream of sanctioning (Kahane & Shackel, 2008). That a battery including such scenarios would be associated with stronger activation in emotional parts of the brain is thus hardly a great discovery about deontological ethics (Kahane & Shackel, 2010).
This issue affects, to varying degrees, much of the original battery of personal dilemmas. Unfortunately, a great deal of subsequent research in this area—including some fairly recent studies—continues to use this problematic original battery of dilemmas to study moral judgment, wrongly classifying the judgment, for example, that it is appropriate to murder the annoying architect as a “utilitarian” judgment. 3
This simple problem has not yet been sufficiently recognized, but some later research has more or less found a way around it. Koenigs, Kruepke, Zeier, and Newman (2012) introduced a distinction between “high” and “low” conflict personal dilemmas (that is, dilemmas on which there is significant disagreement between subjects and dilemmas on which there is near complete consensus), and Greene, Morelli, Lowenberg, Nystrom, and Cohen (2008) recommend focusing only on the former to study the contrast between “utilitarian” and “deontological” judgment. Since a large majority of subjects reject the deeply immoral option offered in some of the most problematic dilemmas (e.g. to murder the architect), there was a strong consensus on these dilemmas and they are classified as “low” conflict, and thus appropriately excluded by later studies that focus only on “high” conflict dilemmas.
However, while the focus on “high” conflict dilemmas is a step forward, it is also misleading, and it only partly addresses the problem. A flaw in the content of a set of dilemmas can only be fully addressed by reclassifying the dilemmas in terms of their content. Whether a dilemma involves a genuine contrast between utilitarian and deontological choices surely depends on the content of the dilemma, not on the degree of consensus about it (Kahane & Shackel, 2010). Opinions may be strongly divided about a moral dilemma even if it doesn’t involve a sharp contrast between utilitarian and deontological options (see below for some examples) while there may be strong consensus against (or for that matter, for) the utilitarian option in dilemmas that do involve a genuine contrast between utilitarian and deontological views. In fact, such a strong intuitive consensus against the utilitarian option is a common feature of many thought experiments that—unlike trolley dilemmas—were specifically devised by critics of utilitarianism in order to highlight utilitarianism’s counterintuitive implications. One such example—the “transplant” case, where one is asked whether to kill one person and use his organs to save five others—was actually included in the original battery of personal dilemmas. One might think that, in terms of its content, this dilemma is highly suitable for studying the contrast between utilitarian and deontological judgments. Yet, because almost no one thinks that such an act is morally acceptable, this dilemma is classified as low conflict, and thus excluded from studies that focus only on high conflict dilemmas.
In their interesting recent paper in this journal, Rosas and Koenigs (2014) highlight further problems with this stimuli set: even after the most problematic or irrelevant dilemmas have been removed, a significant number of high conflict dilemmas still fail to present a clean choice between a utilitarian act that maximizes aggregate welfare and a deontological option. This is because the supposedly utilitarian option in these dilemmas could also be supported by factors that are either irrelevant from a strict utilitarian perspective, or are even opposed to a utilitarian approach.
For example, Rosas and Koenigs point out that some high conflict dilemmas involve a strong component of self-interest: the sacrificial act saves not only the lives of strangers, but one’s own life. If subjects endorse this act, they needn’t be driven by the aim of maximizing the greater good; they might be just concerned about their own good. In other personal dilemmas, the person to be sacrificed would die anyway, so the choice is really between them dying and five others dying as well, or them dying and the five getting saved. This feature of these dilemmas is irrelevant from a simple utilitarian perspective, yet it may offer a strong independent reason to endorse the sacrificial act—a reason that some non-utilitarian ethicists endorse. Finally, in some dilemmas the person to be sacrificed is (directly or indirectly) the source of the threat to those who would be saved by the sacrificial act. Thus, in these dilemmas the person to be sacrificed is far from innocent, and may therefore lose the “moral immunity” normally possessed by an innocent bystander. This, again, is a moral factor that should be irrelevant from a straightforward utilitarian standpoint.
An immediate consequence of the above is that the supposedly “utilitarian” option in many high conflict dilemmas still fails to offer a clear contrast between utilitarian and deontological considerations, since the supposedly utilitarian choice can also be supported by strong self-interested reasons, by considerations (such as inevitability) that are also endorsed by many non-utilitarians, or even by explicitly non-utilitarian (i.e. deontological) moral considerations, relating to the guilt of a threatening agent. This therefore casts some doubt on the interpretation of prior studies reporting a supposed utilitarian bias in clinical populations of patients with damage to the ventromedial prefrontal lobe (Koenigs & Tranel, 2007) and of psychopaths (Koenigs et al., 2012).It would be surprising if psychopaths exhibit an unusually strong concern for the greater good; it is not that surprising that they exhibit an unusually strong concern for their own good (see also Kahane, 2014; Kahane, Everett, Earp, Farias, & Savulescu, 2015).
Rosas and Koenigs make a valuable contribution. But they do not go far enough. They want us to move “beyond utilitarianism,” and use the “impure” sacrificial dilemmas to study not utilitarian judgment but other distinctive patterns of response in clinical populations—I will consider this proposal at the end. But Rosas and Koenigs also give the impression that if researchers would just focus on those personal dilemmas that are “pure,” these dilemmas could be used to study utilitarian decision-making, or to identify a “utilitarian bias” in clinical populations. The problem they highlight is important, but it can be easily addressed by refining the dilemmas we use, weeding out the influence of irrelevant moral factors. Unfortunately however the problem with using sacrificial dilemmas to study utilitarian judgment goes far deeper. It cannot be addressed by any simple refinement of stimuli.
DIGGING DEEPER: MISINTERPRETING ORDINARY MORAL THINKING
In the current literature, sacrificial dilemmas are almost invariably interpreted by reference to the contrast between philosophical theories such as the utilitarianism of Bentham and Mill and Kant’s deontology. But that such dilemmas can be used to highlight this contrast in the philosophical context does not automatically mean that this contrast is an illuminating way to interpret the responses of ordinary folk to such dilemmas. After all, utilitarianism and Kantian ethics are abstract theories that were first proposed a couple of hundred years ago in the West, and have never won the adherence of more than a tiny minority. It is doubtful, to say that least, that the forms of moral thinking that they recommend play much of a role in the moral thinking or ordinary people.
Philosophers sometimes contrast such ethical theories with what they call “commonsense morality”—the pre-theoretical moral views of the folk. Needless to say, commonsense morality is hardly a unity, let alone an abstract theory. But despite its messy diversity, it is characterized by a number of key features:
Commonsense morality is clearly not utilitarian: it obviously does not have as its sole aim the maximization of the aggregate welfare of all sentient beings.
Commonsense morality is pluralist: it recognizes a plurality of fairly specific moral rules and considerations, not a single abstract principle like Bentham’s Principle of Utility or Kant’s Categorical Imperative. (This pluralism is shared by some classical deontological theories, such as Ross, 1930/2002.) 4
Consequently, since these different moral rules sometimes conflict, commonsense morality does not always treat these rules as absolutely binding. In some contexts, one of these rules can outweigh or overrule another. Thus, while commonsense morality is deontological (in the loose sense of not being utilitarian), it is not based on a set of absolute prohibitions. For example, most people think that it is generally wrong to lie, but few believe that it is absolutely wrong to lie, in all circumstances. That rigid Kantian deontological view is as much a departure from commonsense as is the utilitarian view that we should always lie when this would lead to a better outcome (Kahane, 2012; Kahane et al., 2012). The current literature often identifies a deontological approach with such absolute prohibitions. This is a mistake.
Commonsense morality (like many other deontological views) gives great moral significance to the prevention of harm and, more generally, to the promotion of people’s welfare. And it gives moral weight to numbers: saving more lives is morally better than saving less, helping many is better than helping few. This moral concern for others’ welfare has traditionally been referred to as the duty of beneficence, and is a feature even of Kantian ethics. 5
This pedestrian moral idea has little to do with utilitarianism. It is not even correctly described as a utilitarian component within commonsense morality. What is distinctive of utilitarianism is not that it gives moral significance to welfare, not even that it gives weight to numbers. What is distinctive about utilitarianism is, first, that it is a maximizing view, requiring us to always act in the way that would lead to the greatest possible amount of aggregate welfare, and second, that it is a radically impartial view, requiring us to treat the welfare of everyone as of equal importance, regardless of whether they are near or far, our children and friends or absolute strangers, human or animal. This is why utilitarianism is sometimes described as generalized (or universal) benevolence.
Needless to say, these are not features of commonsense morality. Commonsense morality is not a maximizing view: we can often fulfill our moral obligations by doing enough to help others, where enough is significantly less than the maximum possible. And commonsense moralityis, in some respects, profoundly partial, allowing us to give significant priority to our own self-interest and to the welfare of those near and dear to us—to prefer, for example, our family, or compatriots, to distant strangers.
According to commonsense morality it is sometimes permissible to overrule some deontological principle if following it would lead to great harm. This is especially true in emergency situations when the harm which would be prevented is very significant (think of medical triage). To illustrate, very few people would endorse Kant’s counterintuitive claim that it is wrong to lie even if this is necessary to prevent a murder.
In the current literature, when subjects judge that it is acceptable to sacrifice one person to save a greater number, this is classified as a utilitarian judgment, and thought to reflect a utilitarian cost–benefit analysis, which is argued by some to be uniquely based in deliberative processes(So illogical and contrary to actual human behavior!) (Cushman, Young, & Greene, 2010), and even in a distinctive neural subsystem (Greene, 2008; Greene et al., 2004).
This, I will now argue, is a misinterpretation of what underlies such “sacrificial” judgments. In fact, it should now be easy to see that such judgments can be better explained in terms of commonsense morality, without the slightest reference to utilitarianism:
Rejecting a deontological rule is not yet a step in a utilitarian direction
It is typically assumed that when subjects endorse such a sacrificial act, they are rejecting a deontological rule against harming others in a direct and personal manner. But even if subjects making such judgments are really rejecting such a deontological rule, that in itself is not yet a move in a utilitarian direction.
There are very many possible deontological rules, and pretty much everyone rejects at least some, even many: liberals rejects such rules relating to purity or hierarchy, libertarians reject some such rules relating to distributive justice, socialists reject such rules relating to property rights, and so forth. What is distinctive of utilitarianism isn’t that it rejects one or some deontological constraints on the maximization of utility, but that it rejects all of them (Kahane & Shackel, 2010).
To reject a specific rule relating to harming others is perfectly compatible with endorsing extreme deontological rules in other contexts. And we have no reason to think that subjects who supposedly exhibit a “utilitarian bias” reject all (or even more) deontological rules—in fact there is evidence that there is no correlation between rejection of these rules in sacrificial dilemmas and rejecting them in other contexts, for example, relating to lying (Kahane et al., 2012).
It is therefore misleading to speak of a “utilitarian bias,” as if this expresses some general pro-utilitarian tendency. We should, at the very best, speak instead of a utilitarian bias in the context of sacrificial dilemmas, allowing that there may be no such a moral bias (or even a contrary tendency) in other contexts.
Supposedly utilitarian judgments in sacrificial dilemmas lack the impartiality that is distinctive of a genuine utilitarian outlook
Utilitarians reject many conventional moral rules. But this rejection is certainly not the core of a utilitarian perspective. Its core is theimpartial maximization of the good of all.The rejection of various deontological rules is just a consequence of that radical moral goal. In fact, the rejection of conventional moral rules is a feature utilitarianism shares with other views that may otherwise be diametrically opposed to it—such as egoism, which is likely to be the normative view that dominates the thinking of psychopaths (Kahane et al., 2015).
But do we have any reason whatsoever to think that subjects who tend to make supposedly “utilitarian” judgments in sacrificial dilemmas view morality in more impartial terms compared to others? Not really. It is not only psychopaths and vmPFC patients who are more willing to endorse a “utilitarian” act when it also involves an element of self-interest, but also ordinary folk (Moore et al., 2008). And rates of “utilitarian” judgments are strongly influenced by whether they involve sacrificing (or saving) foreigners versus compatriots (Swann, Gómez, Dovidio, Hart, & Jetten, 2010), or strangers versus family members (Petrinovich, O’Neill, & Jorgensen, 1993)—let alone animals versus humans (Petrinovich et al., 1993). In a recent study, we examined this issue more directly by investigating the relation between a tendency to “utilitarian” judgment in sacrificial dilemmas and a wide range of measures of impartial moral concern for the greater good in other contexts—for example, willingness to give some of one’s money to reduce the suffering of people in need in poor countries, rejection of the idea that the needs of one’s family or compatriots have moral priority over those of distant strangers, or generally identifying more with the whole of humanity. We consistently found either no relation or a negative relation between “utilitarian” judgment and such impartial concern for the greater good (Kahane et al., 2015). But it was anyway rather fanciful to suppose that, if psychopaths do exhibit a “utilitarian” tendency in sacrificial dilemmas, then they must also hold that we should give away much of our money to people in need in Africa if that would make the world a better place.
In other words, the judgments that are now routinely classified as “utilitarian” do not actually exhibit one of the key features that distinguishes a genuine utilitarian view from ordinary moral concern for others’ welfare.
Subjects who make “utilitarian” judgments need not be rejecting the opposing deontological rule
To make things worse, it is doubtful that many of the subjects who make “utilitarian” judgments actually reject the deontological rule against direct and personal harm. The common assumption that subjects are rejecting this rule is based on the mistaken conflation of deontology with absolute prohibition: if you are willing to violate some prohibition, you are clearly not treating it as absolute. But as we saw, many of the rules of commonsense morality are not absolute. They can sometimes be outweighed by other moral considerations including, in the context of emergency situations, the harm that will be prevented if these rules are set aside.
That most people who make “utilitarian” judgments do not simply reject that deontological rule (as utilitarianism requires) is clearly shown by the fact that only very few of the participants of studies using sacrificial dilemmas make utilitarian judgments across the board—participants usually make a mix of utilitarian and deontological judgments, changing their mind from case to case. If these participants were simply rejecting a rule against direct and personal harm, such a pattern of response would make no sense (Kahane, 2012).
Overruling a moral rule in emergency context when lives are at stake is part of commonsense morality
Commonsense morality offers no precise formula for deciding when a given moral rule is outweighed by another, and this can often be a matter of considerable disagreement—people will disagree, for example, on how much harm needs to be prevented for a white lie to be permissible. Most (but probably not all) ordinary folk would endorse pushing a man from a footbridge if that would save a thousand lives, or even dozens. Fewer people, it appears, endorse such acts in order to save only five lives. But, given what I’ve said above, it is doubtful that the latter judgments are qualitatively different from the former. They just involve a different understanding of what counts as preventing sufficient harm to justify overruling this moral rule in the context of such an emergency situation (Kahane, 2012).
“Utilitarian” judgments in sacrificial dilemmas do not aim to maximize aggregate welfare
In the current literature, it is widely assumed that when subjects make “utilitarian” judgments, then this is the result of a utilitarian cost–benefit analysis. It is assumed, in other words, that because these subjects endorse the option that would lead to greater welfare (“five lives saved is greater than one life”), then they must be aiming to maximize welfare. It should be obvious by now, however, that this interpretation is not really licensed by the evidence.If subjects who make “utilitarian” judgment are really aiming to maximize utility, they should also judge that we ought to violently sacrifice one person to save two others, or sacrifice fifty to save fifty one—as a genuine utilitarian should judge. But it is very unlikely, to put it mildly, that these views are endorsed by more than a tiny handful. Moreover, while utilitarianism requires us to always maximize utility, most ordinary folks who make supposedly utilitarian judgments appear to merely hold that it is acceptable or permissible to sacrifice one to save five—a far weaker claim (see e.g. Lombrozo, 2009; Royzman, Landy, & Leeman, 2015). 6
It is therefore a mistake to interpret “utilitarian” judgments as based in strict cost–benefit analysis. These judgments are not driven by any such radical maximizing aim, but by the far more mundane duty of beneficence mentioned above or, even more narrowly, by the unremarkable commonsensical idea that, when we are in an emergency situation and can easily save the lives of others, we have a (prima facie) duty to do so (something sometimes known as the “duty of rescue”).
Deliberative processing is needed to weigh competing moral rules, not to perform a utilitarian cost–benefit analysis
This is not the only problem with claims about utilitarian cost–benefit analysis in this empirical literature. One influential strand of research not only ties “utilitarian” judgment to such cost–benefit analysis, but also claims that such analysis is uniquely tied to effortful deliberative processing—to be contrasted with the more primitive emotional responses that supposedly drive opposing deontological judgments (see for example, Greene, 2008; Greene et al., 2004).
It is rather odd however to think that it takes any kind of effortful cognition to calculate that five lives is greater than one life, or to think that only subjects who end up endorsing “utilitarian” conclusions make this trivial calculation. In fact, for a genuine utilitarian, sacrificial dilemmas should require no effort at all—there is no dilemma, and all one needs to do is to identify the course of action that would lead to most utility, an utterly straightforward decision in this context (Kahane, 2012).
If special cognitive effort is involved in arriving at such “sacrificial” moral conclusions, it is not likely to reflect the calculation of which option would lead to the better outcome (a trivial matter) but rather the weighing of several competing moral considerations—a particularly salient deontological principle telling us that we mustn’t cause certain kinds of harm (a component of what is often known as the duty of non-maleficence), and an opposing duty to prevent grievous harm to others (a component of beneficence)—actually a more complex form of moral deliberation than the mechanical application of utilitarian reasoning. 7 It is this conflict of opposing duties or principles that makes these cases genuine dilemmas for most people—but neither of the moral rules involved has much to do with utilitarianism, a view that, as we just saw, denies that such situations involve any kind of genuine dilemma (Kahane, 2012). 8
CONCLUSION: WHAT NEXT?
There is now a large and growing literature using sacrificial dilemmas to study utilitarian decision-making. The real problem with this literature is not that some of these dilemmas are problematic (although this is a serious issue), but that sacrificial dilemmas tell us very little about utilitarian decision-making. The mistake is to artificially project utilitarianism, a radical and demanding philosophical theory, onto the psychology of ordinary folk. This is not merely a pedantic complaint about terminology. As I have argued, the conceptual framework that currently dominates much research in this area is misleading, leading researchers to misinterpret what is really an everyday, non-utilitarian moral concern in terms of a simplistic—and largely irrelevant—opposition between utilitarian and deontological judgments. Ironically, the form of everyday deliberation that I have suggested really underlies sacrificial judgments is actually richer and more complex than the mechanical utilitarian cost–benefit analysis that is mistakenly projected onto it. 9
I do not want to give the wrong impression that past and current research on sacrificial dilemmas is completely misguided, or of no interest at all. I have argued that it tells us very little about utilitarianism (let alone offers any grounds for endorsing a utilitarian approach to ethics), but it certainly tells us something about the structure and psychological basis of certain commonsensical constraints on when it is morally permissible to harm others, and about when and why some people adhere to these constraints and others don’t. This is an interesting if rather narrow and unusual part of ordinary morality—it is not particularly central even to the domain of the ethics of harming, a vast and rich domain that ranges from questions about abortion and euthanasia to self-defense and collateral damage, and many other issues in between.
Moreover, the problematic conceptual framework that currently dominates research in this area obscures some important avenues of research. Instead of classifying judgments as utilitarian or deontological, and seeing these as based in utterly distinct neural subsystems or processes, we should try to investigate how different moral considerations are integrated and (when they are in conflict) weighed against each in moral deliberation. Do such moral rules have fixed, invariant weighs or is the weighing process more ad hoc and contextual? Which processes are involved in deciding that a given moral rule has been outweighed by another, and are they different from the processes that drive the outright rejection of a putative rule? Are there emotional processes that play a role—even an essential role—in everyday moral deliberation, for example, in resolving such moral conflicts? These are just some of the questions that could (and should) be investigated but that, so far as I can see, have been overlooked so far.
Sacrificial dilemmas are a peculiar place to start if one wants to investigate ordinary moral cognition. I have argued here that they are not even the right place to start if one wants to investigate utilitarian decision-making. Where should we start, then, if we want to investigate proto-utilitarian tendencies in everyday moral thinking? We should begin, I would suggest, with what is genuinely distinctive of utilitarian moral thinking. Not with the utilitarian’s willingness to dismiss conventional moral rules and norms—which, as we saw, is not only not the core of the view but is actually something utilitarianism that happens to share with very different views, meaning that research focusing on this dimension of utilitarianism risks ending up studying the psychology of views such as egoism, utilitarianism’s very opposite.
One of the things that are distinctive of utilitarianism is its radical impartiality—utilitarianism asks us to transcend our narrow focus on ourselves and those near and dear to us, and to extend our circle of concern to everyone, however geographically, temporally or even biologically distant. Strangely enough, however, this key aspect of utilitarianism has been virtually entirely ignored by current research on “utilitarian” judgment. The psychological basis of a radically impartial attitude to morality is, I believe, a fruitful area for future research. But it is doubtful that sacrificial dilemmas are a useful way to investigate this issue—and similarly doubtful that the psychological factors that dispose some individuals to adopt a more expansive view of morality are similar to those that drive supposedly utilitarian judgments in sacrificial dilemmas (Kahane et al., 2015).
Let me finally end by remarking on the Rosas and Koenigs (2014) suggestion that since many sacrificial dilemmas turn out not to cleanly pit utilitarian and deontological options due to the presence of interfering factors such as self-interest, inevitability of harm, or the guilt of the person to be sacrificed, we should move “beyond utilitarianism” and use these dilemmas to study the influence of these further factors on moral judgment in clinical (and presumably non-clinical) populations. Rosas and Koenigs (2014) provide suggestive evidence that vmPFC patients and psychopaths may exhibit a distinct pattern of moral judgment when these factors are present, a pattern that may be driven by abnormal affective responses. These preliminary results are certainly intriguing, and call for further research. It seems to me doubtful, however, that convoluted sacrificial dilemmas are the best way to investigate these issues.
Just to illustrate, consider the way considerations of self-interest might affect moral judgment in clinical or healthy populations. They might influence moral judgment covertly, in the form of a moral inconsistency: subjects may reject or endorse the very same moral conclusions depending on whether this is in their self-interest. Or self-interest may influence moral judgment overtly given that, as I explained above, commonsense morality sees certain forms of partiality as legitimate: we are often entitled to refuse to make great sacrifices even when this benefits others, and we are entitled to give priority to family, loved ones, and friends over mere strangers. But different people—and different subject populations—are likely to draw these lines in different places, disagreeing over when, for example, some self-sacrifice is too great, or justified partiality becomes mere nepotism. If considerations of self-interest influence the moral judgment of psychopaths to a greater degree than other populations, is this influence covert or also overt? If, compared to other populations, psychopaths give greater moral priority to their self-interest, might they, given their weaker ties to other people, also at the same time be more impartial when it comes to giving such priority to family and friends over strangers? 10 It is hard to see why, in investigating these and similar questions about moral egocentricity (and partiality more generally), we should rely on sacrificial dilemmas that were, after all, designed to address very different questions, and that involve self-interest (and the other factors highlighted by Rosas and Koenigs) only by oversight; we should not make the error of continuing to use this paradigm simply because it has dominated recent research. 11 We should move, not beyond utilitarianism, but beyond runaway trolleys
For generations the definition of life has eluded scientists and philosophers. (Many have come to recognize that the concept of “definition” itself is difficult to define.3) We can, however, list characteristics of the one example of life that we know—life on Earth:
It is chemical in essence; terran living systems contain molecular species that undergo chemical transformations (metabolism) under the direction of molecules (enzyme catalysts) whose structures are inherited, and heritable information is itself carried by molecules.
To have directed chemical transformations, terran living systems exploit a thermodynamic disequilibrium.
The biomolecules that terran life uses to support metabolism, build structures, manage energy, and transfer information take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and the ability of heteroatoms, primarily oxygen and nitrogen, to modulate the reactivity of hydrocarbons.
Terran biomolecules interact with water to be soluble (or not) or to react (or not) in a way that confers fitness on a host organism. The biomolecules found in terran life appear to have molecular structures that create properties specifically suited to the demands imposed by water.
Living systems that have emerged on Earth have done so by a process of random variation in the structure of inherited biomolecules, on which was superimposed natural selection to achieve fitness. These are the central elements of the Darwinian paradigm.
Various published definitions of life understandably incorporate those features, given that we are the life form defining it. Indeed, because the chemical structures of terran biomolecular systems all appear to have arisen through Darwinian processes, it is hardly surprising that some of the more thoughtful definitions of life hold that it is a “chemical system capable of Darwinian evolution.”4
1.3 IS EVOLUTION AN ESSENTIAL FEATURE OF LIFE?
Many of the definitions of life include phrases like undergoes Darwinian evolution. The implication is that phenotypic changes and adaptation are necessary to exploit unstable environmental conditions and to function optimally in the environment. Evolutionary changes have even been suggested for the hypothesized “clay crystal life” of Cairns-Smith,5 referring to randomly occurring errors in crystal structure during crystal growth as analogous to mutations. Would a self-replicating chemical system capable of chemical transformations in the environment be considered life? If self-replicating chemical compounds are not life, replication by itself is not sufficient as a defining characteristic of life. Likewise, the ability to undergo Darwinian evolution, a process that results in heritable changes in a population, is also not sufficient to define life if we consider minerals that are capable of reproducing errors in their crystal structure to be equivalent to evolution. Although that property of clays may have been vital in the origin of life and particularly in the prebiotic synthesis of organic macromolecules and as catalysts for metabolic reactions, can the perpetuation of “mistakes” in crystal structure result in the selection of a “more fit” crystal structure? It is important to emphasize that evolution is not simply reproducing mutations (mistakes in clays), but also selecting variants that are functionally more fit.
The canonical characteristics of life are an inherent capacity to adapt to changing environmental conditions and to interact with other living organisms (and, at least on Earth, also with viruses).6 Natural selection is the key to evolution and the main reason that Darwinian evolution persists as a characteristic of many definitions of life. The only alternative to evolution for producing diversity would be to have environmental conditions that continuously create different life forms or similar life forms with random and frequent “mistakes” in the synthesis of chemical templates used for replication or metabolism. Such mistakes would be equivalent to mutations and could lead to traits that gave some selective advantage in an existing community or in exploiting new habitats. That random process could lead to life forms that undergo a form of evolution without a master information macromolecule, such as DNA or RNA. It is difficult to imagine such life forms as able to “evolve” into complex structures unless other mechanisms, such as symbiosis or cell-cell fusion, are available.
Evolution is the key mechanism of heritable changes in a population. However, although mutation and natural selection are important processes, they are not the only mechanisms for acquiring new genes. It is understood that lateral gene transfer is one of the most important and one of the earliest mechanisms for creating diversity and possibly for building genomes with the requisite information to result in free-living cells.7 Lateral gene transfer is also one of the mechanisms to align genes from different sources into complex functional activities, such as magnetotaxis and dissimilatory sulfate reduction.8 It is possible that this mechanism was important in the evolution of metabolic and biosynthetic pathways and other physiological traits that may have evolved only once even though they are present in a wide variety of organisms. Coevolution of two or more species is also a hallmark of evolution manifested in many ways, from insect-plant interactions to the involvement of hundreds of species of bacteria in the nutrition of ruminant animals. Organisms and the environment also coevolve, depending on the dominant characteristics of the environment and the availability of carbon and energy sources.
If the ability to undergo Darwinian evolution is a canonical trait of life no matter how different a life form is from Earth life, are there properties of evolving extraterrestrial organisms that would be detectable as positive signs of life? Evolution provides organisms the opportunity to exploit new and changing environments, and one piece of evidence for the cosmic ubiquity of evolution is that on Earth life occupies all available habitats and even creates new ones as a consequence of metabolism. Another hallmark of evolution is the ability of organisms to coevolve with other organisms and to form permanent and obligatory associations. It is highly probable that an inevitable consequence of evolution is the elimination of radically different biochemical lineages of life that may have formed during the earliest period of the evolution of life. Extant Earth life is the result of either selection of the most fit lineage or homogenization of some or all of the different lineages into a common ancestral community that developed into the current three major lineages (domains). All have a common biochemistry based on presumably the most “fit” molecular information strategies and energy-yielding pathways among a potpourri of possibilities.
Thus, one of the apparent generalizations that can be drawn from extant Earth life, and the explanation for the development of a “unity of biochemistry” in all organisms, is that lateral gene transfer is an ancient and efficient mechanism for rapidly creating diversity and complexity. Lateral gene transfer is also an efficient mechanism for selecting the genes that are most “fit” for specific proteins and transferring them into diverse groups of organisms. The results are the addition of genes and the replacement of less-fit genes that have similar functions. Natural selection based solely on mutation is probably not an adequate mechanism for evolving complexity. More important, lateral gene transfer and endosymbiosis are probably the most obvious mechanisms for creating complex genomes that could lead to free-living cells and complex cellular communities in the short geological interval between life’s origin and the establishment of autotrophic CO2 fixation about 3.8 billion years ago and microbial sulfate reduction 3.47 billion years ago on the basis of isotope data.9 An important implication of the existence of viruses or virus-like entities during the early evolution of cellular organisms is that their genomes may have been the source of most genetic innovations because of their rapid replication, high rates of mutation due to replication errors, and gene insertions from diverse host cells.10
Is evolution an essential feature of life? Cells are more than the information encoded in their genomes; they are part of a highly integrated biological and geochemical system in whose creation and maintenance they have participated. The unity of biochemistry among all Earth’s organisms emphasizes the ability of organisms to interact with other organisms to form coevolving communities, to acquire and transmit new genes, to use old genes in new ways, to exploit new habitats, and, most important, to evolve mechanisms to help to control their own evolution. Those characteristics would probably be present in extraterrestrial life even if it had a separate origin and a unified biochemistry different from that of Earth life.
1.4 BRIEF CONSIDERATIONS OF POSSIBLE LIFE FORMSOUTSIDE THE SCOPE OF THIS REPORT
As discussed in the literature,11 chemical models of non-Earth-centric life reveal much about what the scientific community considers possible, particularly regarding ways in which systems organize matter and energy to generate life. Thus, truly “weird” life might utilize an element other than carbon for its scaffolding. Less weird, but still alien to human biological experience, would be a life form that does not exploit thermodynamic disequilibria that are largely chemical. Weirder would be a life form that does not exploit water as its liquid milieu. Still weirder would be a life form that exists in the solid or gas phase.12 In a different direction, yet also outside the scope of life that most communities think possible, would be a life form that lacks a history of Darwinian evolution.
Some features of terran life are almost certainly universal, however. In particular, the requirement for thermodynamic disequilibrium is so deeply rooted in our understanding of physics and chemistry that it is not disputable as a requirement for life. Other criteria are not absolute. Terran biology contains clear examples of the use of nonchemical energy; photosynthesis is the best known, although energy from light is soon converted to chemical energy. Silicon, in some environments, can conceivably support the scaffolding of large molecules. This report explicitly considers nonaqueous environments.
Even Darwinian evolution is presumably not an absolute. For example, depending on how human civilization applies gene therapy, our particular form of life could be able to evolve via Lamarckian,a as opposed to Darwinian, processes. Humankind will be able to perceive and solve problems in human biology without needing to select among random events, thus sparing the species the need to remove unavoidable genetic defects through the death of individuals. That will make the human biosphere no less living, even to those who make Darwinian evolution central in their concept of life.
Likewise, we can easily conceive of robots that are self-reproducing or computer-based processes that grow and replicate.13 Here, information transfer is not based on a specific molecular replication but on a replication involving information on a matrix. Whether such entities will be called life remains to be seen.
What is clear is that the scientific community does not believe that Lamarckian, robotic, or informational “life” could have arisen spontaneously from inanimate matter. At the very least, its matrix would have to be constructed initially by a chemical, Darwinian life form arising from processes similar to those seen on Earth. Again, it is not clear whether those views are constrained by our inability to conceive broadly from what we know or whether they reflect true constraints on the processes by which life might emerge in natural history.
Lamarck recognized a similar principle of evolution referred to as “inheritance of acquired characters,” stating that variations in characteristics seen in organisms were acquired in response to the environment.
Those thoughts introduce a subsidiary theme of this report. It is conceivable that chemistry, structure, or environments able to support life were not suited for the initiation of life. For example, Earth can support life today, but prevailing views hold that life could not have originated in an atmosphere that is as oxidizing as Earth’s today. If that is true, the surface of Earth would be an environment that is habitable but not able to give rise to life.
STRATEGIES TO MITIGATE ANTHROPOCENTRICITY
We have only one example of biomolecular structures that solve problems posed by requirements for life, and the human mind finds it difficult to create ideas truly different from what it already knows. It is thus difficult for us to imagine how life might look in planetary environments very different from what we find on Earth. Recognizing that difficulty, the committee chose to embrace it. The committee exploited a strategy that began with characterization of the terran life that humankind has known well, first because of its macroscopic visibility and then through microscopic observation that began in earnest 4 centuries ago. This, of course, is like life that is associated with humankind. As the next step in the strategic process, the committee assembled a set of observations about life that is considered exotic when compared with human-like life. Exploration of Earth has taken researchers to environments that human-like organisms find extreme, to the highest temperatures at which liquid water is possible, to the lowest temperatures at which water is liquid, to the depths of the ocean where pressures are high, to extremes of acidity and alkalinity, to places where the energy flux is too high for human-like life to survive, to locales where thermodynamic disequilibria are too scarce to support human-like life, and to locations where the chemical environment is toxic to human-like life.
The committee then asked, Can we identify environments on Earth where Darwinian processes that exploit human-like biochemistry cannot exploit available thermodynamic disequilibria? The answer is an only slightly qualified no. It appears that wherever the thermodynamic minimum for life is met on Earth and water is found, life is found. Furthermore, the life that is found appears to be descendant from an ancestral life form that also served as the ancestor of humankind (perhaps we would not necessarily have recognized it if its ancestry were otherwise) and exploits fundamentally human-like biochemistry.
The committee then reviewed evidence of abiotic processes that manipulate organic material in a planetary environment. It asked whether the molecules that we see in contemporary terran life might be understood as the inevitable consequences of abiotic reactivity. Although signatures of such predecessor reactivity can be adumbrated within contemporary biochemistry, they are generally faint.14 Some 4 billion years of biological evolution have attached a strong Darwinian signature to whatever went before; hypotheses regarding evidence of our inanimate ancestry within modern biostructures are the subject of intense dispute.
If life originated first on Earth, it was long ago when conditions on the surface of this planet were very different from what they are today. We do not know what those conditions were, and we may never know. Furthermore, the organisms around today are all highly evolved descendants of the first life forms and probably contributed long ago to the demise of their less fit, more primitive competitors. The historical slate has been wiped clean both geologically and biologically. Finally, because life forms replicate, singular events can have enormous impacts on future developments. Life does not have to be a probable outcome of spontaneous physicochemical processes, although it may well be. Arguments based on probability are not as powerful in this sphere as they usually are in the physical sciences.
The committee surveyed the inventory of environments in the solar system and asked which non-Earth ones might be suited to life of the terran type. Such locales are few, unless there are laws not now understood that could govern the early stages of the self-organization of biochemical structures and processes that could lead inevitably to evolving life forms.15 Subsurface Mars and the putative sub-ice oceans of the Galilean satellites are the only locales in the solar system (other than Earth itself) that are clearly compatible with terran biochemistry.
The committee’s survey made clear, however, that most locales in the solar system are at thermodynamic disequilibrium—an absolute requirement for chemical life. Furthermore, many locales that have thermodynamic disequilibrium also have solvents in liquid form and environments where the covalent bonds between carbon and other lighter elements are stable. Those are weaker requirements for life, but the three together would appear, perhaps simplistically, to be sufficient for life. The committee asked whether it could conceive of biochemistry adapted to those exotic environments, much as human-like biochemistry is adapted to terran environments. Few detailed hypotheses are available; the committee reviewed what is known, or might be speculated, and considered research directions that might expand or constrain understanding about the possibility of life in such exotic environments.
Finally, the committee considered more exotic solutions to problems that must be solved to create the emergent properties that we agree characterize life. It considered a hierarchy of “weirdness”:
Is the linear dimensionality of biological molecules essential? Or can a monomer collection or two-dimensional molecules support Darwinian evolution?
Must a standard liquid of some kind serve as the matrix for life? Can a supercritical fluid serve as well? Can life exist in the gas phase? In solid bodies, including ice?
Must the information content of a living system be held in a polymer? If so, must it be a standard biopolymer? Or can the information to support life be placed in a mineral form or in a matrix that is not molecularly related to Darwinian processes?
Are Darwinian processes and their inherent struggle to the death essential for living systems? Can altruistic processes that do not require death and extinctions and their associated molecular structures support the development of complex life?
Suggested Citation:“1 Introduction.” National Research Council. 2007. The Limits of Organic Life in Planetary Systems. Washington, DC: The National Academies Press. doi: 10.17226/11919.
1. Sagan, C. 1973. Extraterrestrial life. Pp. 42-67 in Communication with Extraterrestrial Intelligence CETI (C. Sagan, ed.). MIT Press, Cambridge, Mass.
2. Ward, P. 2005. Life as We Do Not Know It. Viking, New York.
3. Cleland, C.E. 2001. Historical science, experimental science, and the scientific method. Geology 29:987-990.
4. Joyce, G.F., Young, R., Chang, S., Clark, B., Deamer, D., DeVincenzi, D., Ferris, J., Irvine, W., Kasting, J., Kerridge, J., Klein, H., Knoll, A., and Walker, J.1994. In Origins of Life: The Central Concepts (D.W. Deamer and G.R. Fleischaker, eds). Jones and Bartlett, Boston, Mass.
5. Cairns-Smith, A.G. 1982. Genetic Takeover and the Mineral Origins of Life. Cambridge University Press, Cambridge, U.K.
6. See Brown, J.R., 2003, Ancient horizontal gene transfer, Nature Rev. Genetics 4:121-132; Martin, W., Rotte, C., Hoffmeister, M., Theissen, U., Gelius-Dietrich, G., Ahr, S., and Henze, K., 2003, Early cell evolution, eukaryotes, anoxia, sulfide, oxygen, fungi first (?), and a tree of genomes revisited, IUBMB Life 55:193-204; Ochman, H., Lawrence, J.G., and Groisman, E.S., 2000, Lateral gene transfer and the nature of bacterial innovation, Nature 405:299-304; and Woese, C.R., 2002, On the evolution of cells, Proc. Natl. Acad.Sci. U.S.A. 99:8742-8747.
7. Martin, W., Rotte, C., Hoffmeister, M., Theissen, U., Gelius-Dietrich, G., Ahr, S., and Henze, K. 2003. Early cell evolution, eukaryotes, anoxia, sulfide, oxygen, fungi first (?), and a tree of genomes revisited. IUBMB Life 55:193-204.
8. See Grünberg, K., Wawer, C., Tebo, B.M., and Schüler, D., 2001, A large gene cluster encoding several magnetosome proteins is conserved in different species of magnetotactic bacteria, Appl. Environ. Microbiol. 67:4573-4582; Mazel, D., 2006, Integrons: Agents of bacterial evolution, Nature Rev. Microbiol. 4:608-620; and Mussmann, M., Richter, M., Lombardot, T., Meyerdierks, A., Kuever, J., Kube, M., Glöchner, O., and Amann, R., 2005, Clustered genes related to sulfate respiration in uncultured prokaryotes support the theory of their concomitant horizontal transfer, J. Bacteriol. 187:7126-7127.
9. See Rosing, M.T., 1999, 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland, Science 283:674-676; Shen, Y., Buick, R., and Canfield, D.E., 2001, Isotopic evidence for microbial sulphate reduction in the early Archaean era, Nature 410:77-81; and Shidlowski, M.A., 1988, A 3800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333:313-318.
10. Claverie, J.M., 2006, Viruses take center stage in cellular evolution, Genome Biol. 7:110; Forterre, P., 2006, The origin of viruses and their possible roles in major evolutionary transitions, Virus Res. 117:5-16; Forterre, P., 2006, Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: A hypothesis for the origin of cellular domain, Proc. Natl. Acad. Sci. U.S.A. 103:3669-3674; and Koonin, E.V., and Martin, W., 2005, On the origin of genomes and cells within inorganic compartments, Trends Genetics 21:647-654.
11. Benner, S.A., Ricardo, A., and Carrigan, M.A. 2004. Is there a common chemical model for life in the universe? Curr. Opinion Chem. Biol. 8:672-689.
12. Allamandola, L.J., and Hudgins, D.M. 2003. From interstellar polycyclic aromatic hydrocarbons and ice to astrobiology. In Proceedingsof the NATO ASI, Solid State Astrochemistry (V. Pirronello and J. Krelowski, eds.). Kluwer, Dordrecht.
13. Adami, C., and Wilke, C.O., 2004, Experiments in digital life, Artificial Life 10:117-122; Rosing, M.T., 1999, 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland, Science 283:674-676; Shen, Y., Buick, R., and Canfield, D.E., 2001, Isotopic evidence for microbial sulphate reduction in the early Archaean era, Nature 410:77-81; and Shidlowski, M.A., 1988, A 3800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333:313-318.
14. Benner, S.A., Ellington, A.D., and Tauer, A. 1989. Modern metabolism as a palimpsest of the RNA world. Proc. Natl. Acad. Sci. U.S.A. 86:7054-7058.
15. Kauffman, S.A. 1995. At Home in the Universe: The Search for Laws of Self-organization and Complexity. Oxford University Press, New York.