Heterochrony: Paedomorphosis-Paedogenesis / Neoteny, Progenesis

NOTE: There are many disagreements about developmental timing in Homo sapiens. Research has focused on amphibians, but “neoteny” has become a subject of interest in primates. The processes involved are time relative, so whether or not a change is “slower or faster” than “expected” can depend on the particular development one chooses to observe.
There are 19 other posts about neoteny in this blog – just use search.

Studies in Heterochrony
Heterochrony is a change in the timing of developmental events. For example, a change in timing might slow down the development of the body, but not alter the maturation of the reproductive system. This change yields an adult organism with a form similar to the ancestral juvenile form. Example: Human adult skull resembles juvenile chimpanzee skull: actually, the proper comparison would be with a common ancestor of human and chimps, since chimps are not ancestors of Homo sapiens.

The classic illustration of

The classic illustration – not correct

Paedomorphosis, also spelled Pedomorphosis, is the retention by an organism of juvenile traits into later life. Two processes may lead to paedomorphosis.

Paedogenesis is reproduction by larval or juvenile animal forms

PROGENESIS – the acceleration of sexual maturation relative to the rest of development. 

NEOTENY – retardation of bodily development with respect to the onset of reproductive activity.

The number of features retarded may differ from species to species. Adult humans, for example, display various neotenic body features that other adult primates do not.In other species all morphological development is retarded; the organism is juvenilized but sexually mature. Such shifts of reproductive capability would appear to have adaptive significance to organisms that exhibit it. In terms of evolutionary theory, the process of paedomorphosis suggests that developmental phases of existing organisms may give rise, under certain circumstances, to wholly new organisms (through selective pressures.)
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My speculation: Paedomorphosis may introduce a “fast track” of adaptation to a changing environment, such as the change from hunter-gatherer-foraging-nomad to concentrations of humans in sedentary agricultural environments: younger age of reproduction may have increased population enough to overcome changing climate, dependence on weather, poor diet, diseases, and the reorganization into social hierarchies (redistribution of resources to “top” individuals) which left a newly emerging “slave-peasant” class vulnerable to social predation. Retention of juvenile behavior plus intensive “socialization” (focus on human environment rather than nature) resulted in a “normal modern brain” that rarely  develops adult cognitive capabilities, but which is arrested in childhood magical thinking: belief in the supernatural power of authority (obedience to adult “titles and roles” rather than to specific merit or knowledge; lacking the ability to reason from physical evidence.) 
Magical thinking dominates modern social humans. In a nutshell; domestication of humans occurred in the same way that domestication of certain animals occurred: selective reproduction. Adaptations that helped Homo sapiens to “multiply” much faster and overcome environmental challenges, may have set us on a disastrous path toward overpopulation, when fossil fuels / industrial revolution introduced a shocking increase in world population. The question is open as to how Homo Sapiens will adapt, or not, to this “second great revolution.” 
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Proc Biol Sci. 2000 Jul 22; 267(1451): 1481–1485.
PMCID: PMC1690691

Neoteny and progenesis as two heterochronic processes involved in paedomorphosis in Triturus alpestris (Amphibia: Caudata).

Current theories on the evolution of paedomorphosis suppose that several ontogenetic pathways have appeared according to different selective pressures. The aim of this study was to find out whether two distinct processes can lead to paedomorphosis in the Alpine newt, Triturus alpestris. In this respect, we compared age structures of paedomorphic and metamorphic individuals in two newt populations where the two forms lived syntopically. Whereas paedomorphosis resulted in a slower rate of somatic development in one population, it resulted in an acceleration of sexual maturation in the other population. These processes correspond to neoteny and progenesis, respectively. These results suggest that phenotypic plasticity can result from contrasted ontogenetic pathways between two populations of the same species. They give support to models that consider gonadic development as the target of selection under different environmental pressures.

J Hum Evol. 2007 Jun;52(6):647-62. Epub 2007 Jan 5.

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A geometric morphometric analysis of heterochrony in the cranium of chimpanzees and bonobos.

Abstract

Despite several decades of research, there remains a lack of consensus on the extent to which bonobos are paedomorphic (juvenilized) chimpanzees in terms of cranial morphology. This study reexamines the issue by comparing the ontogeny of cranial shape in cross-sectional samples of bonobos (Pan paniscus) and chimpanzees (Pan troglodytes) using both internal and external 3D landmarks digitized from CT scans. Geometric morphometric methods were used to quantify shape and size; dental-maturation criteria were used to estimate relative dental age. Heterochrony was evaluated using combined size-shape (allometry) and shape-age relationships for the entire cranium, the face, and the braincase. These analyses indicate that the bonobo skull is paedomorphic relative to the chimpanzee for the first principal component of size-related shape variation, most likely via a mechanism of postformation (paedomorphosis due to initial shape underdevelopment). However, the results also indicate that not all aspects of shape differences between the two species, particularly in the face, can be attributed to heterochronic transformation and that additional developmental differences must also have occurred during their evolution.

PMID:
17298840 [PubMed – indexed for MEDLINE
Paedo
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