Philosophical Transactions of the Royal Society / Biological sciences
The evolution and genetics of cerebral asymmetry
This topic does have implications for fundamental “differences” in the Asperger brain.
The “notion” that the human brain is “exclusive” in being organized in right left asymmetrical location for “language” and other functions is dealt with as an oversimplified division between “the rest of” animal life and “Homo sapiens” the Supreme being. The article is long – please see original for detailed discussion. I’ve “clipped” some basics in order to establish how asymmetry may explain Asperger “defects” as differences in the asymmetry of the brain. Again – there is no “standard off-the-assembly line” NORMAL BRAIN. To persist in this SOCIAL fiction is to deny evolution! Evolutionary processes of selection must have “something to work with” for adaptations to “come into existence” – it’s not “magic!
The asymmetry of the brain raises something of a paradox, since, in most respects, the brains and bodies of most organisms, including humans, are strikingly bilaterally symmetrical. As Palmer (2004) put it, bilateral symmetry is the default condition. The midplane of the developing organism is defined by two axes, the anteroposterior and dorsoventral axes, but there is no left–right axis. Instead, the left and right halves of the organism are constructed from separate mediolateral axes. Since these axes are mirror images, the resulting organism will be bilaterally symmetrical, unless there is some symmetry-breaking step. Indeed, most organisms, including humans, belong to the phylum known as Bilateria, which goes back some 600 million years (Chen et al. 2004).
For animals that move freely in the natural world, bilateral symmetry is adaptive, because symmetrically placed limbs, be they legs, wings or flippers, provide for linear movement, which is more efficient than motion in an arc. Directional motion creates a back–front asymmetry, so that eyes and mouth are placed forward, but asymmetry tends to be preserved with respect to left and right. Any sensory asymmetry would create an increased risk of predation from the weaker side. In a world in which left–right parity is largely conserved, then, bilateral symmetry is a natural adaptation.
Against this strong background of bilateral symmetry, our brains and bodies exhibit some striking left–right asymmetries. Indeed, asymmetries are widespread in nature, albeit superimposed on a body plan that is fundamentally bilaterally symmetrical. Many asymmetries are so-called fluctuating asymmetries, which are random variations from symmetry, usually slight, and these are not of concern here. Rather, my focus is on cerebral and behavioural asymmetries in which the direction of asymmetry in the majority of individuals in a population is in the same direction. Such asymmetries suggest that bilateral symmetry is readily and systematically broken if asymmetry proves more adaptive.
This is true of the internal organs, including the heart, lungs, stomach and liver, which are arranged asymmetrically, presumably in the interests of more efficient packaging, and perhaps also of more effective function.
The brain and nervous system, on the other hand, are more directly concerned with sensorimotor activity, and are, for the most part, organized symmetrically. Superimposed on the fundamental symmetry of the brain, though, are a number of systematic asymmetries.
In humans, at least, the most obvious asymmetry is handedness. In the great majority of the human population, one hand is clearly dominant in activities such as writing and throwing, and, in approximately 90 per cent of the population, the dominant hand is the right hand. This asymmetry is not at all obvious in the actual structure of the hands themselves, although there are some differences in muscle strength and bone density favouring the dominant hand; at least some of these are a consequence rather than a cause of greater use of the dominant hand (see Steele & Uomini 2005, for review). Handedness is much more obviously a matter of differential skill and activity between the hands, reflecting a cerebral asymmetry rather than a mechanical one. Since the pioneering discoveries of Broca (1861), it is well established that the left hemisphere of the brain is also dominant for language, especially those aspects of language concerned with production. It is also clear that there are complementary specializations of the right hemisphere (Sperry 1982; Corballis 1991; Mort et al. 2003).
It is widely held that these asymmetries are uniquely human, … but it is becoming increasingly clear that cerebral asymmetry itself is not. Furthermore, many of the lateralized functions documented in non-humans are probably precursors to those functions we regard as uniquely human (e.g. Rogers & Andrew 2002; Corballis 2003)
I will jump to the conclusion section, and then go back to evolutionary implications And possible Asperger Implications in another post.
Recent research has made it abundantly clear that handedness and cerebral asymmetry are not unique to humans. Nevertheless, activities that are uniquely human, such as language, or that are more highly developed in human than in other species, such as manual skill (dexterity), may well have exploited asymmetry in ways not evident in non-human species. Asymmetries are too ubiquitous to be attributed simply to environmental influences, but their genetic bases remain elusive. Single-gene models can provide good fits to phenotypic data, but the location of the putative gene is not known.
A paradoxical feature of the various behavioural and cerebral asymmetries reviewed above is that they do not apply to all members of the species. This suggests that there is some adaptive advantage in maintaining variations in laterality, along with advantages to asymmetry itself. In this respect, these asymmetries contrast with the characteristic asymmetry of the heart and other visceral organs, where reversals and other variations are extremely rare and usually attributed to pathology. In this paper, I have sketched a number of hypotheses as to why variation of lateralization might itself be adaptive. These are not necessarily mutually incompatible, although they do suggest a number of issues that might guide further inquiry. One is whether the genetic component is fixed, and variations arise simply from environmental or epigenetic influences, or whether the variation itself is genetic in origin. If the latter, the question is whether the balance of asymmetries is maintained through frequency-based selection, or through a mechanism such as heterozygotic advantage.
At present, it seems more likely that the balance is one between asymmetry and symmetry, rather than between symmetry and antisymmetry. First, in Bilateria—the phylum to which we and most other animals belong—symmetry is the default condition, and the advantages of asymmetry must be weighed against the long-standing adaptiveness of symmetry in a world lacking systematic left–right biases. Second, genetic models and genetic evidence suggests that genes rarely determine the direction of asymmetry, but influence instead whether or not an asymmetry will be expressed. In research on lateralization in humans, it has been traditional to compare individuals of opposite laterality, as in comparing left- and right-handers or left and right cerebral hemispheres, but, from both evolutionary and genetic perspectives, it may be more fruitful to compare degrees rather than directions of lateralization. We have seen that those without consistent handedness, for example, may differ from both the left- and right-handers in terms of both intellectual abilities and personality characteristics such as magical ideation, delusional behaviour and possibly creativity. This is one avenue, I suggest, that it might be useful to explore further.
Question: Do Asperger’s have more symmetrical (balanced) brains that retain better visual processing than “neurotypicals” without sacrificing verbal processing? Are neurotypical brains highly asymmetrical due to social selection for verbal dominance (magical thinking)?