Saturday, July 11, 2020

Did women jumpstart recent cognitive evolution?



Scatter plots of frequencies of CASC5 variants by sex (Shi et al. 2017). During the last ice age, natural selection favored an increase in the gray matter of ancestral East Asians ... primarily in women.


Back in 2005 there was much interest in genes that regulate brain size, particularly in the ways they varied geographically within our species. It was found that two of these genes, Microcephalin and ASPM, continued to evolve as modern humans spread out of Africa. The latest variant of Microcephalin arose some 37,000 years ago in Eurasia and is still largely confined to the indigenous peoples of Eurasia and the Americas (Mekel-Bobrov et al. 2005). The latest variant of ASPM appeared even later, some 5,800 years in the Middle East (Evans et al. 2005).

Interest fell off when no association could be shown between the new variants and IQ or brain size (Mekel-Bobrov et al. 2007; Rushton et al. 2007; see also Frost 2020). Since then, we have learned that the new ASPM variant is associated with a larger cerebral cortex, and not a larger brain as a whole. Overall brain volume seems to be constrained in modern humans, perhaps by the breadth of a woman's pelvis during childbirth or simply by the high metabolic costs of brain tissue (Ali and Meier 2008; Frost 2020). As for the lack of an association with IQ, we now know that IQ correlates poorly or not at all with some cognitive abilities, like executive function and face recognition.

But what do the new variants actually do? Perhaps a specialized mental task.  It has been suggested that the new ASPM variant assists the brain in processing non-tonal language or alphabetical script (Dediu and Ladd 2007; Frost 2007).


CASC5, another gene for brain growth

Interest has since grown in another gene that regulates brain growth, CASC5. Like Microcephalin and ASPM, it has undergone recent evolution in the modern human lineage:

[...] the CASC5 gene contains mutations in modern humans, but not in Denisovans (Meyer et al. 2012) and this gene also shows distinct sequence divergence between modern humans and Neanderthals (Prufer et al. 2014). These data suggest that CASC5 is an important gene for human neurogenesis, and may harbor modern human specific mutations contributing to the recent evolutionary change of the human brain. (Shi et al. 2017)

Shi et al. (2017) found evidence of recent evolutionary change. Specifically, two nucleotides of CASC5 have been replaced with a new variant in all modern humans. Six other nucleotides have become polymorphic, with some people having the new variants and others not. These polymorphisms show regional differences:

- In four of the polymorphisms, the new variant has a much higher frequency in East Asians than in Europeans or Africans.
- In one polymorphism, it has a much higher frequency in Europeans than in the other two regional groups.
- The remaining polymorphism shows no differences in frequency between the three regional groups.

By and large, the new variants have been under strong positive selection, particularly among East Asians. When the authors examined the six polymorphisms, they found signals of selection for five of them in East Asians and for one in Europeans.


The new variants and brain characteristics

The authors then looked for correlations between the new variants and certain characteristics of the brain, specifically total brain volume, gray matter volume, and white matter volume. To this end, 267 healthy participants were recruited for brain imaging (Han Chinese, 178 females and 89 males, mean age 35.4 ± 12.5 years). All of them were free from mental disorders, drug abuse, alcohol dependence, and brain injury.

Gray matter was significantly larger in participants with the new variant than in those with the ancestral variant at five of the nucleotide sites, including the four polymorphic ones—the same ones that showed differences in variant frequency between East Asians and Europeans. When the authors examined the one polymorphism whose variants were equally common in East Asians, Europeans, and Africans, they found no brain differences between participants with the new variant and those with the ancestral one.

When the authors broke their data down by sex, they found that the new variants were significantly associated with a higher volume of gray matter only in women, not in men, although men seemed to trend in the same direction. The authors suggest that this effect would be significant in men if the number of male participants were larger. Probably. But it seems to me there would still be a sex difference, the number of participants being already large enough.


Ice age origin of the new variants

The authors say the new variants became prevalent "after modern humans migrated out of Africa less than 100,000 years ago." We can narrow down the time range further. The new variants are also present at high frequencies among the indigenous peoples of North and South America; therefore, they must have become prevalent before ancestral Amerindians crossed into North America some 12,000 years ago, apparently in a population that was ancestral both to Amerindians and to East Asians. That would be long before the time of recorded history and even before the Holocene, at a time when northern Eurasia was experiencing glacial conditions.

Did those conditions select for cognitive ability? Cold, seasonal environments did impose new cognitive demands on early modern humans, first by increasing their need to plan ahead over a yearly cycle and second by providing them with new tasks: garment making, needlework, weaving, leatherworking, and kiln operation. Women performed those tasks because the environment offered them few opportunities for food gathering—the usual female activity before the advent of farming. They thus moved into artisanal tasks that not only required greater cognitive ability but also offered much potential for further development. This was the "original industrial revolution" and it was led by women (Frost 2019a).

We can better understand this sexual division of labor by studying northern hunter-gatherers of recent times. According to a cross-cultural study, if women are less involved in food gathering, they specialize in activities unrelated to food procurement, i.e., house building, leatherworking, and burden carrying (Waguespack 2005). A study of two Inuit groups found the highest degree of technological complexity in garment making and shelter building, both of which are wholly or largely women's work (Oswalt 1976). Cold environments thus change the sexual division of labor among hunter-gatherers in a crucial way: while men continue to be food providers, women develop new technologies.

These findings may explain the recent evolution of CASC5: women were the focus of selection for cognitive ability during Ice Age times. But why was the selection stronger among ancestral East Asians than among ancestral Europeans? It looks like the climate at that time was more severe in northern Asia than in northern Europe. Europe benefited from the moderating influence of the Atlantic, which made for a milder and moister climate. Conditions were much colder and drier in northern Asia.

The evolution of human intelligence cannot be reduced to a single unified theory. Cold environments emancipated women from the mental straitjacket of food gathering, thus putting humans on the path to social complexity. That path, however, would take them to latitudes farther south in temperate and even tropical environments where they would be exposed to new cognitive demands. With the end of hunting, men moved not only into farming but also into the artisanal activities that women had developed. The same period saw a decline in brain volume that was greater in women than in men—an indication that cognitive demands were particularly high before the Holocene, and even more so for women (Frost 2019b).

The Holocene thus saw northern populations expand southward and eventually cover almost all of Eurasia, North Africa, Oceania, and the Americas. Those populations had a cognitive advantage that made them better able to exploit the social complexity emerging farther south. This point was made by Darwin's colleague Alfred Russel Wallace:

So when a glacial epoch comes on, some animals must acquire warmer fur, or a covering of fat, or else die of cold. Those best clothed by nature are, therefore, preserved by natural selection. Man, under the same circumstances, will make himself warmer clothing, and build better houses; and the necessity of doing this will react upon his mental organisation and social condition [...] a hardier, a more provident, and a more social race would be developed, than in those regions where the earth produces a perennial supply of vegetable food, and where neither foresight nor ingenuity are required to prepare for the rigours of winter. And is it not the fact that in all ages, and in every quarter of the globe, the inhabitants of temperate have been superior to those of tropical countries? All the great invasions and displacements of races have been from North to South, rather than the reverse.


References

Ali, F. and R. Meier. (2008). Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size. Molecular Biology and Evolution 25(11): 2247-2250.

Dediu, D., and R. Ladd. (2007). Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin. Proceedings of the National Academy of Sciences 104(26): 10944-10949

Evans, P. D., Gilbert, S. L., Mekel-Bobrov, N., Vallender, E. J., Anderson, J. R., Vaez-Azizi, L. M., et al. (2005). Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans. Science 309: 1717-1720.

Frost, P. (2007). The spread of alphabetical writing may have favored the latest variant of the ASPM gene. Medical Hypotheses 70: 17-20.

Frost, P. (2019a). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181

Frost, P. (2019b). Why did brain size decrease after the ice age? Evo and Proud, July 6

Frost, P. (2020). A second look at ASPM. Evo and Proud, April 14

Mekel-Bobrov, N., S.L. Gilbert, P.D. Evans, E.J. Vallender, J.R. Anderson, R.R. Hudson, S.A. Tishkoff and B.T. Lahn. (2005). Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens. Science 309: 1720-1722

Mekel-Bobrov, N., D. Posthuma, S.L. Gilbert, P. Lind, M.F. Gosso, et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Human Molecular Genetics 16(6): 600-608.

Oswalt, W.H. (1976). An Anthropological Analysis of Food-Getting Technology, 1st ed.; John Wiley and Sons: New York, NY, USA.

Rushton, J.P., P.A. Vernon, and T.A. Bons. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK 3(2): 157-60.

Shi, L., Hu, E., Wang, Z. et al. (2017). Regional selection of the brain size regulating gene CASC5 provides new insight into human brain evolution. Human Genetics 136: 193-204. https://doi.org/10.1007/s00439-016-1748-5

Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist 107: 666-676.

Wallace, A.R. (1864). The Origin of Human Races and the Antiquity of Man deduced from the Theory of "Natural Selection." Journal of the Anthropological Society of London, 2, clviii-clxxxvii, Alfred Russel Wallace Classic Writings. Paper 6. http://digitalcommons.wku.edu/dlps_fac_arw/6


4 comments:

Malcolm Smith said...

To what extent, if any, is the new variant found in Australian Aborigines and Melanesians - groups which broke off from the mainstream fairly early after the migration from Africa?

Peter Frost said...

The new variant is absent from New Guineans in four of the five polymorphisms that show regional variation (go to Figure 3 of the paper).

mary said...

Northern Asia was no colder than northern Europe during the Ice Age. In fact, Northern Europe was not inhabited as it was covered by permafrost. It is a certain consensus that Europe suffered a large population reduction during the ice age, perhaps, precisely because it has a good part of its territory unable to sustain life.

Peter Frost said...

Mary,

Northern Asia was drier and colder than northern Europe, largely because Northern Europe was more exposed to the warming and moistening influence of the Gulf Stream. The tundra region of Europe was also farther south because of the ice cap over Scandinavia.

Modern humans inhabited Europe throughout the Ice Age. Even at the glacial maximum, when there was some abandonment of territory, there was still human occupation in much of Eastern Europe. This point is documented by Hoffecker (2002, pp. 192-195)

I can refer you to the following texts:

Goebel, T. (1999). Pleistocene human colonization of Siberia and peopling of the Americas: An ecological approach. Evolutionary Anthropology 8(6): 208-227.
https://doi.org/10.1002/(SICI)1520-6505(1999)8:6<208::AID-EVAN2>3.0.CO;2-M

Graf, K.E. (2009b). Modern human colonization of the Siberian Mammoth Steppe: A view from South-Central Siberia. In M. Camps and P. Chauhan (Eds.) Sourcebook of Paleolithic Transitions, (pp. 484-496). Springer Science & Business Media.
https://doi.org/10.1007/978-0-387-76487-0_32

Hoffecker, J.F. (2002). Desolate landscapes. Ice-age settlement in Eastern Europe. New Brunswick: Rutgers University Press.