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
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