September 09, 2005

Stop the presses... huge papers on brain evolution in recent humans

I am very sure that a set of new papers (one two) in Science will generate a huge amount of buzz. I will blog in more detail about them later, but for now, this Red Nova story covers the findings pretty well. The interesting part:
The team also observed geographic differences. For haplogroup D of ASPM, they found that it occurs more frequently in Europeans and surrounding populations including, North Africans, Middle Easterners, and South Asians, and at a lower incidence in East Asians, New World Indians and sub-Saharan Africans. For microcephalin, the researchers found that haplogroup D is more abundant in populations outside of sub-Saharan Africa.
For now, from the papers:

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Fig. 3. Global frequencies of Microcephalin haplogroup D chromosomes (defined as having the derived C allele at the G37995C diagnostic SNP) in a panel of 1184 individuals.

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Fig. 1. Worldwide frequencies of ASPM haplogroup D chromosomes (defined as having the derived G allele at the A44871G diagnostic polymorphism), based on a panel of 1186 individuals.

UPDATE

Here is what these studies mean:
  • Microcephalin and ASPM are genes involved in regulating brain size
  • A variant of Microcephalin has reached very high frequencies in non-Sub-Saharan Africans in the last 37,000 years.
  • A variant of ASPM has reached very high frequencies especially in Caucasoids but also in some southern Mongoloids and Australoids in the last 5,800 years.
  • It is almost inconceivable that these two factors were caused by random factors (drift). Therefore selection has acted on these two genes, favoring the new Microcephalin variant in non-Sub-Saharan Africans and the ASPM especially in Caucasoids, but also to a lesser extent in some southern Mongoloid and Australoid groups.
  • We know absolutely nothing about what the new Microcephalin and ASPM variants actually do. What we do know is that they confer some substantial advantage that has caused them to grow in numbers. Perhaps, they confer some cognitive or behavioral ability.
These are the facts. The interpretation of the facts must wait until we have more information. However, the dates for the expansion of the two variants are extremely suggestive.

In a recent article, Erik Trinkaus has surveyed the human paleoanthropological record, and wrote that:
The earliest candidates for human anatomical modernity, those between ca. 150,000 and 195,000 years B.P. in Africa, are best considered as bridging a morphological gap between late archaic and early modern humans.

...

The spread of modern humans thoughout Africa and into Eurasia occurred after 50,000 years B.P. and probably after 40,000 years B.P., 100,000 years after their appearance.
Now, it may be a coincidence that the spread of humans throughout Africa and into Eurasia happened at around the same time that the new Microcephalin variant appeared, but the timing is certainly suggestive.

The question is: why did the new variant not get selected in Sub-Saharan Africans? There are only two possible explanations:
  • There is something in the Sub-Saharan African environment which did not allow the variant to be selected; in other words: the variant did not confer an advantage in Africa itself.
  • The gene pool of most Sub-Saharan Africans did not possess the new Microcephalin variant. Hence the variant did not get selected because it was lacking in the Sub-Saharan African gene pool.

Now, it is well known that the greatest difference in modern human genetic variation is between Sub-Saharan Africans and non-Sub-Saharan Africans. This is the result of the fact that humans originated in Africa, and possess only a subset of the variation that exists there.

As I have written before, there is good reason to believe that a human group originated in eastern Africa ("Afrasians") and came to colonize the rest of the world in relatively recent times. But, the rest of the African continent was already inhabited by pre-existing anatomically modern humans ("Paleoafricans") since at least 150,000 years in the past. These "Paleoafricans" were separated from the "Afrasians", as evidenced by the fact that typical "Paleoafrican" markers, originating long before the 40,000BP cutoff date, such as Y-haplogroups A and B and mtDNA haplogroups L0-L2 are not found in Eurasia.

If my theory is correct, then we don't need to propose some unquantifiable peculiarity of the African environment. Rather, the new Microcephalin variant has a low frequency in Sub-Saharan Africans precisely because it emerged in the Afrasians of eastern Africa that started colonizing the world around 40,000BP and was later added to the Paleoafrican populations of Sub-Saharan Africa. It simply has not had enough time to spread in most of Africa!

The second variant (of ASPM) is even more impressive, because it started to spread only 5,800 years ago, although the confidence margins are wide. The only movement which could have affected so many populations of Eurasia, regardless of language, in the last few millennia is the Neolithic expansion, followed by population growth in the first civilizations of the Near East and China.

It seems all by certain that the variant first appeared in Western Eurasia. It could have been carried easily to the east by the Near Eastern Neolithic people who reached India. It would only take a small step to make the jump to the Mongoloid world; once introduced into the population, it would also undergo the same selection process that made it so frequent among Caucasoids. However, agriculture begins much later among Mongoloids and even later among Australoids. So, the low frequency of the new variant in these populations is a consequence of the fact that it has had less time to spread among these populations.

The new ASPM variant is lacking in Sub-Saharan Africans and Native Americans. These results can be easily explained:
  • Sub-Saharan African agriculture is late, and moreover there has been almost no gene flow from Eurasia into Sub-Saharan Africa, with a few occasional exceptions. So, the ASPM variant did not exist in the Sub-Saharan African gene pool, and could thus have not been selected.
  • Native Americans migrated into the New World in Paleolithic times. Naturally, the ASPM variant was not present in their ancestral gene pool yet, so it could not have been selected.
The importance of these new papers is that cognitive evolution in Homo sapiens did not stop after our lineage became anatomically modern, and it did not stop when a subet of anatomically modern humans set out to colonize the world 40,000 years ago from eastern Africa, and it did not stop when Neolithic man appeared 10,000 years ago. Evolution has continued, and the proof for it is in the genetic variation of living human populations.

Science, Vol. 309 No. 5741

Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans

Patrick D. Evans et al.

The gene Microcephalin (MCPH1) regulates brain size and has evolved under strong positive selection in the human evolutionary lineage. We show that one genetic variant of Microcephalin in modern humans, which arose ~37,000 years ago, increased in frequency too rapidly to be compatible with neutral drift. This indicates that it has spread under strong positive selection, although the exact nature of the selection is unknown. The finding that an important brain gene has continued to evolve adaptively in anatomically modern humans suggests the ongoing evolutionary plasticity of the human brain. It also makes Microcephalin an attractive candidate locus for studying the genetics of human variation in brain-related phenotypes.

Link

Ongoing Adaptive Evolution of ASPM, a Brain Size Determinant in Homo sapiens

Nitzan Mekel-Bobrov et al.

The gene ASPM (abnormal spindle-like microcephaly associated) is a specific regulator of brain size, and its evolution in the lineage leading to Homo sapiens was driven by strong positive selection. Here, we show that one genetic variant of ASPM in humans arose merely about 5800 years ago and has since swept to high frequency under strong positive selection. These findings, especially the remarkably young age of the positively selected variant, suggest that the human brain is still undergoing rapid adaptive evolution.

Link

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