NEW variants of two genes that control the development of our brains have swept through much of the world鈥檚 population during the past several thousand years. In short, our brains are still evolving, and the implications of that are far from clear.
The evolution of a large complex brain has been the defining feature of the human lineage, although the size of our brain hasn鈥檛 changed over the past 200,000 years. We don鈥檛 yet know whether the new genetic adaptations discovered in our brains have any effect on brain size, or help to make us any more intelligent.
What鈥檚 more, not everyone has the new gene variants, potentially inflaming the already controversial debate about whether the brains of groups of people from different parts of the world function differently. 鈥淲hatever advantage these genes give, some groups have it and some don鈥檛. This has to be the worst nightmare of people who believe strongly there are no differences in brain function between groups,鈥 says anthropologist John Hawks of the University of Wisconsin in Madison.
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Generally speaking, genes that play important roles in the brain are highly conserved through evolution, with even distantly related species showing little difference in such DNA sequences. This could be because the brain is so complex and crucial to survival that the vast majority of mutations in related genes throw a monkey wrench in the works.
Yet during the few million years of hominin evolution leading to humans, brain size tripled, even though a larger brain uses far more energy and makes childbirth dangerous. Recent genetic studies comparing humans with chimpanzees and other species have also shown that brain-related genes have evolved faster and are more active in the human lineage.
鈥淏rains are very costly, so a large evolutionary increase suggests very strong selection on the brain,鈥 says evolutionary neurobiologist Todd Preuss at Yerkes National Primate Research Center in Atlanta, Georgia. But all the changes discovered so far occurred millions of years ago. Could this trend be continuing today?
Two genetic studies published in Science this week say it may be so. Geneticist Bruce Lahn of the University of Chicago and colleagues analysed the sequences of two genes active in the brain, one called Microcephalin and the other ASPM. Both specifically regulate brain size, as people carrying a non-functioning mutant copy of either of these genes suffer microcephaly, that is, they have brains that are structured normally but are much smaller than usual.
First the researchers sequenced the Microcephalin gene found in 89 ethnically diverse people, and in a single chimpanzee. The team found dozens of variants, or alleles, of the gene, but one particular set stood out. These alleles all carry a specific mutation that changes the protein the gene codes for, and are now in the brains of about 70 per cent of people, half of whom carry exactly the same version. The results suggest the mutation arose recently and spread quickly due to selection, rather than random genetic drift.
By looking at the variations in this gene, the researchers estimated that the distinctive Microcephalin variant arose between 14,000 and 60,000 years ago, with 37,000 years their best estimate. The new mutation is much more common among those from Europe, the Middle East and the Americas than those from sub-Saharan Africa (vol 309, p 1717).
鈥淭he benefits might be counter-intuitive. It could be advantageous to be dumber. I doubt it, but it鈥檚 possible鈥
The team also sequenced the ASPM gene from the same group, and again, among dozens of variants, found a stand-out mutation that alters the corresponding protein. The team estimate that this variant appeared between 500 and 14,000 years ago, with a best guess of 5800 years. It is present in about a quarter of people today, and is most common in Europe and the Middle East (vol 309, p 1720).
鈥淭he evidence for selection is compelling,鈥 says population geneticist Rasmus Nielsen of the University of Copenhagen in Denmark. Yet it remains unclear how these genes work. Many researchers doubt there is any mechanism that selects for greater intelligence today, because human culture has effectively blocked any action natural selection might have on our brains. 鈥淚t seems to me that there is no simple positive relationship between heritable components of intelligence and the number of children people choose to have,鈥 Nielsen says. The Chicago team鈥檚 findings challenge this idea, though it is still a mystery what advantage the gene variants give.
Lahn and his colleagues are now testing whether the new variants give any cognitive advantage. Natural selection could have favoured bigger brains, faster thinking, different personalities or lower susceptibility to neurological diseases, Lahn says. Or the benefits might be counter-intuitive. 鈥淚t could be advantageous to be dumber,鈥 Lahn says. 鈥淚 highly doubt it, but it鈥檚 possible.鈥
However, Lahn points out that since many genes are involved in shaping the brain, knowing what versions of Microcephalin or ASPM you possess will probably tell you little about how your brain works.
Nonetheless, Hawks predicts that the more we sequence active genes in the brains from people worldwide, the more we鈥檒l have to grapple with the meaning of the differences.
