years ago and is now carried by 44% of Caucasians. Both genes are thought to be involved in determining the number of neurons formed in the cerebral cortex in the early embryo. The rapid spread of these two alleles indicates that the human brain has been subject to intense evolutionary pressures in the recent past and may still be evolving.
These instances of recent evolutionary change are probably just the first of many that remain to be discovered. Since all occurred after the dispersal from Africa, the alleles that cause them are present to a different extent in different populations. Human diversity therefore cannot be a purely cultural phenomenon, as many social scientists sometimes seem to believe. It has a genetic component too. The component remains to be defined and quantified, but it could prove to be substantial.
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