Changing Minds

The most interesting kind of genetic changes are those that affect human personality and cognition, and the evidence is good that such changes have indeed occurred.

A number of the new, rapidly spreading alleles found in the recent selection surveys have to do with the central nervous system. There are new versions of neurotransmitter receptors and transporters—neurotransmitters being molecules that relay and influence signals between nerve cells. Several of the new alleles have effects on serotonin, a neurotransmitter involved in the regulation of mood and emotion. Many recreational and therapeutic drugs (particularly antidepressants) modulate serotonin metabolism. And there are new versions of genes that play a role in brain development: genes that affect axon growth, synapse formation, formation of the layers of the cerebral cortex, and overall brain growth. Again, most of these new variants are regional: Human evolution is madly galloping off in all directions.

We see new versions of several genes in factors having to do with muscle fibers and brain function. Dystrophin is a protein (coded by the longest of all known human genes) that has an important structural role in muscle fibers and the brain; the dystrophin complex is a set of proteins that are physically associated with dystrophin. Major defects in the dystrophin gene itself cause Duchenne muscular dystrophy, which has very severe effects, while lesser defects cause Becker's muscular dystrophy, which is milder. These are among the most common genetic diseases, apparently because the extremely large and structurally complex dystrophin gene has so many ways of going wrong. Dystrophin's dual role has medical consequences, in that boys with Duchenne muscular dystrophy suffer reduced IQ as well as muscular weakness.

The dystrophin-associated sweeping alleles that we see in the selection surveys (which do not cause disease) raise the interesting possibility of direct trade-offs between muscle and brain function in the recent past. We have reason to think that humans circa 100,000 BC had stronger muscles than today— and so changes in the dystrophin complex may have sacrificed muscle strength for higher intelligence.

Another very intriguing pattern involves new versions of genes that affect the inner ear.12 We wonder if this is a consequence of recent increases in language complexity sufficiently recent that our ears (and presumably our brains, throats, and tongues) are still adapting to those changes. Or, since some of the sweeping genes involving the inner ear are regional and recent, could some populations be adapting to characteristics of particular languages or language families? It seems that all humans can learn any human language, but we don't know whether everyone is inherently just as good as everyone else at learning every language, communicating in every language, or eavesdropping in every language.

More generally, these sweeping neurological genes could be responses to the new challenges posed by agriculture itself and the dense hierarchical societies it made possible. In the following sections, we discuss those challenges and likely adaptive responses to them.

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