March 10, 2006

More recent selection in humans

Yet another study (full article here) which corrodes the Left and Right versions of Creationism -- respectively, that Darwinian evolution designed a Swiss Army knife mind up through the period of our species' history when everyone was a hunter-gatherer, but then alakazam, froze this Stone Age mind in place; and that some thousands of years ago, abracadabra, God created mankind as we presently know ourselves. The algorithm the scientists used when surveying the genomes of people from various racial groups only sought recent selection; that is, within roughly the last 10,000 years -- since humans adopted agriculture. It could not identify alleles that had already spread to fixation in the population under study, but rather alleles that were just starting to spread. It will of course take some time for curious minds to sort through the wealth of data and consult other disciplines in order to weave a coherent story about what happened and why, but here are some of my off-the-cuff observations:

1) In the past 10,000 years, sub-Saharan Africans especially but also Europeans have been exposed to a nastier pathogen load than Northeast Asians. The new Voight et al study (Table 2) shows that both Europeans and Africans, but not Asians, have recently undergone selection regarding the senses of smell and taste. The obvious guess is that this is to aid in determining whether food (meat for both or harvested foods for Europeans) has been infected by pathogens. If it has, it pays to detect this by perceiving the odor as "funky." Why might this trait have still undergone selection even after tens of thousands of years of pathogen avoidance using smell and taste? Well, new pathogens pop up all the time, and your nose and tongue had better be on their feet if they want to keep up with recognizing the current comestible-corrupting culprits.

2) In the relatively harsher climates of Europe and Northeast Asia, fathers have to invest more in order to see their kids reach maturity to reproduce in their turn, whereas in the more favorable climate of s-S Africa, it's more common for the father to have his wife do most of the child-rearing and hard labor to put food in their mouths. Because s-S African women therefore have little reason to remain faithful to their low-investing husbands, there is a pressure to mate with other men and raise a cuckoo's baby. This would presumably make sperm competition fiercer in s-S Africa. In fact, among Europeans and Asians, Voight et al found recent selection at sites having to do with the basics of reproduction: gametogenesis, spermatogeneis & motility, and fertilization. Perhaps a type of sperm & egg adapted to a region of low paternal investment became maladaptive once there were far fewer rivals' sperm to compete against.

3) Though Voight et al didn't find signs of selection at the two microcephaly genes Bruce Lahn recently identified as having undergone recent selection, they did find selection at other microcephaly sites, as well as other brain genes that are linked to neurotransmitters. Some brain genes are under selection in all three populations, while some differ. Again, it's too early to tell exactly what this means at the phenotypic level, but it does show (once more) that natural selection has not stopped since the broad racial groups diverged, nor has recent selection been kept away from the brain by a magical forcefield.

To anticipate a post I'm writing up -- if recent selection has altered the average brain of diverse populations (to whatever degree), the implications for medicine seem clear enough. But what about an area the most consider beyond the scope of evolutionary biology: aesthetics? Well, almost everyone. Denis Dutton is one of the lone critics willing to take science seriously in accounting for various aspects of artistic creation and criticism, and yet his particular model is inherited from Tooby and Cosmides, who stress what anthropologists have long called "the psychic unity of mankind." Dutton cites Hume, who noted that the "same Homer who pleased at Athens and Rome two thousand years ago, is still admired at Paris and London." That is, since the aesthetic standards seem to largely agree over thousands of years, human nature above the neck could not have changed in the meantime. But with all the incoming evidence of recent selection, some of which targets the brain, this assumption is becoming less intuitively true.

Moreover, if recent selection has fiddled around with the senses of smell and taste between populations, what if some art form were parasitic on these foundations? Say, haute cuisine. Surely we shouldn't expect Yoruba and Japanese critics to largely agree when judging the culinary creations from around the world -- not due to chauvinism but to the difference in the underlying senses employed in judgment. Again, how exactly it all turns out, we'll have to wait and see, but the very possibility that connoisseurs from two different populations could be honestly yet fervently talking past each other in many central aesthetic matters upsets the initial appeal of the "psychic unity of mankind" approach in criticism, even if not the larger evolutionary aesthetics paradigm of which it's a strand.

3 comments:

  1. John S Bolton3/12/06, 4:21 PM

    Music is another area where the psychic unity of mankind appears to have some deep-seated divisions.

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  2. You beat me to the punchline! Well, I might as well give the basics -- IQ profile is a good predictor of how much that group's musical style emphasizes harmony and melody. Lopsidedness toward verbal (Af-Am, Ashkenazim) results in melody-driven music, not as much emphasis on harmony. The converse is true for those lopsided toward spatial (Central Asians), where harmony is complex but not the melody. Where groups are balanced IQ-wise, melody & harmony are roughly equally prominent (Germans).

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  3. Are people getting more or less good at smelling things? If less good, it could be because we offloaded tracking prey animals by scent to dogs when we domesticated them, freeing up valuable brain real estate for other functions.

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