The t-shirts which depict the “ascent of man” from hairy semi-ape to upright Homo sapiens might make you think that human evolution has been trivial since the emergence of our own species. Modern genomics suggests this isn’t so, selection coefficients on the order of 1-10% are probably rather normal, and iterated over hundreds of generations that could result in a nontrivial amount of change on a quantitative trait.
I bring this up because a few days ago in The New York Times a piece was published titled The Twists and Turns of History, and of DNA. Greg Cochran (interest divulgence, a friend of mine) offered the boldest quote:
“Since it looks like there has been significant evolutionary change over historical time, we’re going to have to rewrite every history book ever written,” said Gregory Cochran, a population geneticist at the University of Utah. “The distribution of genes influencing relevant psychological traits must have been different in Rome than it is today,” he added. “The past is not just another country but an entirely different kind of people.”
This is explosive stuff, but it shouldn’t surprise anyone coming from Greg. Earlier this year his “dangerous idea” in response to John Brockman’s question was an elaboration on this concept: the past is undiscovered territory. In their recent book Not by Genes Alone quantitative anthropologists Peter Richerson and Robert Boyd start out with the assumption that it is simply implausible that differences in allele frequences across populations could explain any of the variation in culture we see around us. Richerson and Boyd’s model utilizes population genetic models in a “memetic” context, and appeals to the peculiar character of ideas to put forward an argument for cultural group selection because of the possibility of high intergroup variance in traits. Greg’s ideas would throw another complexifying parameter, gene-culture coevolution. But Richerson and Boyd’s point is well taken insofar as the quicksilver changes in cultural biases are unlikely to have long term genetic implications, rather, the idea that society has a shaping influence on the nature of selection must be drawn from a subset of extremely salient and persistent changes that would drive long term fitness differentials.
What could be a candidate for a cultural-social change would should shift allele frequencies? Consider agriculture. In much of Eurasia, in particular northern Europe, the Neolithic revolution clearly shifted allele frequencies as the capacity for adult lactose digestion became normal, a derived condition. In contrast, societies where cattle husbandry never took root did not exhibit any change from the ancestral genotype (ergo, phenotype). There are non-European societies where adult lactose digestion is normal, in much of northern India for example milk is an important part of the local cuisine, often in the form of a butter (ghee). In contrast adult lactose digestion is less common in southern India, where coconut oil plays a larger role (and the climate is likely less salubrious for cattle). In Africa there are Nilotic cultures which have independently evolved toward the lactose tolerant phenotype, but those agriculturalists who reside within the tsetse fly zone where cattle can not flourish remain in the ancestral condition. The point is that selection can operate over time and space in interaction with cultural innovations, which themselves might be constrained by environment or contingent upon historical vissitudes.
One can see this in our very forms over the last 10,000. Humans have become more gracile during the transition to the settled Neolithic life. Our skulls are smaller and our dentition more delicate. This is a common tendency of all agricultural peoples, as an underlying selection parameter seems to be operative as populations transition toward agriculture away from the hunter-gatherer lifestyle. Our faces seem to be shrinking by at least 1-2% every 1,000 year. There is even evidence that great changes in the size of human faces have transpired over the past 1,000 years.
The lesson is that time does not stand still, and we are not outside of nature. And if this is the trajectory toward which gracility drives us, well, I’m not complaining!