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PDF FormatAccording to the authors, this change also led to the reappearance of significant genetic differences among human populations. Humans who adopted agriculture and experienced the resulting population growth became genetically different from those who did not. “This picture of adaptation to agricultural diets,” so the authors claim, “has two important implications: Populations today must vary in their degree of adaptation to such diets, depending on their historical experience, and populations must have changed over time.” In other words, humankind was again divided in two. Only this time, claim Cochran and Harpending, the genetic gap didn’t close. On the contrary, it widened, and new genetic gaps appeared. Humankind was divided into three, four, five, and so on. The crucial factors in this unprecedented process of genetic differentiation were the adoption of agriculture and population size.
To begin with the former, the authors propose that the earlier a population adopts agriculture, the more time it has to adapt to new selection pressures, and the more time it has to adapt, the more different it will become from the species-wide genetic “starting point,” i.e. the behaviorally modern equilibrium. They explain, “The evolutionary responses to an agricultural diet must differ, since different peoples adopted different kinds of agriculture at different times.” Thus, the descendants of the first agriculturalists, the Near Easterners, are the most genetically distant from the behaviorally modern equilibrium. They are followed by the descendants of later agriculturalists—North Africans, Indians, East Asians, Mesoamericans, etc. These populations, in turn, are followed by the modern descendants of populations who adopted agriculture relatively recently or still live as hunter-gatherers: Aleuts, Australian aborigines, African bushmen, as well as certain Siberians and Amerindians, whose genomes should be quite close to the behaviorally modern type.
Cochran and Harpending argue that the genetic gap between agriculturalists and non-agriculturalists is still with us, though it is closing as the genes of the former spread to the genomes of the latter. But they also claim that new gaps have occasionally appeared within agricultural populations themselves. Some of these populations—civilizations, empires, societies, and nations—have grown very large and very complex. Both of these characteristics increase the probability that some sub population—a territory, class, ethnicity, or profession—will become genetically different. Large, complex populations create the “space,” so to speak, in which sub-populations can become genetically distinct. The force that differentiates them is, of course, natural selection, but here the selection is engineered by other human beings in the same population. Socially induced selection pressure can be subtle, as when one class oppresses another, and not so subtle, as when one race tries to exterminate another. The important point is that socially induced selection pressures accelerate genetic change in the affected sub population and move it toward a new local optimum balance, one that is similar to that of the general population but adapted to local circumstances.
Cochran and Harpending offer the Ashkenazi Jews as an example of this phenomenon. In the Middle Ages, European Christians confined the Ashkenazi to a small set of occupations, most of which required intensive intellectual activity. In other words, Christians imposed a new and unusual set of selection pressures on them. The standard process of evolutionary acceleration, optimization, and genetic homogenization then took place. Since the new selection pressures favored certain intellectual abilities, genes that produced these abilities were selected and spread throughout the Ashkenazi sub population. As a result, the Ashkenazi Jews became different in terms of both their genome and their abilities. “We propose,” Cochran and Harpending write, “that the Ashkenazi Jews have a genetic advantage in intelligence that arose from natural selection for success in white-collar occupations during their sojourn in northern Europe.” Cochran and Harpending hold that this is not an isolated case. In large, complex agricultural populations—just like the ones almost all of us live in—it is happening all the time to one degree or another.
