Cochran and Harpending use the theory of population genetics to tell the story—or rather their story—of human genetic history. It is a tale in three acts.

According to Cochran and Harpending’s speculations, the event in question was a bit unusual: It was, they say, “introgression, that is, the transfer of alleles from another species, in this case Neanderthals.” That’s right, our ancestors mated with Neanderthals. By definition, this “introgression” of Neanderthal genes caused the rate of evolutionary change among the West Eurasians to accelerate. Very quickly after it occurred, they evolved into “behaviorally modern” humans, so-called because they created a more sophisticated material culture than their primitive ancestors. This culture included paintings, sculptures, beads, missile weapons, fish hooks, nets, ropes, baskets, and textiles. For a brief moment, humankind was divided into two genetic types: The new, behaviorally modern humans and the old, primitive humans, each living in separate populations. They coexisted for a time, but the genetic gap between them closed fairly quickly. The behaviorally modern genes enhanced fitness in many environments, and as a result, they spread to primitive populations around Eurasia and Africa. Eventually, the primitive humans disappeared, and humankind was once again united into one genetic type. After this, all behaviorally modern hunter-gatherers underwent the process of genetic optimization and homogenization that populations routinely experience when adapting to new conditions. In this case, the new conditions were behavioral modernity itself. As the new optimum balance was approached, the pace of evolution slowed again, and a new species-wide stasis occurred: the modern hunter-gatherer equilibrium.

Act 3: The Agricultural Disruption: 10,000 years ago to the present. Then, 10,000 years ago, another disruptive event occurred: Some populations in the Near East learned how to grow food. Agriculture, the authors claim, “imposed a new way of life (new diets, new diseases, new societies, new benefits to long-term planning) to which humans, with their long history as foragers, were poorly adapted.” In other words, agriculture brought with it radically new selection pressures. As one might expect, the rate of genetic change initially rose sharply as the Near Easterners struggled to adapt. But then, as the Near Eastern populations worked toward a new optimum balance, something unprecedented happened: Instead of slowing down, the pace of genetic change accelerated. This revolutionary development was not caused by a disruptive event and the new selection pressures that followed. Instead, it was caused by a radical increase in population. Agriculture provided more food than hunting and gathering, and the agriculturalists turned this food into more agriculturalists. The population exploded. This had unexpected genetic consequences. Cochran and Harpending explain it like this: The larger the population, the more genetic mutations it produces; the mutation rate remains the same, but the total amount of mutations increases because there are more humans carrying the mutating genes. The more genetic mutations are produced, the greater the chances that a fitness-enhancing mutation will appear and spread throughout the population. This means that population growth in and of itself accelerates evolution. When a population is expanding, there is always “room for improvement” thanks to an ever-increasing number of possibly beneficial mutations, so even when optimization is reached, the rate of evolution never slows down. Taken together, the adoption of agriculture and the resulting population growth accelerated the rate of evolution as nothing had before.