Anthropologist Peter Frost writes: A synthesis has been forming in the field of human biodiversity. It may be summarized as follows:
1. Human evolution did not end in the Pleistocene or even slow down. In fact, it speeded up with the advent of agriculture 10,000 years ago, when the pace of genetic change rose over a hundred-fold. Humans were no longer adapting to relatively static natural environments but rather to faster-changing cultural environments of their own making. Our ancestors thus directed their own evolution. They created new ways of life, which in turn influenced who would survive and who wouldn’t.
2. When life or death depends on your ability to follow a certain way of life, you are necessarily being selected for certain heritable characteristics. Some of these are dietary—an ability to digest milk or certain foods. Others, however, are mental and behavioral, things like aptitudes, personality type, and behavioral predispositions. This is because a way of life involves thinking and behaving in specific ways. Keep in mind, too, that most mental and behavioral traits have moderate to high heritability.
3. This gene-culture co-evolution began when humans had already spread over the whole world, from the equator to the arctic. So it followed trajectories that differed from one geographic population to another. Even when these populations had to adapt to similar ways of life, they may have done so differently, thus opening up (or closing off) different possibilities for further gene-culture co-evolution. Therefore, on theoretical grounds alone, human populations should differ in the genetic adaptations they have acquired. The differences should generally be small and statistical, being noticeable only when one compares large numbers of individuals. Nonetheless, even small differences, when added up over many individuals and many generations, can greatly influence the way a society grows and develops.
4. Humans have thus altered their environment via culture, and this man-made environment has altered humans via natural selection. This is probably the farthest we can go in formulating a unified theory of human biodiversity. For Gregory Clark, the key factor was the rise of settled, pacified societies, where people could get ahead through work and trade, rather than through violence and plunder. For Henry Harpending and Greg Cochran, it was the advent of agriculture and, later, civilization. For J. Philippe Rushton and Ed Miller, it was the entry of humans into cold northern environments, which increased selection for more parental investment, slower life history, and higher cognitive performance. Each of these authors has identified part of the big picture, but the picture itself is too big to reduce to a single factor.
