Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past

by David Reich

But even if genetic changes—through coordinated natural selection on combinations of many mutations simultaneously—did enable new cognitive capacities, this is a very different scenario from Klein’s idea of a genetic switch. Genetic changes in this scenario are not a creative force abruptly enabling modern human behavior, but instead are responsive to nongenetic pressures imposed from the outside. In this scenario, it is not the case that the human population was unable to adapt because no one carried a mutation that allows a biological capability not previously present. Instead, the genetic formula that may have been necessary to drive the striking advances in human behavior and capacities that occurred during the Upper Paleolithic and Later Stone Ages is not particularly mysterious. The mutations necessary to facilitate modern human behavior were already in place, and many alternative combinations of these mutations could have increased in frequency together due to natural selection in response to changing needs imposed by the development of conceptual language or new environmental conditions. This in turn could have enabled further changes in lifestyle and innovation, in a self-reinforcing cycle.