The Evolutionary Psychology FAQ

Edward H. Hagen, Institute for Theoretical Biology, Berlin

Why is the EEA equated with the Pleistocene?

The EEA is the set of selection pressures faced by a population over evolutionary time. The Pleistocene is a specific period of time beginning about 1.8 million years ago, and ending about 11,000 years ago. So, the two are not equal. However, the set of selection pressures that resulted in the evolution of the human body, including the brain, are almost certainly the selection pressures that acted on humans during the Pleistocene. As noted above, 'evolutionary time' for any species is roughly 1000-10,000 generations. Assuming a human generation to be about 20 years, that translates to 20,000 to 200,000 years. The period of time called the Pleistocene includes this, but is about 10 times longer, so that is a comfortable amount of time for complex adaptations to have evolved. Also, our genus, Homo, emerged in Africa around 2 million years ago, and by 1.8 million years, Homo had spread to Asia--the first hominid to leave Africa. At the end of the Pleistocene, humans invented agriculture, which resulted in a rapid abandonment of hunting and gathering, the means by which humans had survived for the preceding 2 million years. Within a few thousand years after the end of the Pleistocene, some humans were living in cities, a novel form of settlement. In short, the amount of cultural change experienced by humans over the last 10,000 years has been tremendous, possibly exposing humans to novel selection pressures, or eliminating previously important selection pressures. So, the Pleistocene--which (almost) encompasses the origins of our genus, but excludes the recent period of dramatic change--is conveniently identified as the epoch which shaped human physiology and psychology. It is important to note that many of our adaptations--perhaps most--evolved before the Pleistocene. Human anatomy is almost identical to primate anatomy, and, indeed, mammalian anatomy, all of which took their present form well before the onset of the Pleistocene. The reason we can still roughly equate the Pleistocene with the period of time which shaped human adaptations is that, if an adaptation which evolved prior to the Pleistocene, like vision, were not under stabilizing selection during the Pleistocene, that adaptation would have been lost during the 2 million years of the Pleistocene. Stabilizing selection maintains adaptations that have already evolved. For example, assume that the sun blinked out 2 million years ago (but, implausibly, that there were no other changes to the environment and most species did not go extinct). Humans and all other animals with vision would have lost their visual capabilities. Mutations would inevitably have occurred in the genes underlying our visual system, degrading our visual abilities. Since there wasn't any light, however, this degradation would have been inconsequential, and these mutations would not have been selected out of the population. After 2 million years, the visual system would be completely gone (this has actually happened for some species of cave-dwelling fish). Consequently, we can include sunlight as part of our EEA, and our visual system as a product of stabilizing selection for vision during the Pleistocene.

Copyright 1999-2002 Edward H. Hagen