Peter J. Richerson, PhD

Distinguished Professor Emeritus

Department of Environmental Science and Policy

University of California, Davis

Human Evolution in the Plio-Pleistocene:

A World Queerer than We Supposed

The universe is not only queerer than we suppose,

it is queerer than we can suppose.

J.B.S. Haldane (1927)

The evolution of the human capacity for adaptation via cumulative cultural evolution is a major macroevolutionary puzzle. Beginning about 2 million years ago our lineage began to evolve our very large brain. At least two of the things our brain includes are a uniquely sophisticated system of social learning and social psychology that facilitates cooperation with nonrelatives. In the Holocene, humans have undergone a massive adaptive radiation based on cultural adaptations that have made us the earth’s ecologically dominant species. Most similar “killer adaptations,” such as the camera-type eye and internal skeleton of vertebrates, evolved hundreds of millions of years ago. The default, often unarticulated, hypothesis to explain the late emergence of cumulative culture is that it requires complex reorganizations of the brain, and perhaps other features of the basic vertebrate body plan to achieve. Hundreds of millions of years transpired before the critical final reorganization occurred in our species.

       Another possible explanation is that the big brain and extended life history required to sustain cumulative culture is quite costly. Only an environment in which the payoff to cumulative culture is unusually high will favor its evolution. In the last few decades we have learned from ice, lake, and ocean cores that climates have gotten colder, drier, and more variable in space and time over the last 50 million years. With the onset of the Plio-Pleistocene glacial system climates became especially variable, albeit mostly on time scales too long to favor cumulative culture. High-resolution cores began to be recovered and published in the 1990s. They revealed that the last ice age included large amounts of climate variation on century to millennial time scales. These are the time scales that theoretical models suggest can favor a costly system of cumulative culture. In the last decade, longer high-resolution cores suggest that the amount of high-frequency variation has progressively increased over the last several 100,000-year glacial cycles, roughly paralleling increases in human brain size and the increases in sophistication of stone tools. We cannot reject the hypothesis that the human capacity for cumulative culture evolved in the Plio-Pleistocene in direct response to high and rising environmental variation. The expansion of Anatomically Modern Humans out of Africa occurred near the onset of an especially variable part of the last ice age. Agriculture, the economic basis for our Holocene dominance, began to evolve when the intense millennial and submillennial scale variation stopped.

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