The importance of gene flow in human evolution

Abstract

By the latter half of the 20th century, there were three dominant models of human evolution. All three accepted an African origin of humans at the Homo erectus stage, with H. erectus expanding out of Africa and colonizing Eurasia near the beginning of the Pleistocene. The candelabra model had H. erectus splitting into mostly isolated geographical lineages that independently evolved into the modern African, European and Asian “races”. The out-of-Africa replacement model starts out like the candelabra model, but then posits that Homo sapiens first evolved in Africa and then expanded out of Africa in the late Pleistocene and replaced all of the archaic Eurasian populations without interbreeding with them. Neither of these models assign an important role to gene flow (genetic interchange). In contrast, the multiregional model regarded the human populations in Africa and Eurasia as experiencing gene flow throughout the Pleistocene and evolving as a single human lineage with some local differentiation. Studies on mitochondrial DNA in the 1980’s claimed to support the out-of-Africa replacement model and to falsify both the candelabra and multiregional models by mistakenly equating the two. In fact, the mitochondrial DNA studies were fully compatible with both the replacement and multiregional models. The first statistically significant discrimination between these two models appeared in 2002 and revealed a hybrid model in which there was a mid-Pleistocene and a late Pleistocene expansion of humans out of Africa that resulted in limited genetic interchange with Eurasians rather than complete replacement. Moreover, significant gene flow and population movements led to genetic interchange throughout the mid-Pleistocene to the present. Studies on genomic data and ancient DNA have strongly confirmed these inferences. Moreover, our modern species of humans was forged in an African multiregional metapopulation rather than arising from one local area of Africa. Thus, gene flow has played a dominant role in human evolution since the mid-Pleistocene whereas splits and isolation have not. This undercuts the idea that human races are biologically real categories or separate branches on an evolutionary tree.