DNA's evolutionary dilemma: Genetic studies collide with the mystery of human evolution

Abstract

Just a few years ago, genetic researchers assumed the status of a scientific Supreme Court in the debate over humanity's prehistoric roots. The coils of human DNA appeared to have hardened into a molecular gavel with which these scientists could issue a final ruling on how best to explain the evolution of modern Homo sapiens. During the late 1980s, initial studies of global DNA diversity encouraged a vigorous bout of gavel pounding. One after another, investigators concluded that modern humans probably arose in Africa around 200,000 years ago and then spread elsewhere, replacing Neandertals and any other species in our evolutionary past. Analyses of DNA-inherited only through the maternal line-in people from different parts of the world traced the origin of these genes back to one or a few African women dubbed mitochondrial Eve. Decades of anthropological debate over modern human origins that was based on interpretations of measurements and a host of bumps and grooves on ancient fossils teetered on the verge of irrelevance. However, even some admirers of Eve now say that the jury is out on whether state-of-the-art DNA studies can live up to their early billing. Accumulating data prove compatible with either of the two main theories of how H. sapiens came about. The recent African-origin model championed by many genetic researchers relies on genetic findings that fit just as easily into a contrasting multiregional model. That is, populations of H. sapiens living in different parts of Africa, Asia, and Europe interbred enough over at least the past 1 million years to evolve collectively as a single species. The various populations around the world derived from even older ancestors of H. sapiens in this scenario. Practitioners of what has been dubbed anthropological genetics now operate with a sense of caution and a hunger for better explanations of how evolutionary forces produce genetic diversity among individuals and groups. A lot of us have been too eager to assume that a strict out-of-Africa model is correct because it's compatible with the genetic data, without considering that the data also fit with the multiregional theory, says anthropologist John H. Relethford of the State University of New York at Oneonta. It's time to go back to the drawing board on this issue.