Because of the scarcity of fossils and the discovery of important new finds every few years, researchers disagree about the details and sometimes even basic elements of human evolutionary history. While they have revised this history several times over the last decades, currently, researchers agree that the first species of the genus Homo, Homo habilis, evolved in Africa around two million years ago, and that members of the genus migrated "out of Africa" somewhat later. The descendants of these ancient migrants, which probably included Homo erectus, have become known through fossils uncovered far from Africa, such as those of "Peking man" and "Java man." The Neanderthals are also considered the descendants of early migrants.
According to the single-origin model, however, every species of the genus Homo but one was driven extinct: Homo sapiens. This species evolved in Africa between 100,000 and 200,000 years ago and, in a second important exodus from Africa, began colonizing the rest of the world some time afterwards. According to the single-origin model, these more recent migrants did not interbreed with the scattered descendants of earlier exoduses. For this reason, the model is sometimes called the "replacement scenario." In support of it, advocates have drawn from both fossil and DNA evidence, in particular from mitochondrial and Y-chromosome DNA sequences.
The opponents of a single origin argue that interbreeding indeed occurred, and that the characteristics of modern humans, including those that have been and still are perceived by some to distinguish races, reflect genetic contributions from several lineages that evolved semi-independently in different parts of the world. This is the "multiregional model.
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2 Further reading