Scholars have long debated the exact origins of our species. Some have maintained that modern humans probably arose simultaneously in several regions of the globe. They base these arguments largely on the fossil record, which shows that some of our immediate predecessorsHomo neanderthalensis and Homo erectus, for instancehad migrated out of Africa as early as two million years ago. But a new study published in today's issue of Nature lends more credibility to the rival theorynamely that Homo sapiens emerged only in Africa, and only later infiltrated other regions, wiping out the Neanderthals already living there over time.

Instead of tracking our family tree with fossils, the study's authorsUlf Gyllensten and Max Ingman of the University of Uppsala, and Henrik Kaessmann and Svante Pbo of the Max Plank Institute for Evolutionary Anthropologyturned to the living. They completed the most extensive analysis of mitochondrial DNA (mtDNA) ever done, taking samples from 53 people having diverse geographical, racial and linguistic backgrounds. Unlike the DNA within chromosomes, which is inherited from both parents and so changes slightly with each generation, mtDNA is passed directly from mother to child; only random mutations alter its sequence. As a result, it offers a powerful measure of how closely different populations are related.

Earlier work on mtDNA had focused exclusively on the control regions of the sequence, which account for a scant 7 percent of its total. In contrast, Gyllensten and his colleagues worked out the entire mtDNA sequencea string of some 16,500 base pairsfor each individual in their study. From this data, they weeded out stretches of DNA that had mutated unusually quickly and so very likely represented little more than twigs on the family tree. When they compared the rest, a trunk and major branches became clear: the most recent common ancestor of all subjects in the study lived in sub-Saharan Africa some 171,500 to 50,000 years ago, suggesting that we are all descended from a single group there. In addition, they found a major branch separating most Africans from non-Africans, which probably represents H. sapiens' first major exodus from the continent, 52,000 to 27,500 years ago.