By at least a million years ago (and probably closer to two million), early humans were able to expand their range outside Africa. Fossil evidence for this has been found in southern Europe, Russia, the Middle East, Asia and Indonesia, but not in the Americas or in Australia. This dispersal is sometimes called ‘Out of Africa I’.
There are two competing theories as to what happened next. The first is the ‘multiregional’ hypothesis that modern humans evolved over the past million years from hominid populations already living in Eurasia via gene flow. It postulates that isolated bygeography, populations evolved independently, but with occasional interbreeding. The result was separate populations of modern humans, each with distinct physical features. It is characterised by regional continuity of anatomical features and argues for deep genetic divisions between ‘races’.
The second is the ‘Out of Africa’ hypothesis, which argues that modern humans originated in Africa until as recently as 150 000 years ago and then spread out from there. The genetic and fossil evidence for this is more convincing. It contends that the people descended from the first migration out of Africa were replaced by a second, more recent migration of anatomically modern people about 80 000 years ago. Instead of interbreeding with the earlier population, modern humans replaced them, driving them to extinction. The study of mtDNA shows that an unbroken genetic train can be traced back to a common ancestor from Africa. So far there is little evidence of a genetic contribution from non-African populations.
