Out of Africa: a genetic predisposition?
Center for Human Genetics, University of Liège, Belgium
Since its emergence more than 2 million years ago in Africa, the genus Homo has shown a strong predisposition to migrate: fossil deposits of archaic human lineages such as Homo erectus are on record in all continents except for America. Migrations of modern Homo sapiens out of Africa began more than 70,000 years ago. Why did Homo sapiens leave Africa is open to discussion and probably involves several factors. But undoubtedly his gene pool gave him the ability to adapt himself to a large variety of environments. Moreover, he has seized on the opportunity, given by exceptional climatic events, to migrate into territories temporarily reachable, such as some islands or the American continent during glacial periods with lowering of the sea level.
Besides the usual classical methods, peopling of the planet by modern humans is documented today by mitochondrial, Y chromosome and genomic DNA studies. We shall briefly present a general view of main human migrations out of Africa during the last 70,000 years as suggested by DNA analyses. We shall comment longer on two specific points.
The first is the peopling of the American continent by small groups of humans, whose number is estimated to be the same as the one proposed today for any species on its way to extinction. Nevertheless, in a relatively short period of time, about 10,000 years, some 5,000 people had several millions of descendants, foreseeing the large reproductive human potential leading to the actual 7 billion individuals on Earth. Moreover, almost 100% of the “natives” of Central and South America show the blood group O. Among the several hypotheses to explain this condition, a founder effect seems very likely.
The second point concerns the special place of Africa. Some African populations never left the continent, nor had any gene admixture with humans “back to Africa”, carriers of archaic Neandertal or Denisovan DNA. They also show a greater genetic diversity, in contrast with the out of, or back to, Africa populations. The most concerned groups are the hunter-gatherers Khoesans and the Pygmies. DNA studies in these lines are of great interest and Africa becomes progressively the field of many research programs, related to the general context of human migrations.
In conclusion, our species has always been on the move. The genetic determinism of migration is well documented in many animal species. Of course, modern human migrations show a very different pattern, but the persistence of this behaviour, observed soon after Homo sapiens appeared, suggests the possibility of a genetic component. Should this be the case, Homo sapiens will continue to migrate, whatever the circumstances.