Man left Africa three times
Early humans came out of Africa again and again.
There were at least three major waves of early human migration out of Africa, our DNA suggests. Apparently the wanderers made love, not war: gene patterns hint that later emigrants bred with residents.
Human origins are contentious. Most researchers agree that there have been several major migrations out of Africa. Some hold that human populations in many regions evolved in parallel after Homo erectus left Africa around two million years ago. Others think that a wave of modern humans from Africa replaced all previous Eurasian populations perhaps as recently as 50,000 years ago.
The truth lies somewhere in the middle, proposes geneticist Alan Templeton of Washington University, St Louis1
. "Africans have had a huge genetic impact on humanity," he says. "But my analysis really isn’t compatible with complete replacement."
But this will not be the last word on the matter. Researchers are divided over what sorts of genetic information are most useful, and how it should be analysed. Believers in the replacement hypothesis remain unconvinced by Templeton’s arguments.
Templeton compared DNA sequences of populations around the world. He combined information from ten genetic regions on regular chromosomes, sex chromosomes and mitochondria, the cellular powerhouses with their own genomes. By analysing many different genes, he hoped to clarify the sometimes contradictory results from individual sequences.
Variation in genes from different places enabled him to reconstruct the story of human movement. He saw where particular mutations arose, and how they spread through mating or migration.
Templeton’s reading of the genetic runes is that, post Homo erectus’ exit, there was a second major human migration out of Africa between 400,000 and 800,000 years ago and a third about 100,000 years ago. He also sees a more recent movement back into Africa from Asia, and huge amounts of genetic interchange between groups.
And this is just a start. "It’s the big picture on a very coarse timescale," Templeton says. "The potential for adding more details is truly immense."
"It is very significant work - it fits the genetic, fossil and archaeological evidence," says anthropologist Jonathan Relethford of the State University of New York, Oneonta.
But geneticists who believe that recent African emigrants replaced older Eurasian populations remain sceptical. This hypothesis is based on studies of mitochondrial and Y-chromosome DNA. Martin Richards, of the University of Huddersfield, UK, who did some of these studies, thinks that genes on normal chromosomes, called autosomes, only confuse the picture.
"The data from autosomal genes are very, very impoverished," he says. The picture the Y chromosome gives, on the other hand, is "absolutely watertight", he says.
"The tree’s root is in Africa, and one branch contains all the non-Africans," says Richards. "You don’t get deeper lineages popping up all over the world."
Some archaeologists also dispute Templeton’s conclusions. Richard Klein, of Stanford University, California, says there is scant archaeological evidence of a permanent human presence in Europe before 500,000 years ago. Migrants before this point "may have found no one to interbreed with".
- Templeton, A. R. Out of Africa again and again. Nature, 416, 45 - 51, (2002).
JOHN WHITFIELD | © Nature News Service