Until recently, scientists studying human genetic history had to rely on relatively few genetic markers. Now, however, they can study human evolution by comparing the entire genomes of individuals from different backgrounds such as Africans, Asians and Europeans.
Two researchers, from Cambridge, UK and Cambridge, US, used an elegant statistical technique to track differences in the genomes of two contemporary Asians, three Europeans, and two Africans back in time.
Their findings confirmed studies that showed that as early modern humans left Africa to colonize Europe and Asia, they squeaked through a "genetic bottleneck" that reduced the number of individuals who passed their genes on into the present to as few as 1200 humans. The early modern human population in Africa did a bit better – about 5700 people managed to transfer their genes on to the present humans. Both of these early populations appear to have fallen to about one-tenth their pre-bottleneck size during those difficult times.
The new research paints a much clearer picture of the "out of Africa" migration. Rather than involving a single group of founders at a particular moment in time, it now seems that many groups of early modern humans left Africa, with later groups mingling and mating with earlier arrivals until quite recently by archaeological standards.
" . . . when we look at non-African individuals from Europe and East Asia, we see a shared history of a dramatic reduction in population, or bottleneck, starting about 60,000 years ago, as others have also observed," says Richard Durbin, head of the Genome Infomatics Group at the Sanger Institute in the UK. "But unlike previous studies we also see evidence for continuing genetic exchange with African populations for tens of thousands of years after the initial out-of-Africa bottleneck until 20,000 to 40,000 years ago."
Interweaving genetics and archaeology
Scientists have had difficulty, until recently, in tracking human genetic markers into the past and matching that "genomic archaeology" with fossilized bones and stone tool data. The current findings, however, seem to fit well with the archaeological record.
For example the range of time during which the African and non-African branches humanity now seem to have separated--from 120,000 to as recently as 40,000 years ago, includes the time when archaeologists begin to find signs of early modern humans in Europe and Asia.
"This evidence . . . is potentially consistent with the archaeological evidence of anatomically modern humans found in the Near East around 100 kyr [100 thousand years] ago," the authors write.
Interestingly, the researchers found two earlier "bumps" in population size, one before 60,000 years ago, the other one to three million years ago. They speculate that the more recent of these surges may represent population growth following the origin of anatomically modern humans, and the earlier the period after humans and chimpanzees diverged.
The authors say that this new approach promises to clarify human genetic history.
Source:
Heng Li and Richard Durbin. "Inference of human population history from individual whole-genome sequences," Nature, 2011. DOI: 10.1038/nature10231
Robert Adler - Robert Adler is a science and technology reporter and book author. He divides his time between Santa Rosa, California and Oaxaca, ...
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