DNA evidence backs up 'out-of-Africa' human origin theory
WASHINGTON (Reuters) Genetic research unveiled Thursday provides compelling support for the theory that anatomically modern humans rose out of Africa in the past 100,000 years and swept aside populations of prehistoric man, with no interbreeding.
A team of Chinese and American geneticists obtained blood samples from more than 12,000 men from across East Asia and examined characteristic DNA sequences called markers on the Y chromosome (the male chromosome).
The Y chromosome is considered one of the most powerful molecular tools for tracing human evolutionary history because it remains unchanged over eons when passed from father to son.
The researchers found that every one of the men could trace his ancestry to forefathers who lived in Africa over the past 35,000 to 89,000 years. They also found absolutely no genetic evidence that modern people, Homo sapiens, mated with archaic humans, Homo erectus, that already lived in Asia, having migrated from Africa about 1 million years ago.
The findings, appearing in the journal Science, appeared to confirm the so-called out-of-Africa theory that modern people originated in Africa about 100,000 years ago and then migrated outward, replacing Homo erectus around the globe.
"Our work not only provided the evidence that supports the out-of-Africa theory, but also showed that such a replacement is complete," human population geneticist Li Jin of Fudan University in Shanghai and the University of Texas in Houston, who led the study, said.
Li added that the absence of any genetic signature from archaic humans in the huge sample of men studied meant there was no support for the idea that Homo sapiens mated and produced babies with Homo erectus.
Asked about any evidence of interbreeding, Li said: "Zilch. None. Nada."
Some dissident scientists have expressed the view that people living today descended from several indigenous archaic human populations in the Old World, such as the Neanderthals who resided in Europe or so-called Java man or Peking man in Asia. This theory is called multiregionalism.
But the evidence is mounting against this view. Several studies have shown that modern human mitochondrial DNA, passed down by the mother, is of African origin. And when scientists sequenced the DNA from the mitochondria, tiny structures within a cell but outside the nucleus that contain genes, of a Neanderthal four years ago, they found it was vastly different from that seen in people today.
"The genetic evidence implies a recent common origin of our species. The Y chromosome really makes that argument bulletproof," Stanford University molecular biologist Peter Underhill, a study coauthor, said in an interview.
Li said the researchers devised a simple way to make the out-of-Africa theory a testable hypothesis. He said the team wanted to answer the question of whether there was any trace of Y chromosomes of non-African origin in East Asia, where Homo erectus and Homo sapiens fossils have been found in abundance.
The researchers screened 12,127 genetic samples from men in 163 populations from different regions in Asia - in such places as Iran, China, New Guinea and Siberia - for three specific Y chromosome mutations that are derived from a single earlier mutation seen in African populations.
All of them carried one of the three mutations, suggesting that archaic humans did not contribute to the origin of modern man. "All these people trace their roots back to a common ancestor who lived in Africa maybe 100,000 years ago," Underhill said.