A study comparing the DNA of people around the world has yielded what could be the best evidence yet that modern man first evolved in Africa and scattered to populate the planet as recently as 50,000 years ago.
Such a view suggests that the first Homo sapiens held such dramatic evolutionary advantages - perhaps stronger powers of reasoning - that they replaced other early humans with virtually no interbreeding.
This is not the first time DNA technology has been applied to the question of when and where modern humans emerged. But the researchers said they analyzed the longest strand of DNA ever examined for a human lineage study.
They said their findings strongly favor the "out-of-Africa" theory of modern human origin. Advocates of the rival multiregional theory say modern humans evolved simultaneously in Africa, Europe and Asia from multiple early humans, maybe including Neanderthals and Homo erectus who left Africa in a much earlier wave.
"I think people are going to stop testing those two theories and say, `Let's look at the details of the out-of-Africa hypothesis,"' said evolutionary biologist Blair Hedges at Pennsylvania State University, who did not take part in the study. "I think people are not going to be too much concerned with the multiregional."
Others, though, said the latest findings could allow for a theory that merges both models: a core of modern humans from Africa later mating in limited numbers with other early humans in distant places.
The study, published in Thursday's issue of the journal Nature, was carried out by Swedish and German researchers. They analyzed the genetic material inside little structures known as mitochondria within the cells of 53 people of various modern nationalities, ethnic groups and races.
Earlier researchers studied only 7 percent of the mitochondrial DNA. Taking advantage of techniques worked out in the Human Genome Project, the project to decipher the human genetic blueprint, the Swedish-German team looked at the entire length of mitochondrial DNA, or about 16,500 chemical base pairs.
The researchers determined how heavily mutations scrambled the DNA across the generations. They found that Africans are about twice as diverse in their genetic makeup - and thus older - than other groups.
The scientists used a chimpanzee's DNA to establish a theoretical rate of change from mutations. They then calculated that a common ancestor of chimps and humans might have lived about 5 million years ago. And a common ancestor of all modern humans might have lived about 170,000 years ago somewhere in Africa.
With their calculations, they estimate that modern humans left their African homeland relatively recently, perhaps 50,000 years ago. Other out-of-Africa theorists have put the exodus at around 100,000 years ago.
The Swedish-German team also found that about 38,000 years ago, the population of modern humans began exploding. The out-of-Africa theorists say the modern humans were replacing early human competitors with little or no interbreeding, presumably by dint of better powers of survival.
"There was probably a fairly small group that migrated out of Africa and that population probably spread in several directions and grew pretty quickly," said geneticist Ulf Gyllensten, the study's chief researcher at the University of Uppsala, in Sweden.
Hedge said in an accompanying commentary that the initial wave may have numbered only several thousand.
However, University of Michigan anthropologist Milford Wolpoff, a multiregional theorist, said mitochondrial DNA more poorly reflects the distant past than some DNA within the nucleus of a cell. He said late Neanderthal fossils suggest that they were evolving toward modern humans in some ways, developing chins and losing their low, sloping foreheads.
Paleontologist Fred H. Smith at Northern Illinois University argued for an "assimilation" model with elements of both theories.
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