Proof that man is 'Out-of-Africa'
2000-12-07 12:09
Springfield, Massachusetts - 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 analysed the longest strand of DNA ever
examined for a human lineage study.
They said their findings strongly favour the "out-of-Africa" theory
of modern human origin. Advocates of the rival multi-regional 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: æLet's look at the details of the out-of-Africa hypothesisÆ,ö
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 multi-regional."
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 analysed 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.
Hedges said in an accompanying commentary that the initial wave may have numbered only several thousand.
However, University of Michigan anthropologist Milford Wolpoff, a
Multi-regional 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.
Palaeontologist Fred H Smith at Northern Illinois University argued
for an "assimilation" model with elements of both theories.
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- SAPA