Our Species Mated With Other Human Species, Study Says

A new piece of evidence—one sure to prove controversial—has been flung into the human origins debate.
A study published March 7 in Nature presents genetic evidence that humans left Africa in at least three waves of migration. It suggests that modern humans (Homo sapiens) interbred with archaic humans (Homo erectus and Neandertals) who had migrated earlier from Africa, rather than displacing them.

Ancient Origins
In the human origins debate, which has been highly charged for at least 15 years, there is a consensus among scientists that Homo erectus, the precursor to modern humans, originated in Africa and expanded to Eurasia beginning around 1.7 million years ago.
Beyond that, opinions diverge.
There are two main points in contention. The first is whether modern humans evolved solely in Africa and then spread outward, or evolved concurrently in several places around the world.

The second area of controversy is whether modern humans completely replaced archaic forms of humans, or whether the process was one of assimilation, with interbreeding between the two groups.
"There are regions of the world, like the Middle East and Portugal, where some fossils look as if they could have been some kind of mix between archaic and modern people," said Rebecca Cann, a geneticist at the University of Hawaii.
"The question is," she said, "if there was mixing, did some archaic genetic lineages enter the modern human gene pool? If there was mixing and yet we have no evidence of those genes—as is indicated from the mitochondrial DNA and y chromosome data—why not?"
Alan Templeton, a geneticist at Washington University in St. Louis who headed the study reported in Nature, has concluded that yes, there was interbreeding between the different groups. "We are all genetically intertwined into a single long-term evolutionary lineage," he said.
To reach his conclusion, Templeton performed a statistical analysis of 11 different haplotype trees. A haplotype is a block of DNA containing gene variations that researchers believe are passed as a unit to successive generations. By comparing genetic differences in haplotypes of populations, researchers hope to track human evolution.
Templeton also concluded that modern humans left Africa in several waves—the first about 1.7 million years ago, another between 800,000 and 400,000 years ago, and a third between 150,000 and 80,000 years ago.
Alison S. Brooks, a paleoanthropologist at George Washington University, is more cautious about Templeton's conclusions. "Archaeological evidence supports multiple dispersals out of Africa," she said. "The question has always been whether these waves are dead ends. Did all of these people die? Templeton says not really, that every wave bred at least a little bit with those in Eurasia.
"This has not been the majority viewpoint of geneticists up to this point," said Brooks.

Dueling Theories
The fossil record shows that about 100,000 years ago, several species of hominids populated Earth.
Homo sapiens could be found in Africa and the Middle East; Homo erectus, as typified by Java Man and Peking Man, occupied Southeast Asia and China; and Neandertals roamed across Europe.
By about 25,000 years ago, the only hominid species that remained was Homo sapiens. Scientists have conducted a considerable amount of both genetic and archaeological research in an effort to understand how this outcome occurred.
....More at link......

(Thanks for the post. Here's one on the same subject from The Independent UK)

Early man evolved by spreading love, not war

By Steve Connor, Science Editor
07 March 2002

The came, they saw and they made love, not war. This is the story of how our human ancestors spread across the world, according to the most detailed study of our genetic heritage attempted so far.

Alan Templeton, professor of biology at Washington University in St Louis, Missouri, has found convincing evidence to suggest that the history of human evolution is one of sexual interchange rather than the physical elimination of one group by another.

"Humans expanded again and again out of Africa, but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world," he said.

The study, a computer analysis of the DNA from people living in 10 different regions of the world, was more extensive than any previous research, Professor Templeton said.

The findings, published today in the journal Nature, add a new twist to the long-running dispute over whether modern humans are the result of a single migration out of Africa some 100,000 years ago, or the product of a series of migrations extending back over one million years to several regional homelands in Asia as well as Africa.

"The main conclusions are that human populations in Africa and Eurasia have not been genetically isolated from one another, but rather have been interchanging genes for least 600,000 years," Professor Templeton said.

"This 'gene flow' was restricted, primarily by geographical distance, which meant that local populations could and should show genetic differences, as they do today. But over a long time there was sufficient genetic interchange to insure that all humanity evolved as a single species."

Professor Templeton's research indicates that there were two important waves of migration out of Africa – one about 600,000 years ago when humans were represented by "archaic" species such as Homo heidelbergensis and the Neanderthals, and the other about 95,000 years ago, soon after the rise of anatomically modern Homo sapiens.

Professor Templeton said the earlier migration coincided with a significant expansion in brain size and the latter with the appearance of "modern" traits, such as smaller brow ridges, a rounded skull, a vertical forehead and a pronounced chin.

"This later set of traits is difficult to reconcile with a population replacement, but is compatible with this most recent out-of-Africa expansion event being characterised by interbreeding," he said.

Eight of the 10 DNA regions Professor Templeton studied possessed characteristics that dated back well before 100,000 years ago, meaning that they predated the migration of Homo sapiens. Those features could only be accounted for by interbreeding between the newly arrived migrants and the resident Asian population, he said.

Professor Chris Stringer, a palaeontologist at the Natural History Museum in London and a prime proponent of the "replacement" theory, said Professor Templeton's study was interesting but needed to be scrutinised by geneticists who had come to different conclusions, albeit based on smaller samples of DNA.

(I agree with Professor Templeton)

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