Our Species Mated With Other Human Species, Study Says
for National Geographic News
|March 6, 2002|
A new piece of evidenceone sure to prove controversialhas 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.
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 genesas is indicated from the mitochondrial DNA and y chromosome datawhy 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 wavesthe 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.
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.
The two primary theories in the human origins debate are the "Out of Africa" theory and the multi-regionalism theory. Each has its own variations, and there are intermediate models, such as one favoring assimilation among the different groups. Credible evidence exists to support each theory.
The multi-regionalism theory, which relies on fossil evidence, holds that after members of Homo erectus first left Africa roughly 1.7 million years ago, they settled in different regions of the world and evolved separately but concurrently into Homo sapiens. Despite the vast distances, there was enough gene exchange between groups that an entirely new species did not evolve.
The "Out of Africa" theory relies considerably on DNA evidence. This scenario also holds that Homo erectus first left Africa around 1.7 million years ago. Evolution continued, and anatomically modern humans appeared in Africa between 200,000 and 100,000 years ago.
Beginning about 100,000 years ago, these modern humans expanded outside the continent, making their way across Asia and Europe, where they completely replaced the older species, Homo erectus.
Unlike Templeton's assertions, the "Out of Africa" theory does not support the idea of interbreeding between archaic and modern humans.
Explaining his contention that interbreeding occurred, Templeton said humans "have long shown a pattern of isolation by distance," and at any given time there is some degree of genetic difference between human populations.
"However," he added, "genetic interconnections have long existed among human populations, and this was accentuated by the latest 'Out of Africa' expansion, not eliminated, as under the replacement model."
Templeton's view is "a kind of compromise," said Brooks. "Africa was still the major source of all modern humans, but there was a limited amount of interbreeding with other populations already living in Eurasia," she said.
Fred Smith, a paleoanthropologist at Northern Illinois University who proposed the assimilation model of human evolution, said Templeton's data support the idea that modern humans evolved in Africa, spread to other continents, and interbred with archaic populations.
"I argued for the assimilation model based on morphologywhat could be seen in the fossil record, rather than on genetic evidence. But I'm in agreement with what Templeton has found," Smith said.
Cann, in an accompanying article in Nature, said Templeton's attempt to view the data from a global perspective is over-ambitious given problems with genetic studies of small-scale modern populations.
"I want to see [his methodology and analysis] validated in an area of the world where a variety of scientists from different disciplines think they understand how humans spread and when," she said.
Examples of human migration that might help demonstrate the validity of Templeton's analysis and its limitations, she suggested, include the relatively recent expansion to Polynesia, the spread of farmers from Turkey into Northern Europe, and the migration of Vikings to Iceland.
"We need lots of different tools to study human evolution," Cann pointed out. "Scientists get into trouble when they expect one tool will do everything. Sometimes you need a hammer to attach things, sometimes a screwdriver, and sometimes Velcro works as well!
"Better to keep exploring these different methods with an open mind," she added, "since there are things only fossils can tell you, and things only genetics can reveal."
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