- A new analysis of human genetic history deals a blow
to the theory that early people moved out of Africa and completely replaced
local populations elsewhere in the world. The findings suggest there was
at least limited interbreeding between our African ancestors and the residents
of areas where they settled.
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- "The new data seem to suggest that early human pioneers
moving out of Africa starting 80,000 years ago did not completely replace
local populations in the rest of the world," says Henry Harpending,
a University of Utah anthropology professor and co-author of the new study.
"There is instead some sign of interbreeding."
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- If that conclusion is correct, it contradicts the "replacement
theory" of human evolution - a theory Harpending has advocated for
more than a decade.
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- "Hypotheses are called into question by data every
day in science. That's the way it works," he says.
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- The journal Proceedings of the National Academy of Sciences
is publishing the new findings in its online edition the week of Dec. 23,
2002. The study's 20 co-authors include three from the University of Utah:
Harpending; Alan Rogers, also a professor of anthropology; and Stephen
Wooding, a postdoctoral researcher in human genetics.
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- The study was led by anthropologist Stephen Sherry and
mathematician Gabor Marth of the National Center for Biotechnology Information
at the National Institutes of Health in Bethesda, Md. Sherry is a former
student of Harpending's when both were at Pennsylvania State University.
Other co-authors of the new study are from the Washington University School
of Medicine in St. Louis, The Johns Hopkins University School of Medicine
in Baltimore and the University of California, San Francisco.
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- Most anthropologists agree human ancestors first spread
out of Africa roughly 1.8 million years ago, establishing new populations
in Europe, Asia and elsewhere. The "multiregional theory" holds
modern humans evolved from those multiple populations. The competing "replacement
theory" says that the local populations, including Europe's Neanderthals,
went extinct when they were replaced roughly between 80,000 and 30,000
years ago by another wave of human immigrants from Africa.
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- Scientists can analyze ancient genetic mutations in modern
people to learn about how humans evolved and the size of the human population
over time. Mutations occur at a relatively steady rate over time. If the
human population were large at a specific point in prehistoric time, more
mutations would occur, resulting in greater diversity in genetic mutations
found in modern people. A small population of human ancestors would result
in fewer mutations, so modern humans would display less genetic diversity.
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- So a person's genetic material "contains the whole
history of the population from which you descended," Harpending says.
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- Earlier studies of genetic material known as mitochondrial
DNA and microsatellites supported the notion that a small group of perhaps
5,000 to 20,000 primitive humans migrated from East Africa, spread around
the world, a rapidly expanded in population as they replaced other human
populations elsewhere in Africa 80,000 years ago, and in Asia 50,000 years
ago and Europe about 35,000 years ago.
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- The new study, however, analyzed mutations called SNPs
(single nucleotide polymorphisms) in DNA from the nucleus of human cells
studied for the Human Genome Project, the effort to map the entire human
genetic blueprint. The analysis indicates there was a bottleneck in the
human population - what looks like a sharp reduction in the number of people
- when ancestors of modern humans colonized Europe roughly 40,000 years
ago.
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- Researchers are not sure what this means because it conflicts
with studies of other kinds of human genetic information, which support
the idea that a rapidly expanding African population spread globally and
replaced local populations elsewhere.
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- "If Africans moved out of Africa and then populated
the whole world, we would see that in the genetic evidence as an expansion
in population size," yet the new study indicated the population shrank
instead, Rogers says.
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- The evidence five years ago indicated migrating Africans
did not interbreed with local populations, while the new study indicates
they did, Rogers notes, adding that the conflicting genetic data mean "the
question is still open."
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- Harpending says one possible explanation for the new
data is that there was a large population of humans who migrated from Africa,
yet they kept largely to themselves and mated only to a limited extent
with local populations in Europe and elsewhere. Because interbreeding still
was uncommon, only a few of the prehistoric European genes were incorporated
into the modern human genetic blueprint, giving a false impression that
the prehistoric human population collapsed or shrank in size, Harpending
says.
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- Another possibility is that the prehistoric African population
was large 100,000 years ago, but only a very small number - perhaps a few
dozen - of those Africans migrated to other areas some 80,000 years ago,
ultimately replacing local populations. That would explain why the human
genetic blueprint could give a false impression that the human population
collapsed in size even if it did not. But Harpending believes it is unlikely
that such a small number of migrants from Africa could spread globally
and ultimately replace other populations.
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- Editor's Note: The original news release can be found
<http://www.utah.edu/unews/releases/02/dec/genome.html>here.
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- Note: This story has been adapted from a news release
issued for journalists and other members of the public. If you wish to
quote any part of this story, please credit University Of Utah as the original
source. You may also wish to include the following link in any citation:
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- http://www.sciencedaily.com/releases/2002/12/021226071610.htm
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- Copyright © 1995-2002 ScienceDaily Magazine | Email:
editor@sciencedaily.com
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- http://www.utah.edu/unews/releases/02/dec/genome.html
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