Public release date: 27-May-2003
Contact: Mark Shwartz mshwartz@... 650-723-9296
Stanford University
http://www.stanford.edu/dept/news/

Scientists use DNA fragments to trace the migration of modern humans

Human beings may have made their first journey out of Africa as recently as
70,000 years ago, according to a new study by geneticists from Stanford
University and the Russian Academy of Sciences. Writing in the American Journal
of Human Genetics, the researchers estimate that the entire population of
ancestral humans at the time of the African expansion consisted of only about
2,000 individuals.

"This estimate does not preclude the presence of other populations of Homo
sapiens sapiens [modern humans] in Africa, although it suggests that they were
probably isolated from one another genetically, and that contemporary worldwide
populations descend from one or very few of those populations," said Marcus W.
Feldman, the Burnet C. and Mildred Finley Wohlford Professor at Stanford and
co-author of the study.

The small size of our ancestral population may explain why there is so little
genetic variability in human DNA compared with that of chimpanzees and other
closely related species, Feldman added.

The study, published in the May edition of the journal, is based on research
conducted in Feldman`s Stanford laboratory in collaboration with co-authors Lev
A. Zhivotovsky of the Russian Academy and former Stanford graduate student Noah
A. Rosenberg, now at the University of Southern California.

"Our results are consistent with the `out-of-Africa` theory, according to which
a sub-Saharan African ancestral population gave rise to all populations of
anatomically modern humans through a chain of migrations to the Middle East,
Europe, Asia, Oceania and America," Feldman noted.

Ancient roots

Since all human beings have virtually identical DNA, geneticists have to look
for slight chemical variations that distinguish one population from another.
One technique involves the use of "microsatellites" - short repetitive
fragments of DNA whose patterns of variation differ among populations. Because
microsatellites are passed from generation to generation and have a high
mutation rate, they are a useful tool for estimating when two populations
diverged.

In their study, the research team compared 377 microsatellite markers in DNA
collected from 1,056 individuals representing 52 geographic sites in Africa,
Eurasia (the Middle East, Europe, Central and South Asia), East Asia, Oceania
and the Americas.

Statistical analysis of the microsatellite data revealed a close genetic
relationship between two hunter-gatherer populations in sub-Saharan Africa -
the Mbuti pygmies of the Congo Basin and the Khoisan (or "bushmen") of Botswana
and Namibia. These two populations "may represent the oldest branch of modern
humans studied here," the authors concluded.

The data revealed a genetic split between the ancestors of these
hunter-gatherer populations and the ancestors of contemporary African farming
people - Bantu speakers who inhabit many countries in southern Africa. "This
division occurred between 70,000 and 140,000 years ago and was followed by the
expansion out of Africa into Eurasia, Oceania, East Asia and the Americas - in
that order," Feldman said.

This result is consistent with an earlier study in which Feldman and others
analyzed the Y chromosomes of more than 1,000 men from 21 different
populations. In that study, the researchers concluded that the first human
migration from Africa may have occurred roughly 66,000 years ago.

Population bottlenecks

The research team also found that indigenous hunter-gatherer populations in
Africa, the Americas and Oceania have experienced very little growth over time.
"Hunting and gathering could not support a significant increase in population
size," Feldman explained. "These populations probably underwent severe
bottlenecks during which their numbers crashed - possibly because of limited
resources, diseases and, in some cases, the effects of long-distance
migrations."

Unlike hunter-gatherers, the ancestors of sub-Saharan African farming
populations appear to have experienced a population expansion that started
around 35,000 years ago: "This increase in population sizes might have been
preceded by technological innovations that led to an increase in survival and
then an increase in the overall birth rate," the authors wrote. The peoples of
Eurasia and East Asia also show evidence of population expansion starting about
25,000 years ago, they added.

"The exciting thing about these data is that they are amenable to a combination
of mathematical models and statistical analyses that can help solve problems
that are important in paleontology, archaeology and anthropology," Feldman
concluded.


###
The research was supported by grants from the National Institutes of Health,
the National Science Foundation and the Russian Foundation for Basic Research.

By Mark Shwartz

CONTACT: Mark Shwartz, News Service: (650) 723-9296, mshwartz@...
COMMENT: Marc Feldman, Biological Sciences: (650) 725-1867,
marc@...

EDITORS: The study, "Features of Evolution and Expansion of Modern Humans,
Inferred from Genomewide Microsatellite Markers," by Lev A. Zhivotovsky, Noah
A. Rosenberg and Marcus W. Feldman, appears in the May 2003 edition of the
American Journal of Human Genetics. A copy can be obtained from Professor Marc
Feldman. His photo is available at http://newsphotos.stanford.edu (slug:
"Humans_Feldman.jpg").

Relevant Web URLs:
http://www-evo.stanford.edu/
http://news-service.stanford.edu/news/2003/january8/genetics-18.html
http://news-service.stanford.edu/news/november8/chromosome-1108.html
http://www.neanderthal-modern.com/genetic1.htm

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http://www.eurekalert.org/pub_releases/2003-05/su-sud052703.php




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