BY MARK SHWARTZ
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.
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.
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.
research was supported by grants from the National Institutes of
Health, the National Science Foundation and the Russian Foundation
for Basic Research.
Stanford Report, May 28, 2003