March 18, 2004 • Volume 12 Number 13
Out of Africa? BC Biologist Marth Believes It's So
Recent study shows evidence of African origin for human life
Research by a Boston College biologist and colleagues at the National Institutes of Health finds evidence for a recent African origin for all humans.Asst. Prof. Gabor Marth (Biology) and Eva Czabarka, Janos Murvai and Stephen Sherry of the NIH recently completed a study on variation in human DNA that provides new insight into the distant past of human populations.
In a research article in the January 2004 issue of the journal Genetics, Marth and his colleagues report strong evidence for a large population bottleneck in the distant past in European and Asian populations. They found no evidence for a similar bottleneck in African populations.
Their data supports the hypothesis that multiple waves of humans migrated out of Africa, and that members of the last wave out of Africa replaced humans from the previous waves. In summary, Marth and his collaborators say, their study indicates a recent African origin for all modern humans.
Although the DNA sequence of every human being is very similar, say the researchers, many subtle differences exist. In their study, Marth and his colleagues looked at tens of thousands of these differences, DNA variations geneticists call "single-nucleotide polymorphisms."
Combining innovative new techniques with mathematical modeling, the scientists were able to sketch out the shapes of prehistoric past of three large world populations: Europeans, Asians, and Africans.
The polymorphism "signatures" found in DNA indicate that European and Asian populations experienced dramatic reductions in population size - known as "bottleneck" events - roughly 50,000 to 80,000 years ago.
The data also indicate that the population size in both populations has been steadily increasing after the bottleneck, up to the present. In contrast, African populations appear to have undergone uninterrupted population growth without bottlenecks.
Much of what is known about human origins from molecular evidence is based on variation in the DNA of mitochondria, organelles present in every human cell.
Mitochondria have a tiny amount of DNA, but very high mutation rates, and therefore record only the most recent events in our prehistory. In contrast, the nucleus of human cells contains a great deal of DNA, but a very low mutation rate, and as a result, nuclear DNA records population events far into the past.
Whereas previous studies on human prehistory looked at mitochondrial DNA, Marth looked at variations in nuclear DNA. Mitochondrial DNA data only shows growth in every world population, with no bottlenecks. The higher resolution nuclear DNA suggests the growth seen in mitochondrial data is only the most recent phase of population history, and that scientists must use nuclear DNA to resolve population events of the more distant past.
Wide agreement exists among scientists that all human populations originally evolved in Africa, but competing hypotheses have arisen about exactly when the direct ancestors of modern humans in Europe and Asia left Africa.
One explanation, the "multiregional" hypothesis, states that after an ancient, initial colonization of the habitable parts of Europe and Asia, geographically dispersed humans evolved simultaneously, in multiple regional centers, into the modern human populations we find today.
Another explanation, the "replacement" hypothesis, states that multiple waves of "out-of-Africa" migrations occurred, but all modern human populations are descendants of the last wave, which replaced humans from previous waves.
The data presented by Marth and his colleagues support the "replacement" hypothesis, which is consistent with recent archeological evidence pointing to the extinction of Neanderthals - members of a previous wave of humans - 20,000 to 40,000 years ago.
If this study has implications for the understanding of human prehistory, the scientists say it also offers significant medical applications. In laboratories around the world, genetic polymorphisms are used extensively as the key to identifying disease-causing genes. Knowledge of the differential structure of polymorphisms in human world populations offers a way to design scientific and medical resources that are inclusive in scope, and that are most likely to benefit all modern humans.
-Biology Department Assistant Director John P. Roche