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Prehistoric Migration

Lost Routes
The maps might lie inside us

Very high on the list of big unknowns in science (number one being, of course, Why is there something rather than nothing?) is the mystery of prehistoric human migration. How did human beings spread across the planet? Who went where, and when? Did anatomically modern humans evolve first in Africa? When did people colonize the Americas and the Pacific islands?

The fossil record is patchy at best. "It's just not fine grained enough to allow more than broad generalizations," says Ian Tattersall, an anthropologist at the American Museum of Natural History.

Paleontologists have to tease an elaborate tale from a fragmentary text of bones and teeth. But in the past decade clues to our early history have come from a completely new source: mitochondrial DNA.

Think of our genes as fossils. In addition to ordinary DNA, we have within our cells a separate ring of DNA inside tiny energy-producing engines called mitochondria. Unlike ordinary nuclear DNA that's sliced and diced and recombined during sexual reproduction, mitochondrial DNA (mtDNA) is passed whole from mother to child.

Mutations in mtDNA form recurring and recognizable patterns that scientists can use to calculate how long different population groups (for example, Inuit and Australian Aborigines) have been separated. The mtDNA record from living people can link modern humans through matrilineal lines to a common female ancestor who lived less than 200,000 years ago, probably in Africa. That conclusion jibes nicely with the fossil record.

"The fossil people are going directly back in time and looking at the actual record," points out Sarah Tishkoff, a geneticist who studies mtDNA at the University of Maryland. But the mtDNA approach requires an inferential leap. "We have to take the present data and infer about the past."

"There are so many ways to explain the same [mtDNA] pattern that it's often too easy to tell the story one likes and get away with it," says Lounès Chikhi, a French evolutionary biologist and critic of the mtDNA approach.

And mtDNA tells only the history of females. Men are invisible. Except in a few very rare cases, male mtDNA is lost in the shuffle because it doesn't enter the egg during conception. A fellow could cross half the planet, then sire a child, and his migration would go unnoted in the mtDNA record.

Individually the two strategies, fossils and genes, have some drawbacks. But what if they could be combined? What if we could extract mtDNA directly from ancient bones? The inferential leap into the past would no longer be necessary. Unfortunately ancient mtDNA—when a researcher is lucky enough to find some—is invariably degraded, though scientists report some progress on this front. As for the mystery of human migration, Tattersall notes: "There are a lot of ways of trying to solve it, but we haven't yet found the silver bullet."

Now if only those old bones could talk.

 —Joel Achenbach
    Washington Post staff writer


Web Links

The British Society for Cell Biology: Mitochondrion
www.kcl.ac.uk/kis/schools/life_sciences/biomed/bscb/softcell/mito.html
Get the lowdown on the multitasking of mighty mitochondria: They're "much more than an energy converter."

Mitokor
www.mitokor.com/science/
Get more background on the function of mitochondria, and learn about some of the diseases associated with their dysfunction.

More Articles by Joel Achenbach

Military Theory and the Force of Ideas
www.washingtonpost.com/wp-dyn/articles/A12208-2003Mar22.html
As military technology becomes more and more advanced, there is less room for valor on the battlefield.

Rough Draft
www.washingtonpost.com/wp-dyn/nation/columns/roughdraft
Writer Joel Achenbach's column is gaining a cult following. It takes a sometimes humorous, sometimes eye-squinting, but always intelligent look at today's headlines, personal interests, and the little life-annoyances we all live with.


Free World Map
Bibliography

Bower, Bruce. "Stone age genetics: Ancient DNA enters humanity's heritage," Science News  (May 17, 2003), 307.
 
Kahn, Patricia, and Ann Gibbons. "DNA From an extinct human," Science (July 11, 1997), 176.
 
Cann, Rebecca, and Allan C. Wilson. "The Recent African Genesis of humans," Scientific American (August 25, 2003), 54-61.
 
Cann, Rebecca, and others. "Mitochondrial DNA and human evolution," Nature (January 1, 1987), 31-36.
 
Schwartz, Marianne, and John Vissing. "Paternal Inheritance of Mitochondrial DNA," New England Journal of Medicine (August 22, 2002), 576-80.
 
Tattersall, Ian. The Fossil Trail: How We Know What We Think We Know About Human Evolution. Oxford University Press, 1995.
 
Thorne, Alan, and Milford Wolfpoff. "The Multiregional evolution of humans," Scientific American (August 25, 2003), 46-53.




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