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MARGINALIA

We Are All Africans

Pat Shipman

Evidence for "Eve"

In April of this year, Sarah Tishkoff of the University of Maryland and a team of coworkers reported genetic analyses of more than 600 living Tanzanians from 14 different tribes and four linguistic groups. They analyzed mitochondrial DNA (mtDNA)—the tool of choice for tracing ancestry because it is inherited only through the mother as part of the ovum. The number of mutations that have accumulated in mtDNA is a rough measure of the time that has passed since that lineage first appeared. The owner of the first modern human mtDNA (by definition, a woman) is often referred to as "Eve," although many women of that time are likely to have shared similar mtDNA.

Figure 1. Genetic diversityClick to Enlarge Image

Tishkoff and her colleagues chose to investigate East African peoples for specific reasons. The number of linguistic and cultural differences is unusually high in the region, as is the variation in physical appearance—East Africans are tall or short, darker-skinned or lighter-skinned, round-faced or narrow-faced, and so on. This observation suggested that the genetic composition of the population is highly diverse, and as expected, the team found substantial variation in the mtDNA. In fact, members of five of the lineages showed an exceptionally high number of mutations compared with other populations, indicating that these East African lineages are of great antiquity. Identified by tribal affiliation, these are: the Sandawe, who speak a "click" language related to that of the Bushmen of the Kalahari desert; the Burunge and Gorowaa, who migrated to Tanzania from Ethiopia within the last five thousand years; and the Maasai and the Datog, who probably originated in the Sudan. The efforts of the University of Maryland group reflect a substantially larger database and more certain geographic origins for its subjects than earlier mtDNA studies. Further, the work by Tishkoff's team reveals that these five East African populations have even older origins than the !Kung San of southern Africa, who previously had the oldest known mtDNA.

"These samples showed really deep, old lineages with lots of genetic diversity," Tishkoff says. "They are the oldest lineages identified to date. And that fact makes it highly likely that 'Eve' was an East or Northeast African. My guess is that the region of Ethiopia or the Sudan is where modern humans originated."

By assuming that mtDNA mutates at a constant rate, Tishkoff's team estimated that the oldest lineages in their study originated 170,000 years ago, although she cautions that the method only gives an approximate date. Nonetheless, this finding is neatly congruent with new fossil evidence.








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