To pinpoint Eve's origins, geneticists Himla Soodyall and Trefor Jenkins of the South African Institute for Medical Research and the University of the Witwatersrand in Johannesburg analyzed mitochondrial DNA (mtDNA), genetic material from the energy-producing organelles in all cells. Because after conception only the egg's mitochondria survive, mtDNA forms a continuous thread running back in time through the maternal lineage. Knowing mtDNA mutation rates, scientists can infer, by comparing DNA segments from different populations, which are the most ancient patterns.

The researchers drew blood from 100 people from two Khoisan groups and compared the mtDNA sequences with those from 50 other sub-Saharan Africans. Soodyall says the study found that some 84% of the mtDNA types they looked at were "unique" to the Khoisan and could be dated back to 120,000 years ago. This demonstrates that "some of the most ancestral signatures in mtDNA are still found in living Khoisan people," she says. The same sequences have been lost due to random mutations in other, later populations.

The findings, presented at a recent human evolution meeting at Cold Spring Harbor Laboratory in New York, complement data from the male side: Y chromosome studies had previously pegged the Khoisan among a handful of groups with Y chromosomes most closely resembling those of a common ancestor who lived in Africa 145,000 years ago (Science, 31 October 1997, p. 804). Mike Hammer of the University of Arizona, Tucson, who took part in the Y chromosome study, says the latest mtDNA work provides "important confirmation" of the team's work.