African Eve gets lost in the 'trees.' (mitochondrial DNA research)

Since 1987, an influential group of molecular biologists has published several reports indicating that analyses of mitochondrial DNA -- genetic material located outside the cell nucleus and inherited only from the mother -- track the maternal lineage of all humans back to one or possibly several women who lived in Africa around 200,000 years ago (SN: 9/28/91, p.197). That controversial contention gets the statistical rug pulled out from under it in two reanalyses of the most extensive sample of mitochondrial DNA studied earlier. The new findings appear in Feb. 7 SCIENCE.

One study, directed by S. Blair Hedges of Pennsylvania State University in University Park, builds five mitochondrial DNA trees but fails to pinpoint statistically the geographic origins of humanity. Particular sequences of chemical components of mitochondrial DNA appear about as often in all geographically separated populations, Hedges' team argues. This, they say, suggests that widely separated human groups have shared, or conserved, specific chemical arrangements of mitochondrial DNA since those populations diverged from a single group. Analyses of DNA sequences from within the cell nucleus may offer a better possibility of establishing a valid evolutionary tree for humanity, the scientists hold.

A widely cited 1991 study used a computer-driven statistical analysis that created 100 possible evolutionary "trees" based on the similarity of mitochondrial DNA sequences among 136 people from around the world. The computer program then identified several trees containing the fewest branches, or changes, in DNA sequences. The researchers regarded those trees -- with purely African roots -- as most representative of evolution. However, that study relied on a single computer analysis whose outcome depends on the order in which individual genetic sequences are examined. Tree construction that begins with the mitochondrial DNA of other randomly selected individuals in the sample yields a mixture of African and non-African origins, Hedges' group asserts.

Alan R. Templeton of Washington University in St. Louis also reports generating from the existing mitochondrial DNA data a tree that displays both African and non-African roots.

Most researchers agree that Africans possess more mitochondrial DNA mutations than do inhabitants of other regions, indicating an older evolutionary history. But the rate at which these mutations occur remains controversial, clouding the question of whether all these changes arose in modern humans.