Eve Takes Another Fall
Source: Newsweek (March 2, 1992): 58.
They called her Eve. She was born not of Adam's rib but out of a marriage of silicon and DNA. A computer compared the genes from scores of people around the world and, in 1987, proclaimed that all of humankind descended from one woman who lived in Africa some 200,000 years ago. Eve's discovery was a sensation -- NEWSWEEK put her on its cover. But now Eve has gone the way of all flesh. In a terse "technical comment" this month in the journal Science, geneticist Alan Templeton of Washington University torpedoes the Eve hypotheses. "The inference that the tree of humankind is rooted in Africa is not supported by the data," he concludes. Eve's creators (the group's leader, Allan Wilson, died last year) gracefully concede that their analysis is fatally flawed.
In the original paper, Wilson and colleagues at the University of California, Berkeley, analyzed DNA from 147 people in five continents. They chose mitochondrial DNA (mtDNA), which codes not for height or other traits but for cogs in cells' energy factories. The wonder of mtDNA is that a child inherits it solely from his or her mother. It passes down through the generations cleanly, not scrambled up with paternal DNA as regular genes are. This makes it a handy measure of people's relatedness, and a fertile seed for constructing a family tree. When the researchers ran their computer, they got a branching lineage that led back to a single mother who lived 140,000 to 290,000 years ago in sub-Saharan Africa. Last year the team ran a more rigorous analysis, using exact sequences of molecules in mtDNA from 189 people, and concluded again that we are all descended from one woman who was, give or take, our 10,000th-great-grandmother.
Family tree: Now the DNA theory has had its genes knocked out from under it. The problem lies in how the biologists used the computer program that turns mtDNA data into a family tree. The program tries to find the family tree that is most "parsimonious." In this case, parsimony means a tree based on the fewest genetic mutations -- one that resembles topiary and not a scraggly oak, and is thought to best mimic what actually happened during evolution. The trouble is that there are millions of equally parsimonious trees, and no algorithm to guarantee the computer has found the best one. Which trees grow in the computer depends on the number of runs and the order in which data are entered. The Berkeley group assumed the order of data irrelevant. "They weren't using the program in an adequate way," says Templeton. A tree with African roots, it turns out, is no more probable than one with Asian or European roots.
Why did it take five years to discover this? One reason is that it takes months of dedicated mainframe time to grow family trees from mtDNA data. Another is that the stones-and-bones anthropologists who disdained the gene jocks' work didn't have the mathematical savvy to understand the statistical traps in the computer. Also, no researchers tried to replicate the results until Templeton entered the fray serendipitously (a journal asked him, as an outsider, to write about Eve).
The Out-of-Africa school lives, however. Fossils support an African Eden. "An African origin still provides the best explanation," argues Mark Stoneking, a member of the Wilson team now at Pennsylvania State University. The debate centers on whether humankind had a single origin, an Eve whose children left other early humans in the dustbin of prehistory -- and whether Homo sapiens will ever find it.
HOME | SYLLABUS | READINGS | ASSIGNMENTS
REFERENCE | DISCUSSION SECTIONS | CONTACT