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Science: Everyone’s Genealogical Mother

3 minute read
Michael D. Lemonick

If family trees were charted indefinitely backward, they would eventually converge on a small group of ancients who were ancestors of us all. Now biologists suggest in a report to Nature that a single female living between 140,000 and 280,000 years ago in Africa was an ancestor of everyone on the earth today. Inevitably — and to the probable delight of creationists — many scientists are calling her “Eve.”

The authors point out that the hypothetical Eve, unlike the biblical one, was in no sense the one ancestral mother of all humans. There were other females reproducing at the time who have modern descendants. But Eve is the only one who appears in everyone’s genealogy, a conclusion the biologists reached by studying mitochondrial DNA (mtDNA).

Most of the DNA in human cells is in the cell nucleus, in the form of chromosomes. Since chromosomes come from both parents, this nuclear DNA is reshuffled with each generation, confusing the line of inheritance. But there is also DNA outside the nucleus, in mitochondria, substructures within each cell that are responsible for producing energy the cell needs. Since the sperm’s mitochondria do not survive fertilization intact, mtDNA is inherited solely through the mother. The only way it can change over the generations is through mutation. And that mutation, evidence suggests, proceeds at a steady, known rate. To calculate how much time has elapsed since the mutations that gave rise to today’s variations began, the scientists need merely determine how much change has taken place.

To measure this change, the biologists — Allan Wilson, currently on sabbatical from the University of California, Berkeley; Rebecca Cann, now of the University of Hawaii; and Mark Stoneking, at Berkeley — examined mtDNA from 147 individuals representing five broad geographic regions. The scientists analyzed the samples by mixing them with restriction enzymes, proteins that cut strands of DNA at specific sites. After comparing the resulting fragments, the scientists used a computer to analyze the differences between the mtDNA samples and construct a “family” tree. Those differences were so small that they could be explained by assuming the existence of one ancestral mtDNA. Then the biologists extrapolated backward to calculate when that mtDNA existed — in other words, when Eve lived.

The facts that mtDNA is maternally inherited and that Eve’s alone has survived mean that she was the only one among her female contemporaries whose descendants included some females in all succeeding generations. Descendants of the other females alive during Eve’s time eventually included generations that produced no children or only males, thus halting propagation of their mtDNA.

By considering the geographic origins of the 147 people, the biologists were even able to determine where Eve lived: samples from those of sub-Saharan African descent showed the most intragroup differences, implying that their mtDNA had had more time to change and thus that their ancestors arose earliest. This finding plus the structure of the family tree indicated sub- Saharan origin, a conclusion that agrees with current archaeological and anthropological theory.

The researchers concede some uncertainties: about the mutation rate, for example, and whether it is constant. Still, Stoneking says, the evidence shows that “mtDNA is a good molecule for tracing relationships between populations in general.” And, adds Cann, “it is a way of welding molecular biology and anthropology. Sometimes fossils are misleading. We’re trying to build better pictures of how humans evolved.”

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