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Thursday, March 11, 1999 Published at 17:57 GMT


Genetic clock runs slow

The "African Eve", from whom all women are believed to be descended could be twice as old as first thought.

Two new pieces of research challenge the widely accepted view that modern humans are descended from a common ancestor that lived about 200,000 years ago.

The new studies at the Universities of Cambridge and Sussex suggest one of the tools used to track our genetic lineage may be seriously flawed.

This is based on the way mitochondrial DNA (mtDNA) is thought to change over time.

Mitochondria are the energy factories in our cells. They contain circular DNA molecules that are believed to be the genetic leftovers of free-living organisms.

Changes over time

mtDNA mutates at a constant rate. Uniquely, these changes are not thought to get recombined, or shuffled, as they do in nuclear DNA.

The mtDNA is believed to be handed down unchanged from mother to daughter.

This simple pattern of inheritance together with the rapid mutations has made mtDNA a very useful "molecular clock" to study evolution - right back to the hypothetical "Eve", the most recent common ancestor of all women.

But the two studies reported by New Scientist Magazine now suggest that there is a degree of paternal mtDNA inheritance.

The Cambridge team, led by Erika Hagelberg, came to their conclusions after looking at the distribution of one mtDNA mutation on the island of Nguna in the Melanesian archipelago of Vanuatu.

The Sussex research, by John Maynard Smith and colleagues, focused on mutations in mitochondrial proteins from populations around the world.

Paternal influence

They suggest that recombination with paternal mtDNA must be responsible for some of the variation seen in the mtDNA.

This does not invalidate the molecular clock technique, but does have the effect of making it run much slower.

It would mean "Eve" must have lived longer ago than the 100 000 to 200 000 years of current estimates.

Laurence Hurst, an evolutionary geneticist at the University of Bath, told New Scientist that the two studies would provoke heated debate.

"These papers are going to create some waves."

It is difficult to prove that paternal recombination, rather than just mutation, is involved, he said. "But I think recombination is the best explanation for what they've found."

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