US-POLAND: Original mother lived 200,000 years ago

After considering the results of 10 different genetic models, Polish and US researchers say the maternal ancestor of all living humans lived 200,000 years ago.

The models used by scientists at Rice University in Houston, Texas, and the Silesian University of Technology in Gliwice, Poland, were based on different assumptions about how humans migrated, expanded and spread.

The work was based on differences in human's mitochondrial DNA - inherited through the mother - and aimed to put a date on 'mitochondrial Eve' or 'mtEve' - the woman from whom all living humans are descended.

The researchers explain that mitochondria - tiny organelles that serve as energy factories inside human cells - have their own genome. They contain 37 genes that rarely change, but also a 'hypervariable' region, which changes fast enough to provide a molecular clock calibrated to times comparable to the age of modern humanity.

"You have to translate the differences between gene sequences into how they evolved in time," said co-author Krzysztof Cyran, vice-head of the Institute of Informatics at Silesian University of Technology.

"And how they evolved in time depends upon the model of evolution that you use. So, for instance, what is the rate of genetic mutation, and is that rate of change uniform in time? And what about the process of random loss of genetic variants, which we call genetic drift?"

Within each model, the answers to these questions take the form of coefficients - numeric constants that are plugged into the equation that returns the answer for when mtEve lived.

Cyran said human genetic models have become more complex as researchers try to correct for invalid assumptions. But some of the corrections are complicated and that raises the question of whether less complex models might do equally well in capturing what is occurring.

"We wanted to see how sensitive the estimates were to the assumptions of the models," said co-author Professor Marek Kimmel of the department of statistics at Rice University.

"We found that all of the models that accounted for random population size - such as different branching processes - gave similar estimates. This is reassuring, because it shows that refining the assumptions of the model, beyond a certain point, may not be that important in the big picture."

The research is available online in the journal Theoretical Population Biology.