Washington: Mitochondrial Eve, the maternal ancestor to all living humans, most likely lived about 200,000 years ago, a new study has claimed.
Researchers at the Rice University have conducted what they describe as the most "robust" statistical comparison of 10 human genetic models to confirm that the mother of all living humans lived at the same time when modern humans are believed to have emerged.
The study based on the analysis of mitochondrial DNA -- found in the energy-producing centres of human cells -- discovered that the genome is only passed down the maternal line, and can be traced back to one woman.
However, they said, this doesn`t mean she was the first modern woman, rather it indicates that only her descendants survive to the present day.
"There is always some other female that predated mitochondrial Eve, whose DNA didn`t make it up to modernity," Professor Marek Kimmel, lead author of the study, was quoted as saying by LiveScience.
"So the age of the mitochondrial Eve is always less than the age of the true, first female modern human."
As part of the three-year project, Kimmel and Krzysztof Cyran, a Polish researcher, compared the estimates produced by about 10 genetic models intended to determine when mitochondrial Eve lived.
Because the entire human genome has more than 20,000 genes and comparing the differences among them for distant relatives is a huge task, difficult even for the most advanced computers, researchers used mitochondrial genomes to find common ancestors.
This method is somehow less complicated as mitochondria have their own genome and each person`s mitochondrial genome is inherited from the mother, so all mitochondrial lineages are maternal.
Mitochondria also have only 37 genes than seldom change and they contain a "hypervariable" region that changes fast enough to provide a molecular clock calibrated to times comparable to the age of modern humanity, the scientists said.
In order to find out the age on mitochondrial Eve, they have to convert the measure of relatedness between random blood donors into a measure of time. The models make different assumptions about growth and extinction rates, which had the potential to change the estimate of mitochondrial Eve`s age, the researchers found.
One type of model makes the less realistic, but more manageable assumption that the human population has increased at a smooth, nearly exponential rate. Another more realistic, but more technically challenging type of model assumes the human population has grown in discrete random episodes.
But, regardless, all of the models produced estimates placing this ancient mother`s age at around 200,000 years -- the time when moder humans are believed to have emerged.
"We actually show if one uses different models, one comes up with a very similar estimate, so this makes the estimate more robust," Kimmel said.
The estimates produced by models that assume population growth occurred in discrete, random bursts fell within 10 percent of each other. When taking into consideration models that assumed smooth growth, that range expanded by up to 20 percent. These models also tended to estimate that mitochondrial Eve lived earlier, said Kimmel.
The research was published in the journal Theoretical Population Biology.