One of the ancestors of all humans, the so-called "mitochondrial Eve," lived about 200,000 years ago, according to statisticians and geneticists at Rice University. The researchers compared 10 different human genetic models to make their calculations, some of which assumed that population rates have changed deterministically, and others of which assumed that populations have changed randomly, says Rice statistician Marek Kimmel.
"No matter which stochastic approach we took, we got basically the same estimate, ranging from 180,000 to 200,000 years. It's a reasonable ballpark figure," Kimmel adds.
The researchers began with the concept of the molecular clock — if mutations occur at a relatively constant speed within the genome, then by measuring the amount of mutations a given subject has accumulated, researchers should be able to work backwards to estimate the amount of time that it took to accumulate all these mutations. The researchers at Rice — who teamed up with researchers at Institute of Informatics at Silesian University of Technology in Poland — measured the divergence in the genome and divided it by the rate of mutation to estimate the time.
This, Kimmel says, is no different from any of the other methods researchers have used in the past to estimate the age of "mitochondrial Eve." However, he adds, there is a catch. "To properly estimate divergence, one has to remember that individuals are embedded in populations, and populations change and sometimes change very rapidly," he says, referring to genetic drift.
Applying the 10 models, with their different assumptions of population demographics over time, allowed Kimmel and his team to get the most accurate estimate possible for the age of the common ancestor.
But pinpointing the age of "Eve" was not just an exercise in statistics. "The mtEve story has become a benchmark in genetics," Kimmel says. "We want to know how genetic diseases evolved, how they came into being, and how it happened that they were stabilized in a population."
To find the factor that stabilizes a genetic disease to the point where it is passed on to subsequent generations, it is necessary to figure out the early evolution of the disease, including how long ago it showed up in the human genome. "There is a direct link between the evolutionary history of a disease and the way it is structured in the population, and this is important for medicine," Kimmel says.
If researchers can figure out which mutations caused a certain disease, and which subsequent mutations forced it to take hold, then they could, perhaps, better target the disease.