A study published by a group from Rice University (USA) in 'Theoretical Population Biology' proposes that a woman who lived in Africa approximately 200,000 years ago was the last common female ancestor to everyone alive today. Pieces of her mitochondrial DNA can be found in all modern human populations.
     The scientists performed statistical analyses of mitochindrial DNA from individuals representing many different countries. They then compared calculations from 10 models with different assumptions about human migration, population growth and genetic mutation rates. These assumptions are represented by mathematical coefficients in the equations of the models.
     The goal was to find the 'Mitochondrial Eve', the woman who connects us all genetically. The study found that all of the models that allowed for random population size gave approximately the same result of about 200,000 years.
     The process and calculations are extremely complex and require simplifying assumptions that may or may not accurately represent human populations. "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 in Gliwice, Poland. "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?"
     Mitochondrial DNA is found in the energy-producing mitochondria of cells and contains 37 genes. Under normal circumstances, mitochondrial DNA comes directly to male and female children from the mother. When a sperm cell fertilizes an egg, the father's chromosomes from the nucleus of the cell join with the mother's nuclear DNA. But, the mitochondrial DNA of the sperm cell remains outside the egg. So the child inherits only the mother's mitochondrial DNA, which came from her mother, which came from her mother, down through history.
     'Mitochondrial Eve' is something of a misnomer. The findings of the study do not mean that she is the ancestral mother of all living humans. Many woman who lived during her time and before have contributed to our nuclear gene pool. But at some time during history, the genetic tree of all of her contemporaries produced a family of only male children so that their mitochondrial lines ended while hers continued.
     
Source: ScienceDaily (17 August 2010)