In the beginning, there was Eve…
But when was the beginning? Statisticians have calculated that the mother of all human beings, the so-called 'mitochondrial Eve' (mtEve), lived on Earth about 200 000 years ago. The team from Poland and the US based its estimation on the most comprehensive statistical examination ever conducted to date that included genetic models and assumptions based on human migration and proliferation. Findings from the study are published in Theoretical Population Biology.
The study was conducted by Dr Krzysztof Cyran from the Silesian University of Technology in Gliwice, Poland, and Dr Marek Kimmel from Rice University in the US. The statisticians compared 10 human genetic models to pinpoint mtEve's existence through a set of assumptions on the way human beings moved and spread across the planet.
In the study, the authors define mtEve as the 'root of the mitochondrial polymorphisms of the modern humans based on the DNA (deoxyribonucleic acid) from humans and Neanderthal fossils'.
Mitochondria are tiny organelles in the cytoplasm of cells that produce energy. All mitochondrial lineages are maternal because every person's mitochondrial genome is inherited from their mother.
Scientists use this genome as a way to gauge the relationship between one human and another. At 37 genes that rarely change, mitochondria are a far easier option than comparing the some 20 000 genes contained in the human genome.
So identifying the right age of the human being's original maternal ancestor is a pursuit that brings scientists closer to our past by way of genetic processes that are key players in disease such as mutation and selection.
'This is why we are interested in patterns of genetic variability in general,' explained Dr Kimmel. 'They are very important for medicine.'
The genetic profiles of random blood donors allow scientists to understand the degree to which one donor is related to another. The secret to determining mtEve's age lies in converting the measures of relatedness of these random donors into a measure of time.
According to Dr Cyran, differences between gene sequences must be translated to determine how they evolved over time. 'And how they evolved in time depends upon the model of evolution that you use,' he explained. '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?'
Answers to these key questions take the form of coefficients (numeric constants) within each of the 10 models incorporated by Drs Cyran and Kimmel, and are added to the equation to determine mtEve's age.
'We found that all of the models that accounted for random population size — such as different branching processes — gave similar estimates,' Dr Kimmel said. '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.'
Dr Kimmel added that his and Dr Cyran's findings underscore the importance of considering the random nature of population processes such as growth and extinction. 'Classical, deterministic models, including several that have previously been applied to the dating of mtEve, do not fully account for these random processes.'