Polygamy left its mark on human genome

The rise of the single mother may seem a rather modern phenomenon. But even before the first humans walked out of Africa 70,000 years ago, mothers have consistently outnumbered fathers, DNA analysis suggests.

'Imagine a population of 100 females and 100 males', explained the study's lead author, Professor Mark Stoneking, to the Guardian. 'If all the females but only one of the males reproduced, then while the males and females contribute 50:50 to the next generation, the male is all from just one male'.

In this case the next generation would all have the exact same Y-chromosome sequence, since this is inherited paternally, but up to 100 different sequence variants for their mitochondrial DNA, which is maternally inherited.

Using a new technique, Professor Stoneking and his team at the Max Planck Institute for Evolutionary Anthropology in Germany looked at the genetic variation in the Y-chromosome and the mitochondrial DNA of over 600 people from 51 populations worldwide. Based on their findings, they could then make inferences about our evolutionary history.

Consistent with previous reports, the researchers found a greater diversity in the paternally inherited Y-chromosome DNA than the maternally inherited mitochondrial DNA, although they suggest that technical limitations in prior studies led this difference to be overestimated in the past.

According to their computer simulations, this genetic diversity could be explained by a dramatic difference in the number of men and women breeding throughout human history. The report suggests that the ancestral human population in Africa consisted of around 60 breeding females and 30 breeding males, while of those that migrated out of Africa only around 15 men and 25 women reproduced.

'More of the women were reproducing than men. This often happens in human societies, because not all men are able to afford wives, or sometimes a few men will have many wives', Professor Stoneking told LiveScience.

Social practices may also have a big impact on the results. In some cultures women have tended to move in order to be with their husbands, resulting in their mitochondrial DNA spreading between populations. This may help to explain why in Europe and East Asia there is less difference in mitochondrial DNA than Y-chromosome DNA between distinct populations.

By contrast, the team found lower paternal variation in the African samples, which they suggest may be due to the rapid expansion of Bantu people into eastern and southern Africa around 3,000 years ago.

'What we've found is that there are significant differences in the history of human males and females in different parts of the world', Professor Stoneking told the Guardian. 'Understanding why that's the case and what are the social historical processes that led to those differences are what we want to investigate now'.

The study was published in the journal Investigative Genetics.