The practice of monogamy — the most popular mating practice on the planet today — is nothing new. In fact, anthropologists have found evidence of monogamous relationships in Homo erectus, a human ancestor that lived nearly 2 million years ago.
But the alternative to monogamy — polygamy — though not nearly as popular in today’s world, has long been a part of our species’ history as well. In this week’s issue of PLoS Genetics, human geneticist Michael Hammer and his team have found evidence that polygamy – and specifically polygyny, the practice of one man taking multiple wives – was widespread enough in the ancient past to leave its mark in the human genome.
The X chromosome is a very important bit of DNA. Having one versus two copies of the X chromosome determines whether you are a boy or a girl, and many famous diseases can be traced to the inheritance patterns on X. Scientists have often assumed that because the X gets passed down at a slightly lower rate than the 22 paired chromosomes (women inherit two X chromosomes, but men get only one) it would be proportionately less diverse.
Upon closer examination however, the opposite appears to be the case. As part of the study, researchers examined the DNA of isolated populations around the world, including the Biaka Pygmies of Central Africa and French Basque from the Pyrenees Mountains. What they found was that there is in fact more genetic diversity worldwide of the X chromosome compared to the biparentally inherited parts of the genome. Not only that, the authors found that this genetic diversity is heavily skewed towards females.
Hammer and his team propose that this high amount of X chromosome diversity can be directly related to polygyny. Their reasoning:
In polygynous societies, because single men have children by multiple wives, the number of males contributing to the genome is lower than it would be if each woman chose a different mate.
The effect of that decrease in genetic diversity should be roughly equal for the 22 biparentally inherited chromosomes, because half of the DNA they contain comes from the paternal side.
But that’s not true of the X, because although girls get one X from each parent, boys get only one – and it comes from mom. So all other things being equal, the X chromosome is a primarily “female” chromosome. Any decrease in genetic diversity due to the under-representation of men in the breeding population will have less of an effect on it compared to the rest of the chromosomes.
This is exactly what Hammer and his colleagues have found – increased genetic diversity on the X chromosome compared to the rest of the genome. That pattern, they argue, is a smoking gun that proves the existence of polygyny in the human past.