In the world of genetic anthropology, mitochondrial DNA and the Y-chromosome are the major players. They are regions of our genome scientists use most frequently when tracing both ancient and historical human migrations, and are an important tool for genealogists using DNA to piece together their family trees.
But another part of the human genome has recently started to prove itself as a window into our species’ past: the X-chromosome. Like both the mtDNA and the Y-chromosome, the X is passed from parents to children in such a way that scientists can use it to to trace the deep ancestry of our species. But it is only recently that the X-chromosome has been used this way, and some of the early research has been rather inconsistent.
Now, the authors of a paper published in this week’s Nature Genetics believe they have perfected a way of using the X-chromosome to unravel details about the initial migration of humans out of Africa tens of thousands of years ago. Their results suggest that more men than women were involved in the exodus that initiated the peopling of the entire globe.
One of the reasons analysis of the X-chromosome has not proven straightforward compared to the mtDNA or the Y-chromosome is that the way the X is passed down from one generation to the next differs depending on the sex of the child. Fathers pass on their X-chromosomes to their daughters, but not their sons, while mothers pass one X-chromosome to their children of both sexes. So while any person’s X-chromosome came down to them along a specific lineage – just like their Y-chromosome and mtDNA – that path is impossible to trace.
But the X-chromosome does have one distinctive quality. Since men have one and women have two, any population with a 50/50 sex ratio will have three X chromosomes for every four of the 22 paired chromosomes. And that means that all other things being equal, genetic diversity on the X-chromosome should be about three-quarters as much as it is on the non-sex chromosomes.
To see if all has been equal in the history of the human species, researchers from Harvard, the Broad Institute and the National Human Genome Research Institute analyzed the genotypes of people from around the globe and looked for differences among the populations of Africa, Europe and East Asia. Though they found the expected 75% X-chromosome diversity ratio within Africans, there was considerably less diversity among European and East Asians on the X compared to their 22 non-sex chromosomes.
What could all this mean? First of all, a decrease in genetic variation is usually a sign of decreased population size. Because it shows up in East Asians and Europeans but not Africans, the authors believe the decreased variation is a signal of a population bottleneck that occurred after humans left Africa for the first time nearly 60,000 years ago – but before the non-African populations diverged from each other.
But the decreased genetic variation only seems to show up on the X-chromosome. This, the authors speculate, may be due to some sort of sex-biased migration. In other words, when humans first ventured outside of Africa and into Europe and Asia, there may have been more men on the move than women. Scientists have long observed the same phenomenon when comparing the Y-chromosome to mtDNA; now researchers are seeing the same phenomenon here, with the X.