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Around 50,000 years ago, a new X chromosome appears to have been introduced into modern humans that had not long left Africa.

There was probably exceptionally strong selection for聽parts of this chromosome because, today, most people of聽less-recent African ancestry have inherited those regions.

at Aarhus University in Denmark, who led the research, thinks these聽regions may contain bits of聽so-called selfish DNA that promote their own spread by聽killing sperm that carry Y聽chromosomes. Such sperm lead聽to male offspring if they fertilise an egg. The selfish DNA may therefore result in the birth of聽more daughters. Exactly when this 鈥渒illing鈥� occurs is unclear.

All this would lead to a higher proportion of people inheriting the X chromosome that carries the聽selfish DNA, causing it to spread rapidly in a population, in聽an example of meiotic drive, a聽kind of gene drive.

鈥淔or some reason, there鈥檚 a聽reluctance to entertain the thought that meiotic drive is a thing in humans,鈥� says Terkelsen. Yet meiotic drives are being found in all the organisms we study closely, he says.

Terkelsen and his colleagues analysed X chromosomes from male humans in a database of genomes from around the world.

They found that people with long-standing African ancestry have an even diversity across their X聽chromosomes, meaning their DNA is a mix of genetic material from many different ancestors.

But in people without recent African ancestry, there are regions that are tens of thousands of DNA letters long with hardly any diversity. This suggests that each region derives from a single ancestor. Five of the regions were found in more than 75 per cent of people without recent African ancestry.

鈥淭he X chromosome is crazily enriched in these low-diversity regions,鈥� says Terkelsen.

These X chromosome regions are also present in the genome of聽the Ust鈥�-Ishim man, a modern human who lived in what is now Siberia around 45,000 years ago.

The team also found that the聽regions displaced bits of Neanderthal DNA that entered the聽population to which the Ust鈥欌�慖shim man belonged around 10,000 years before his birth. This聽means these regions spread 55,000 to 45,000 years ago.

Terkelsen thinks the source of the regions was an X chromosome that arose in an east Asian population. This group may have then bred with the Ust鈥�-Ishim man鈥檚 ancestors around this time.

The reason why the regions spread so rapidly could be to do with a lack of defence mechanisms on the Y chromosomes of the Ust鈥欌�慖shim people, says Terkelsen. Meiotic drives result in an evolutionary battle between the聽sexes or, more specifically, the聽sex chromosomes.

The cells that give rise to sperm divide by a process called meiosis, which should result in half the sperm carrying an X chromosome and half a Y chromosome. But genetic variants can skew this and聽result in a greater or smaller number of boys or girls being born. In other words, a meiotic drive comes about because X or Y聽chromosomes selfishly favour their own survival, even if those variants are detrimental to the population as whole.

X and Y chromosomes can evolve mechanisms to counter the聽effects of meiotic drives on rival chromosomes, resulting in a聽more even balance of sons and daughters being born over time, which may be the case now among聽modern humans. But when a chromosome with a strong聽meiotic drive enters a new聽population with no counter mechanisms, that chromosome can rapidly spread.

This is just speculation, however, as Terkelsen makes clear.聽鈥淭his paper does not show anything other than a striking, puzzling observation,鈥� he says.

鈥淸Meoitic] drive seems like quite聽a reasonable hypothesis,鈥� says at the Stowers Institute for Medical Research in Missouri. 鈥淒rive does lead to these type of sweep signatures.鈥�

Several mechanisms that can lead to meiotic drive in humans have already been identified, says Zanders, at least one of which is linked to male infertility.

Future studies should look at聽whether the X chromosome regions the team identified are linked with male infertility, says Terkelsen, which would provide more evidence that these regions harbour a meiotic drive.