Neanderthals lost their Y chromosome to modern humans

Interbreeding led to Neanderthals having their male sex chromosome replaced by that of modern humans, researchers have found.

Since the first Neanderthal DNA was sequenced in 1997, scientists have used genetics to gain insights into this extinct group of humans – our closest evolutionary relatives.

So far, the best-preserved Neanderthal specimens have tended to be female, so it hadn’t been possible to study the Neanderthals’ Y chromosomes, which are only possessed by males. These chromosomes pass from father to son, so they can be traced back to reveal useful information about the male lineage and history of the species.

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Now, an international team of researchers has developed a technique that’s able to ‘fish out’ Y chromosome molecules from the DNA that contaminates ancient bones and teeth.

The researchers were able to piece together the Y chromosome sequences of three male Neanderthals who lived around 38,000 to 53,000 years ago – close to the time when they became extinct. The team also reconstructed the Y chromosomes of two male Denisovans – another extinct group of humans who were relatives of the Neanderthals in Asia.

When the team compared the ancient Y chromosomes to those of modern humans, they were surprised to find that Neanderthal and modern human Y chromosomes are more similar to each other than they are to Denisovan Y chromosomes.

“We know … that Neanderthals and Denisovans were closely related and that humans living today are their more distant evolutionary cousins,” said lead author Martin Petr at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. “…we expected that their Y chromosomes would show a similar picture.”

So it seems that interbreeding between modern human men and Neanderthal women eventually led the modern Y chromosomes to replace the Neanderthal ones. This is evidence that DNA flowed from modern humans into Neanderthals, as well as vice versa.

The researchers speculate that the Y chromosome replacement may have been due to the relatively small size of Neanderthal populations, which may have caused the Y chromosomes to accumulate harmful mutations.

“Given the important role of the Y chromosome in reproduction and fertility, the lower evolutionary fitness of Neanderthal Y chromosomes might have caused natural selection to favour the Y chromosomes from early modern humans, eventually leading to their replacement,” said Petr.