By now, it’s firmly established that modern humans and their Neanderthal relatives met and mated as our ancestors expanded out of Africa, resulting in a substantial amount of Neanderthal DNA scattered throughout our genome. Less widely recognized is that some of the Neanderthal genomes we’ve seen have pieces of modern human DNA as well.

A new study in the journal Science reveals that interbreeding between Neanderthals and humans was strongly sex-biased. This social dynamic explains why Neanderthal DNA is missing from our X chromosome.