Evidence for multiple matings already exists, in the form of a 40,000 year old human fossil from Romania with a recent Neanderthal ancestor, whose Neanderthal DNA nonetheless did not become part of modern-day human genomes.

“If it happened there,” says Schraiber, “why wouldn’t it be happening in other places?”

To find out which scenario – dilution or multiple matings – was more likely, he and his colleague Fernando Villanea first looked at European and Asian populations separately. They analysed the distribution of Neanderthal chunks across genomes in the 1000 Genomes Project, a large public database of human genetic variation.

In both Europe and Asia, the patterns of inheritance pointed to multiple periods of mating, rather than just one.