Now a study by scientists at the University of Cambridge has questioned this conclusion, hypothesising instead that the DNA overlap is a remnant of a common ancestor of both Neanderthals and modern humans.

When the genetic sequence of Homo neanderthalensis was published in 2010, one of the headline findings was that most people outside Africa could trace up to 4% of their DNA to Neanderthals. This was widely interpreted as an indication of interbreeding between Neanderthals and early Homo sapiens just as the latter were leaving Africa. The two species would have lived in the same regions around modern-day Europe, until Neanderthals died out about 30,000 years ago.

But Andrea Manica said the analysis had over-estimated the amount of shared DNA between Neanderthals and humans that could be explained by interbreeding. The analysis had not taken into account the genetic variation already present between different populations of the ancestors of modern humans in Africa.

"The idea is that our African ancestors would not have been a homogeneous, well-mixed population but made of several populations in Africa with some level of differentiation, in the way right now you can tell a northern and southern European from their looks. The mixing is not complete within continents."

Taking these population differences, known as "substructuring", into account for early humans living in Africa, Manica and his colleague Anders Eriksson worked out that modern humans and Neanderthals must have shared a common ancestor some 500,000 years ago and that the subsequent evolution of this species was enough to account for the DNA crossover.