It's now clear that our ancestors, upon leaving Africa, interbred with the Neanderthals and Denisovans, archaic humans that occupied Europe and Asia.
The first offspring of those pairings would have had equal amounts of their two ancestral legacies: one chromosome in each pair would be modern human, the other archaic.
People have been considering a number ideas.
These range from the simple—a large and growing modern human population simply swamped the archaic genes—to more complex situations such as limited reproductive compatibility between the two genomes.
But a new paper suggest something in between the two.
It's difficult to understand the fate of Neanderthal genes, since we don't have any sense of how well the offspring of these matings did.