It’s weird to think that tens of thousands of years ago, humans were mating with different species—but they were. That’s what DNA analyses tell us. When the Neanderthal genome was sequenced in 2010, it showed that as much as 1 to 4 percent of the DNA of non-Africans might have been inherited from Neanderthals. (Given that no African populations are known to have Neanderthal DNA, the matings must have occurred as modern humans moved into Europe and Asia). Scientists also announced last year that our ancestors had mated with another extinct species, and this week, more evidence is showing how widespread that interbreeding was.