News From The Stone Age
Neanderthals, one of the last extant hominid species other than our own, left Africa somewhere between 400,000 and 800,000 years ago and settled mostly in Europe until they went extinct 30,000 years ago. Early modern humans left Africa about 80,000 to 50,000 years ago, meaning they overlapped with Neanderthals in time and place for at least 20,000 years.
... answer came with the first sequencing of the Neanderthal genome last year. Dr. Labuda compared 6,000 chromosomes from all over the world to the corresponding part of the Neanderthal sequence. With the exception of people from sub-Saharan Africa - whose ancestors would have been unlikely to come into contact with Neanderthals, since their territories didn't overlap - every chromosome featured evidence of the Neanderthal sequence.
That even includes particularly far-flung groups of humans like native Australians, who are thought to have reached the island continent by as far back as 40,000 years ago. For that sequence to show up even in such geographically isolated groups, it suggests that there was a lot of interbreeding between the two hominid species, and that pretty much all ancient humans that left Africa passed through Neanderthal territory and had close interaction (read: a ton of sex) with their evolutionary cousins.