"The incompleteness of the fossil record makes it hard to test these ideas. But in Europe, the only place with a relatively complete archaeological record, fossils show that within a few thousand years of our arrival, Neanderthals vanished. Traces of Neanderthal DNA in some Eurasian people prove we didn't just replace them after they went extinct. We met, and we mated. Elsewhere, DNA tells of other encounters with archaic humans. East Asian, Polynesian and Australian groups have DNA from Denisovans. DNA from another species, possibly Homo erectus, occurs in many Asian people. African genomes show traces of DNA from yet another archaic species. The fact that we interbred with these other species proves that they disappeared only after encountering us. But why would our ancestors wipe out their relatives, causing a mass extinction – or, perhaps more accurately, a mass genocide?"