Paleogenetics is helping to solve the great mystery of prehistory: how did humans spread out over the earth? ... Before the Second World War, prehistory was seen as a series of invasions, with proto-Celts and Indo-Aryans swooping down on unsuspecting swaths of Europe and Asia like so many Vikings, while megalith builders wandered between continents in indecisive meanders. After the Second World War, this view was replaced by the processual school, which attributed cultural changes to internal adaptations. Ideas and technologies might travel, but people by and large stayed put. Today, however, migration is making a comeback. ... Much of this shift has to do with the introduction of powerful new techniques for studying ancient DNA. ... Whole-genome sequencing yields orders of magnitude more data than organelle-based testing, and allows geneticists to make detailed comparisons between individuals and populations. Those comparisons are now illuminating new branches of the human family tree. ... In five years, we’ve gone from thinking we shared no DNA with Neanderthals, to realising that there was widespread interbreeding, to pinpointing it (for one individual) within 200 years – almost the span of a family album. But the use of ancient DNA isn’t limited to our near-human relatives. It is also telling us about the dispersal of humans out of Africa, and the origin and spread of agriculture, and the peopling of the Americas. It is also helping archaeologists crack one of the great mysteries of prehistory: the origins of the Indo-Europeans.