Starting about 200,000 years ago, humans began moving out of Africa. As the current theory goes, they never looked back. Those who reached Europe mated with Neanderthals, human relatives residing on the continent. Echoes of this tryst show up today in the genomes of those of European and Asian descent — those populations are a few percent Neanderthal by DNA. It also explains why Africans were thought to have little to no Neanderthal DNA, as their ancestors never met the lineage.