Human genetic map. Many human populations carry genetic markers which can be traced back through generations. This can produce a genetic tree and the diverse branches can be followed back to a common African root. On the map the blue arrows represent human migration routes based on evidence from Y-chromosome markers (passed down from father to son) and the brown arrows represent migration routes based on mitochondrial DNA markers (passed down from mother to child, both male and female). It appears that members of one branch of Homo sapiens left Africa around 100,000 years ago, and spread out across the continents.