The argument over when our lineage split from chimps is about to be settled, with colossal consequences for prehistory

(Image: Andreas Preis)

LINE them up in your head. Generation after generation of your ancestors, reaching back in time through civilisations, ice ages, an epic migration out of Africa, to the very origin of our species. And on the other side, take a chimp and line up its ancestors. How far back do you have to go, how many generations have to pass, before the two lines meet?

This is one of the biggest and hardest questions in human evolution. We know that at some point we shared a common ancestor with chimps, but exactly when – and what that ancestor was like – have been maddeningly hard to pin down. Palaeontologists have searched for fossil remains, and geneticists have rummaged through the historical documents that are human and chimp DNA. Both made discoveries, but they did not see eye to eye.

No more. New estimates for when our lineage and chimps went their separate ways suggest that some of our established ideas are staggeringly wrong. If correct, they demand a rewrite of human prehistory, starting from the very beginning.

When was that beginning? The obvious first place to look for answers is in the fossil record. But fossil humans – or more strictly hominins, the group that includes us and all our extinct relatives from after the split – are notoriously thin on the ground and difficult to interpret.

Geneticists have more to work with. DNA contains telltale traces of events in a species’ past, including information about common ancestry and speciation. In theory, calculating the timing of a speciation event should be straightforward. As two species diverge from a common ancestor their …