Rewriting Human Evolution: Not One Lineage of Man, but Two?

A new study from a team of researchers at the University of Cambridge has upended centuries of theory as to the origins and evolution of… us. We are not who we thought we were, it seems.

Previously the longstanding scientific consensus was that Homo sapiens was one of a family of human and near-human species who evolved together in a series of branching parallels. However all the other human species died out in prehistory, leaving us as the sole survivor of a larger family.

Scientists were also pretty sure that we originally evolved in Africa, maybe 300,000 years ago, before spreading across the globe: up into Europe and Asia, then eastwards into south east Asia and Australasia before crossing the frozen wastes of the Bering Sea and, finally, colonizing America. But this part of the story appears to be wrong.

Or, at least, incomplete. The new study, published in Nature Genetics, totally changes the picture of this earliest point in our shared history. According to the study, we do not descend from a single lineage in Africa, but from at least two distinct populations which had been separated genetically for more than a million years.

A million years is a long time (nothing but the facts from All That History!) and unique species have evolved in much shorter time periods: ourselves for example, given we have only really been around as a recognizable species for a small fraction of that time. Does this mean we come from a co-mingling of two distinct species which evolved in isolation?

The answer is “kind of” mixed with a touch of “we already knew that” which at first seems unhelpful. We have existing evidence that we are a mix of species from known earlier interactions, interbreeding and genetic mingling with other species, cousins to humans. This mixing, most notably with the Neanderthals of Europe and the Denisovans of Asia, occurred some 50,000 years ago.

But this new study talks of another, much wider intermingling of genes way before any of this, some 300,000 years ago. Nor is it a mere smattering of genetic traits (neanderthal DNA tops out at around 2% of modern genetic makeup in any individual): this errant population which joined with ancient proto-humans contributed as much as a fifth of modern genes.

The team adopted a top-down approach to their study, using modern DNA analysis to infer ancient populations which otherwise have been completely lost without a trace. Data from a large and wide-ranging sample of human DNA was assessed using a computational algorithm known as “cobraa” and the results give us our clearest ever view of our most ancient history.

According to the results, the human population split millions of years ago, the two populations dividing and becoming geographically remote from each other. One of the populations then suffered some sort of catastrophe of unknown origin, which almost wiped them out.

This devastated population was a long time coming back from this, growing slowly over the next million years. It was these people, with their genetic bottleneck, who are the 80% ancestors of Homo sapiens, as well as the forefathers of the Neanderthals and Denisovans.

The other population may only have contributed a smaller portion of our modern genes, but their contributions are among the most important genetic traits we have and are essential to what makes us, conceptually, human. The genes related to neural functions and the processing of information, literally how we think, came from this smaller population.

So, who were these mysterious other humans, who gave us the power to think for ourselves? Well, we have a fair few candidates from around the time, proto humans who could be this unknown second population.

But they could just as easily have been lost forever, all evidence gone except this new reconstruction. All we can say for now is that evolution, and in this case human evolution, is one of interbreeding, mingling of genetic traits, and that we came from a melting pot, not some elevated lineage. We are not special, we are everyone.

Header Image: We are not one, but many. New research shows an 80/20 split in our genes from two distinct proto-human lineages, millions of years ago. Source: Sérgio Valle Duarte / CC BY 3.0.