Origin of our own species pushed back by half a million years

The history of our own species has just had a major shake-up.

Fossil and genetic data have long pointed to our lineage splitting from that of the Neanderthals and Denisovans around 500,000 years ago, before these two ancient human species themselves diverged a little later on.

But new analysis of two ancient human craniums discovered in China and then compared to other human fossils hints at a much older history for our species. It would mean that our origin goes back at least 400,000 years earlier than thought, and possibly even further.

One of our experts in human evolution Professor Chris Stringer has been involved in this new study published in Science.

“Our analysis suggests that all large-brained humans from the last 800,000 years or so can probably be put into one of five groups,” explains Chris. These are the groups of Asian Homo erectus, Homo heidelbergensis, Homo neanderthalensis, the Homo longi group which likely contains the Denisovans, and of our own species Homo sapiens.

“What’s revolutionary about our analysis is that it suggests all these five lineages trace their ancestry back more than a million years, which is much older than almost everyone has said, including me. And there are a couple of aspects that suggest that it could be an even more ancient divergence.”

An evolutionary thicket

The story of our own evolution is proving to have been incredibly messy and frequently revised.

Historically it has typically focused on a range of hominin species emerging in the grasslands and forests of the African continent over the past seven million years. Some of these animals, which would have shown a mixture of human and ape-like characteristics, stayed within Africa, whilst others migrated out into Asia and Europe.

As these populations moved into different environments and split apart, some evolved into separate species. On the conventional view, by around two million years ago, Homo erectus appeared in Africa and then soon after in Eurasia. However, it’s still unclear whether all the fossils dating to between 1 and 1.5 million years ago can be assigned to this species.

Regardless, by around 600,000 years ago another species of human, Homo heidelbergensis, was living in Africa and Europe, and presumably the regions in between. The reasoning suggests that by 400,000 years ago, this species had given rise to the Neanderthals in Europe, and by 300,000 years ago to our own species, Homo sapiens, in Africa.

Finally, it is thought that the Denisovans branched off from the Neanderthal lineage somewhere in the regions between Europe and Asia within the last half a million years.

This has been the very general picture that has emerged over the last 25 years or so. But as more fossils are found, analytical and dating techniques improved and with the addition of sampling ancient DNA, our understanding of this story has also changed.

Among the ancient human fossils that have been unearthed over this time are a few from China known as Yunxian 1, 2 and 3. These ancient human skulls were initially discovered in the banks of the Han River over a period of 30 years and are thought to date to about a million years old.

“These fossils have usually been called Homo erectus by researchers,” explains Chris. “But for a long time I haven’t thought that they were Homo erectus as I could see that the shape was not typical, although I’d never had a closer look at them.”

“Then Xijun Ni, my collaborator on the Dragon Man paper, managed to get hold of high-quality CT data of the Yunxian 1 and 2 skulls.”

This allowed Xijun and his colleagues to reconstruct the skulls, and working with Chris, to reveal something striking about the skulls and as a result our own origins.

Uncovering the identity of the Yunxian fossils

The fossil skulls from China were distorted during the fossilisation process, meaning that Chris’s colleagues needed to make some corrections.

To do this, they built a 3D model of the skulls, tentatively filling in the gaps in one with the surviving material from the other. What they arrived at was something which looked less like Homo erectus and more like those of a fossil known as Dragon Man.

The Dragon Man fossil, also unearthed in China, was described by Chris and his colleagues in 2021 and has been named as the species Homo longi.

“Our analysis suggests that the Yunxian skulls are actually an early member of the same group as Dragon Man,” says Chris. “And because Dragon Man increasingly now looks like it’s a Denisovan, there’s a lot of evidence pointing to the fact that the Yunxian fossils also belong to the Denisovan group.”

This rather convoluted series of events, combined with the age of these fossils, suggests that the Denisovan linage of humans had already split off from other humans by a million years ago. On the surface this might appear like a fairly minor discovery, but its ramifications could be huge.

This is because the new analysis of the craniums also puts the Denisovans as the most closely related extinct human species to our own lineage. Therefore, if the Denisovans split off over a million years ago, it means that ours did too and that the Homo sapiens lineage is equally as ancient.

When did our species diverge from Neanderthals?

Considering that it was previously thought that our lineage and of Neanderthals last shared a common ancestor around 500,000 years ago, this new finding would at least double that split.

“Some people probably won’t accept it, so we’ll have to see how that goes,” says Chris. “But it does have some implications.”

“It means that there must be some early members of the lineages of H. heidelbergensis, H. neanderthalensis and H. sapiens that we haven’t found yet, or which we have found but we haven’t recognised yet. There must be some million-year-old proto-sapiens, proto-neanderthals and proto-heidelbergensis out there.”

It also opens the door for the potential that our own lineage first emerged somewhere in Eurasia, before populations migrated into Africa where Homo sapiens then evolved. Chris, however, is quick to point out that this conclusion still needs checking against million-year-old human fossils unearthed on the African continent that were unavailable for the current study.