It's not known exactly when humans and Neanderthals split off from their last common ancestor, but the estimated window is very wide, between 300,000 and 800,000 years ago. Now a new study has found evidence that this split took place towards the earlier end of that range, and may have taken place even longer ago still.

The research, conducted by anthropologist Dr. Aida Gomez-Robles at the University College London, focused on the teeth of hominins found in a cave called Sima de los Huesos in Spain. At this site, the remains of 28 people have been discovered and dated to about 430,000 years ago. Previous studies have identified the bones as belonging to Homo heidelbergensis, an ancestral species of the Neanderthals.

Some studies have suggested that H. heidelbergensis is the last common ancestor of the human and Neanderthal lines, which then diverged roughly 400,000 to 600,000 years ago. But others say there's no evidence that humans are descended from H. heidelbergensis at all, and instead the species sits on the Neanderthal branch of the family tree, after humans split off.

The new study seems to agree with that latter idea. Both DNA analyses and features of the teeth suggest the Spanish hominins are closely related to Neanderthals.

"Sima de los Huesos hominins are characterized by very small posterior teeth (premolars and molars) that show multiple similarities with classic Neanderthals," says Gomez-Robles. "It is likely that the small and Neanderthal-looking teeth of these hominins evolved from the larger and more primitive teeth present in the last common ancestor of Neanderthals and modern humans."

But this discovery has larger implications for human and Neanderthal evolution. Generally, for these teeth to be this advanced down the Neanderthal track, evolution must have either worked extremely fast, or had more time than is currently believed. That pushes back the timeline of when humans and Neanderthals split off from the last common ancestor.

The evolution of dental shapes seems to occur at a pretty steady rate across the board. For this study Gomez-Robles used quantitative data to measure how long it might take for teeth to look like those in the Sima de los Huesos specimens, given different projected starting points. She concluded that the last common ancestor must have lived more than 800,000 years ago.

"Any divergence time between Neanderthals and modern humans younger than 800,000 years ago would have entailed an unexpectedly fast dental evolution in the early Neanderthals from Sima de los Huesos," says Gomez-Robles.

Of course, Gomez-Robles acknowledges that the discrepancy could be explained through other factors. These Neanderthals were isolated from other populations in mainland Europe, and under the right circumstances evolution has been known to work overtime. But the simplest explanation seems to be that humans and Neanderthals split off earlier, which is backed up by other studies.

That said, the study of human evolution is murky, and the story is constantly changing as new research comes to light. We might never know all the details, but scientists won't stop looking.

The research was published in the journal Science Advances.