At some point during the Pleistocene epoch, 2.6 million to 11,700 years ago, the common ancestor between humans and Neanderthals walked the Earth. Although we have no decent, fossilized remains of this creature, researchers from the University of Cambridge have circumvented this problem by using novel algorithms to simulate what its skull would have looked like. This incredible 3D rendering has been showcased in the Journal of Human Evolution.

The fossil record of any evolutionary lineage is far from complete. This is partly because we haven’t found every single fossil yet, and partly because not all creatures’ remains are preserved. Consequently, certainly evolutionary “gaps” exist, including within our own species’ evolutionary tree.

We know that Homo sapiens and Neanderthals both evolved from Homo heidelbergensis, but from two different populations of this ancestral species. The African group of H. heidelbergensis evolved into H. sapiens 130,000 years ago, who then rapidly migrated into Europe and Asia afterwards. The pre-existing Eurasian H. heidelbergensis group evolved into Neanderthals (Homo neanderthalensis), who eventually became extinct.

However, the fossil evidence for H. heidelbergensis is quite poor, so we don’t have a good idea of what they may have looked like. This team of researchers decided to create a virtual fossil of the skull of the last common ancestor between humans and Neanderthals using a novel technique called “digital morphometrics.”

A wide range of skulls spanning the evolutionary history of our shared Homo genus were digitally scanned, including H. neanderthalensis skulls, modern human skulls, and a 1.6 million-year-old Homo erectus fossil – the species preceding the emergence of H. sapiens, H. heidelbergensis, and H. neanderthalensis. Distinct features of the skulls were highlighted.

 

 

^{The video above shows the evolutionary lineage of the two species. University of Cambridge}

These 797 “landmarks” provided the researchers with an evolutionary framework – key physical traits whose emergence could be tracked through time. They were then fed into a computer program, where the virtual skulls of both branches of the Homo tree were analyzed, and compared with the limited fossil data belonging to ancestral family members back in the Pleistocene period.

“This allowed us to… recreate virtually skull fossils of the last common ancestor of modern humans and Neanderthals, using a simple… 'tree of life' for the genus Homo,” said the study's lead author Dr. Aurélien Mounier, a researcher at Cambridge University's Leverhulme Centre for Human Evolutionary Studies (LCHES), in a statement.

In terms of the skull of our last common ancestor, the program produced three possibilities. By comparing these skulls to the limited original fossils and bone fragments from the Pleistocene, the researchers chose the one skull they felt had the best fit.

Containing hallmarks of both species, in particular the bulge at the back of the skull (the “occipital bun”), it also features some unexpected differences. For example, the virtual skull shows the strong indention underneath the cheekbones that modern humans have. Neanderthals did not share this trait; they had a thicker bone around this area, meaning that their face would have been more outwardly protruding.

^{Image credit: The 3D render of our last common ancestor's skull. Dr. Aurélien Mounier}

Overall, the fossils look more like Neanderthals than modern humans, but the researchers say this isn’t surprising. Looking at our own species’ evolutionary history, it is clear to see that we deviated far more from our common ancestors over a relatively short period of time compared to our Neanderthal cousins.

Significantly, this technique has given another estimate for the date at which the two species emerged. H. sapiens and H. neanderthalensis first split 700,000 years ago – 300,000 years earlier than previously thought – and this last common ancestor emerged in the African group of H. heidelbergensis.