About 40,000 years ago, our closest relatives, the Neanderthals, went extinct. But the mystery of how they died is a paleoanthropological cold case, with many clues but no definitive answers. Cannibalism, a rapidly changing climate, natural disasters, and disease have all been blamed. A study published Thursday in Scientific Reports introduces yet another culprit to the list: the structure of the Neanderthal brain itself.

What the paper argues, in other words, is that ancient humans likely had a leg up in terms of cognitive abilities compared to their Neanderthal relatives. It was a fundamental difference in brain morphology, argue the international team of scientists that wrote the paper, that allowed Homo sapiens to thrive and damned Neanderthals to extinction. While ancient humans and Neanderthals had brains of a similar size, previous research showed that human brains are more globular while Neanderthal brains were more elongated horizontally. In the new study, the scientists posit that the humans living alongside Neanderthals had brains with a larger cerebellum — which may have given them a social and cognitive advantage.

Brains, Neanderthal, humans
The top row are the reconstructed Neanderthal brains.

The evolutionarily ancient cerebellum, or “little brain,” makes up 10 percent of the human brain’s volume but contains about 50 percent of its neurons. This very important region of the brain has long been linked to physical activity, like standing and breathing, but recent studies have suggested it’s important in shaping conscious human behavior as well. “As the cerebella hemispheres are structured as a large array of uniform neural modules, a larger cerebellum may possess a larger capacity for cognitive information processing,” the scientists write. “Such a neuroanatomical difference in the cerebellum may have caused important differences in cognitive and social abilities between the two species and might have contributed to the replacement of Neanderthals by early Homo sapiens.”

The study authors came to this conclusion after using the CT scans of the skulls of four Neanderthals and four ancient humans to construct virtual 3D casts of the skulls. They then gathered MRI data from the brains of 1,185 study volunteers to create a model of the average human brain, which was then “deformed” to fit into the virtual skull casts. Because the genetic divergence between Neanderthals and anatomically modern humans took place relatively recently, the study authors argue that the present-day human brains are a reasonable stand-in for early human brains. Virtually squishing the human brain model into the virtual Neanderthal skulls, they found “that early Homo sapiens had relatively larger cerebellar hemispheres but a smaller occipital region in the cerebrum than Neanderthals long before the time that Neanderthals disappeared.”

And because cerebellar volume is linked to abilities like cognitive flexibility, language processing, and working memory capacity, the scientists argue larger cerebellar hemispheres may have helped humans survive and adapt to a dangerous world while Neanderthals could not.

That’s not to say that Neanderthals were just sacks of meat in comparison — we know now that they buried their dead and created art, cultural touchstones signifying symbolic thinking — but the differences in brains does hint that our direct ancestors may have advantageous cognitive abilities. Still, that difference in brains didn’t keep ancient humans from hooking up with them — which has allowed Neanderthals, in a small way, to live on.