A freshly unearthed 20-million year old skull may have belonged to a common ancestor of humans and the other great apes.

A team led by Martin Pickford, a paleoanthropologist at the College de France in Paris, and Bridgette Senut, at the National Museum of Natural History in Paris, discovered the fossils last month, while excavating the semi-arid region surrounding the Napak Volcano in northeastern Uganda.

The team attributed the partially complete skull to the species Ugandapithecus major, a hulking Miocene era ape known mostly by its other body parts. “It was a pretty big animal, almost as large as a gorilla, about the size of a chimpanzee,” Pickford says.

His team plans to analyze the fossil more closely in France and describe it in a Ugandan scientific journal. But he says its small brain already stands out. “You’re talking about an animal that has the muzzle of a gorilla with the brain size that would go with a baboon,” Pickford says.

Its small brain and other facial features such as its teeth and palette could suggest that modern chimpanzees and gorillas have evolved substantially from their ape ancestors, Pickford says. The skull shares a number of features with modern-day orangutans, suggesting that the evolutionary lineage that gave rise to modern orangutans changed less.

But the evolutionary origins of great apes – humans, chimpanzees, bonobos, gorillas and orangutans – are poorly understood, Pickford says. “Finding the skull of Ugandapithecus is really going to focus the debate on that particular linage, but we must not forget there were quite a few other species running around at the same time.”

Closer examination of the skull should firm up Ugandapithecus’ position. “My gut feeling at the moment is that it’s not far from the ancestor of modern African apes and orangutans,” Pickford says. “I’ve been waiting for about 30 years for this kind of discovery.”

Image courtesy of Martin Pickford, Uganda Museum, Kampala and the Uganda Palaeontology Expedition