Meeting a New Ancestor

Mind the gap: Churchill holds a cast model of a prehistoric skull found in South Africa.

Chris Hildreth

Three years ago, during excavations in the region of South Africa known as the "Cradle of Humankind," paleoanthropologist Lee Berger uncovered two hominin skeletons that, when the digging was done, were each in the neighborhood of 40 percent complete. The rare find demanded immediate attention—and Steven Churchill was ready to offer it.

Churchill, an associate professor of evolutionary anthropology, was part of an international team of scientists recruited to study the specimens. Found together in a cave, the two skeletons—a juvenile male and an adult female—are estimated to be more than 1.9 million years old and belong to an ancient species known as Australopithecus sediba. The age of the remains—falling around the time scientists believe the genus Homo emerged— and their combination of primitive and modern features are strong indications that they represent a direct human ancestor, Churchill says.

"The interesting thing about [the juvenile] is that in a lot of respects, he's like other Australopiths," Churchill explains. "He has a very, very small brain. It's about the size of a chimpanzee's brain. But it is already showing some of the asymmetries that you see in a modern human brain. We don't see those asymmetries in a chimpanzee brain, and we don't appear to see them in other species of Australopithecus."

In addition, Churchill says, the juvenile "has relatively long arms. He has features in the arms that make it look like he was good at climbing. But he's got some features in the hip bone that look like he is becoming a better biped, more like you would see in modern humans."

The researchers also noted features of the specimens' teeth and hands that indicated they may have fashioned and used stone tools to process food—like other contemporaneous species. Although more research is needed, those characteristics offer promise that Australopithecus sediba may prove to be a long-sought direct ancestor of modern humans. "Sediba may very well be the Rosetta Stone that unlocks our understanding of the genus Homo," Berger says.