Some of our ancestors were not a pretty sight, their ugly mugs featuring tall jaws and bony skull crests. Now a researcher argues in tomorrow's Science that the three species of robust australopithecines that looked this way were not a closely related hominid family, but grew to resemble each other thanks to the shapes of their unusual teeth.
Researchers have identified 50 or more skull features shared by all the robust australopithecines that seem to indicate a close evolutionary relationship. To analyze the way australopithecine faces grew, anatomist Melanie McCollum of Case Western Reserve University in Cleveland, Ohio, studied how the differently shaped skulls and faces of living hominoids--humans, chimpanzees, gorillas, and orangutans--grow during postnatal development.
The comparison shows that teeth drive the shape of much of the rest of the face. For example, the australopithecines' massive molars require a tall back jaw, along with big jaw muscles and the skull-crowning crests that serve to anchor them. And their small front teeth change the configuration of the floor of the nose. In order to balance the competing demands of the growing mouth and nose, including the tall back jaw, the palate--the boundary between all these areas--thickens, forming a massive bone in the center of the face. The rest of the face then has to adjust to this bone, with the net result being a face so tall that it almost rises above the brain.
The analysis "shows that if you have similarities in dental pattern, then you're going to get similarities in facial features," says McCollum. Even if the robust australopithecine species evolved separately on opposite sides of Africa, "as long as they have big molars and small front teeth, their faces will look alike," she says.
Some scientists welcome the work's larger implication: that any traits used in organizing a genetic family tree should be scrutinized from a developmental perspective. "That adds a dimension that's not usually thought about," says developmental biologist Rudy Raff of Indiana University, Bloomington.