An early human ancestor from South Africa called Australopithecus sediba didn’t have the teeth and jaws to survive on a steady diet of hard foods like nuts and seeds. According to tests on computer models of a 2-million-year-old skull described in Nature Communications this week, A. sediba couldn’t generate the high bite forces needed to be a nutcracker like other members of its genus.
Discovered in a cave 40 kilometers (25 miles) west of Johannesburg back in 2008, A. sediba is thought to be a close relative of our genus, Homo. Unlike other australopiths – who could process foods that were difficult to crack open – this species had small molars and premolars, and it also lacked large muscle markings and other features that would have increased muscle leverage and facial buttressing. Yet, a 2012 analysis of microscopic wear on the teeth surfaces of two individuals suggested that the A. sediba diet consisted of hard foods mixed with fruits, leaves, and bark. The dietary evidence seems to go against interpretations of their functional anatomy.
University of New England’s Justin Ledogar and colleagues wanted to see if the facial skeleton of A. sediba is well configured (or even adapted) to consume hard foods. The team created a 3D virtual reconstruction of the cranium of a key A. sediba specimen known as MH1, and then they subjected the computer model to a series of biomechanical tests to study the chewing mechanics.
The team discovered that A. sediba didn’t have the jaw and tooth structure needed to exist on hard foods. So while they might have eaten nuts and seeds, this limitation on their ability to bite powerfully meant that they likely weren't adapted to consume such hard foods. "If it had bitten as hard as possible on its molar teeth using the full force of its chewing muscles, it would have dislocated its jaw," Ledogar says in a statement. The foods important for their survival must have been eaten relatively easily without high forces.
These findings could help us better understand the environmental changes that played a role in the origin of our own genus. "Humans also have this limitation on biting forcefully and we suspect that early Homo had it as well, yet the other australopiths that we have examined are not nearly as limited in this regard," Ledogar adds. "This means that whereas some australopith populations were evolving adaptations to maximize their ability to bite powerfully, others (including A. sediba) were evolving in the opposite direction."