A possible early human ancestor, Australopithecus sediba, didn't have the jaw and tooth structure necessary to exist on a steady diet of hard foods, suggesting that dietary changes were shaping the evolutionary paths of early humans, according to a study.
Australopithecus sediba, a diminutive pre-human species that lived about two million years ago in southern Africa, has been heralded as a possible ancestor or close relative of Homo, according to an international team of researchers who carried out the study.
"Australopithecus sediba is thought by some researchers to lie near the ancestry of Homo, the group to which our species belongs," said Justin Ledogar, a researcher at the University of New England in Australia.
"We find that A. sediba had an important limitation on its ability to bite powerfully; 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 added.
Australopiths appear in the fossil record about four million years ago, and although they have some human traits like the ability to walk upright on two legs, most of them lack other characteristically human features like a large brain, flat faces with small jaws and teeth, and advanced tool-use.
"Most Australopiths had amazing adaptations in their jaws, teeth and faces that allowed them to process foods that were difficult to chew or crack open. Among other things, they were able to efficiently bite down on foods with very high forces," said team leader David Strait, professor of anthropology in arts and sciences at Washington University in St. Louis.
Humans in the genus Homo have almost certainly descended from an Australopith ancestor, and A. sediba is a candidate to be either that ancestor or something similar to it.
The new study does not directly address whether Australopithecus sediba is indeed a close evolutionary relative of early Homo, but it does provide further evidence that dietary changes were shaping the evolutionary paths of early humans.
"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 said.
"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," he added.
"Some of these ultimately gave rise to Homo," Strait said. "Thus, a key to understanding the origin of our genus is to realise that ecological factors must have disrupted the feeding behaviours and diets of Australopiths. Diet is likely to have played a key role in the origin of Homo."
The study findings were published on February 8 in the journal Nature Communications.
