Australopithecus africanus, a hominid that lived about three million years ago, was believed to have eaten a diet much like modern chimpanzees. They forage mostly in wooded areas for fruits and plant material from trees and bushes.

Later hominids, on the other hand, looked for food in more open environments such as grasslands and ate the meat of animals they killed with stone tools.

But after studying the fossilised teeth of Australopithecus, Matt Sponheimer and Julia Lee-Thorp from the University of Cape Town now think its diet may have been much more diverse - small mammals that could be caught without tools may even have been on the menu.

Isotopic analysis

Analysing teeth from Australopithecus africanus revealed a diverse diet

The teeth were subjected to isotopic analysis. This technique relies on the knowledge that different types - isotopes - of a particular atom exist in the environment in a specific ratio to each other.

For example, grasses and sedges display a different isotopic ratio of carbon atoms to that found in the plants typical of woodland areas, like herbs. And since animals absorb some of the carbon they eat into their teeth, a study of the isotopic ratio in the enamel should say something about the environment in which the animals lived.

In the case of Australopithecus, the isotopic content of its teeth is consistent with a diet that included both the grasses and sedges found on open savannahs and the woodland plants the creature was supposed to have dined on exclusively.

Common assumtpions

A pattern of scratching and pitting on the creature's molars also hinted at meat being a likely source of the unexpected isotopic ratios.

The scientists say the common assumption that our ancestors in the genus Homo developed their large brains after they began eating the nutrient- and energy-rich animal foods necessary to fuel the larger brains may now need reappraisal.