About 3 million years ago the diet of our very early ancestors in central Africa is likely to have consisted mainly of tropical grasses, researchers say.

A new study led by Oxford University of the fossilized teeth of three early hominim Australopithecus bahrelghazali individuals analyzed carbon isotopes in the teeth and found the signature of a diet rich in foods derived from tropical grasses and sedges, flowering plants resembling rushes, a university release reported.

The discovery suggests early hominins experienced a shift in their diet relatively early, at least in Central Africa, and could explain how early humans were able to survive in open landscapes with few trees, rather than sticking only to types of terrain containing many trees.

Previously it had been thought early human ancestors acquired tougher tooth enamel, large grinding teeth and powerful muscles so they could eat foods such as hard nuts and seeds.

This research finding suggests the diet of early hominins diverged from that of the standard great ape at a much earlier stage.

"We found evidence suggesting that early hominins, in central Africa at least, ate a diet mainly composed of tropical grasses and sedges," Oxford researcher Julia Lee-Thorp said. "No African great apes, including chimpanzees, eat this type of food despite the fact it grows in abundance in tropical and subtropical regions.

This dietary shift allowed early humans to move out of the first ancestral forests or denser woodlands and occupy and exploit new environments much farther afield, the researchers said.