An international research team led by Oxford University extracted information from the fossilised teeth of three Australopithecus bahrelghazali individuals - the first early hominins excavated at two sites in Chad.
Professor Julia Lee-Thorp with researchers from Chad, France and the US analysed the carbon isotope ratios in the teeth and found the signature of a diet rich in foods derived from C4 plants.
"We found evidence suggesting that early hominins, in central Africa at least, ate a diet mainly composed of tropical grasses and sedges, said Lee-Thorp, a specialist in isotopic analyses of fossil tooth enamel.
"No African great apes, including chimpanzees, eat this type of food despite the fact it grows in abundance in tropical and subtropical regions.
"The only notable exception is the savannah baboon which still forages for these types of plants today. We were surprised to discover that early hominins appear to have consumed more than even the baboons," Lee-Thorp said in a statement.
The discovery demonstrates how early hominins experienced a shift in their diet relatively early, at least in Central Africa.
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The finding is significant in signalling how early humans were able to survive in open landscapes with few trees, rather than sticking only to types of terrain containing many trees.
This allowed them to move out of the earliest ancestral forests or denser woodlands, and occupy and exploit new environments much farther afield, says the study.
The fossils of the three individuals, ranging between three million and 3.5 million years old, originate from two sites in the Djurab desert.
Researchers observe that at the time when Australopithecus bahrelghazali roamed, the area would have had reeds and sedges growing around a network of shallow lakes, with floodplains and wooded grasslands beyond.
Researchers suggest that these early hominins may have relied on the roots, corms and bulbs at the base of the plant.
The study was published in the Proceedings of the National Academy of Sciences.