13 november 2012

3.5 million years ago our early ancestors ate tropical grasses

Society | Science

Australopithecus bahrelghazali teeth.
Australopithecus bahrelghazali teeth used in the analyses.

Researchers involved in a new study led by Oxford University have found that between three million and 3.5 million years ago, the diet of our very early ancestors in central Africa is likely to have consisted mainly of tropical grasses and sedges.

The findings are published in the early online edition of Proceedings of the National Academy of Sciences.

An international research team 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 from Oxford University along with 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.

Professor Lee-Thorp, a specialist in isotopic analyses of fossil tooth enamel, from the Research Laboratory for Archaeology and the History of Art, said: 'We found evidence suggesting that early hominins, in central Africa at least, ate a diet mainly comprised of tropical grasses and sedges. 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.' 

The research paper suggests this discovery demonstrates how early hominins experienced a shift in their diet relatively early, at least in Central Africa. 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. Today this is a dry, hyper-arid environment near the ancient Bahr el Ghazal channel which links the southern and northern Lake Chad sub-basins. However, in their paper the authors 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.

It was previously widely believed that early human ancestors acquired tougher tooth enamel, large grinding teeth and powerful muscles so they could eat foods like hard nuts and seeds. This research finding suggests that the diet of early hominins diverged from that of the standard great ape at a much earlier stage. The authors argue that it is unlikely that the hominins would have eaten the leaves of the tropical grasses as they would have been too abrasive and tough to break down and digest. Instead, they suggest that these early hominins may have relied on the roots, corms and bulbs at the base of the plant.

Professor Lee-Thorp said: 'Based on our carbon isotope data we can’t exclude the possibility that the hominins' diets may have included animals that in turn ate the tropical grasses. But as neither humans nor other primates have diets rich in animal food, and of course the hominins are not equipped as carnivores are with sharp teeth, we can assume that they ate the tropical grasses and the sedges directly.'