CAPE TOWN - Australopithecus sediba, the 2-million-year-old South African human ancestor, probably survived on a diet that included bark, leaves, and fruit, according to analysis of its fossilised teeth.

The results are surprising, as they suggest eating habits quite different to most early African hominins studied so far, which typically ate tropical grasses, sedges and the animals that ate these plants.

The food Au. sediba put in its mouth was similar to that eaten by modern-day chimpanzees, and suggests they lived in forests - rather than the open savannah favoured by many other hominid species, said Amanda Henry, lead author of a letter in this week's edition of the journal Nature, describing an analysis of the fossilised Au. sediba's teeth.

This argument is consistent with previous work on the fossils, which suggested that they were tree climbers. "Diet can tell us a lot about how they behaved and lived. We know that among primates, the ones living in closed forest environments tend to live in smaller groups than those living in grasslands. We could hypothesise that perhaps sediba lived in small groups . and travelled long distances to find food," she said.

Au. sediba was discovered by Wits professor Lee Berger in 2008 at a site called Malapa, near Johannesburg. He has controversially argued that it evolved from Au. africanus (which some scientists think was an evolutionary dead-end) and is a direct predecessor to our early human ancestor Homo erectus, which lived between 1,89-million and 70000 years ago, and is the oldest known early human to have the body proportions of modern humans.

"I found the evidence for bark consumption the most surprising," said Prof Berger. "While primatologists have known for years that primates, including apes, eat bark as a fallback food in times of need, I really had not thought of it as a dietary item on the menu of an early human ancestor."

Dr Henry and her colleagues used three different techniques to analyse Au. sediba's well-preserved teeth and hone in on its likely diet. They examined the ratio of two carbon isotopes, carbon-12 and carbon-13, in the tooth enamel, studied the wear patterns on the tooth surfaces, and scrutinised tiny plant fragments called phytoliths embedded in preserved tartar on the teeth.

Plants take up different ratios of carbon-12 and carbon-13 during photosynthesis depending on their access to water; plants that have ready access to water in forests take up more carbon-12 than plants growing in drier savannah areas, and the analysis of the teeth indicated that the hominins ate plant material from forests. The microscopic scratches and pits on the teeth-wear patterns suggested Au. sediba ate roots, nuts and seeds.

Phytoliths are minute bits of silica that take on the shape of the epidermal cells of the plants they come from. Since plants have evolved more slowly than our ancestors, the scientists could compare the shape of the 2-million-year-old phytoliths with those in modern plant libraries to identify the broad groups of plants eaten by Au. sediba, said co-author Prof Marion Bamford.