A short, gangly relative of man that lived in South Africa two million years ago ate bark and wood, the only early hominid known to have done so, a report said Wednesday.

While its ancient relations opted for softer grasses and shrubs, Australopithecus sediba, an upright-walking tree climber, "included quite a large amount of hard food in its diet," anthropologist Amanda Henry told AFP.

Henry, from the Max Planck Institute for Evolutionary Anthropology in Germany led an international team that examined fossilized plaque, known as calculus, on the teeth of A. sediba to determine its diet.

"Just like a dentist does, I used a dental pick to remove this calculus from the surface of the tooth and then, after some processing, I looked under a microscope for the phytoliths," she explained.

The phytoliths, tiny silica remains of plants, were then compared to those of modern plants to identify what A. sediba ate, said the study published in Nature.

"This is the first time anyone has looked for phytoliths in the dental calculus" to determine the diet of a species of hominin -- a sub-grouping of hominids most closely related to man, added Henry.

Remains of four A. sediba skeletons have been discovered in South Africa's Malapa cave, north of Johannesburg, since 2008. The individuals are believed to have fallen into a pit in the cave and died.

It is not certain whether the species, which had long arms, a small brain and a thumb possibly used for precision gripping, was a direct ancestor of our genus, Homo, or simply a close relative.

Henry said its strange diet was just one of A. sediba's many oddities.

It walked upright on two legs, but its foot structure suggests it probably also spent a lot of time climbing trees.

"So there seems to be a mix of very modern and somewhat primitive features in the skeleton of this species."

Scientists had expected A. sediba's diet to more closely resemble that of other species in the genus Australopithecus, said Henry.

"However, what we see is that A. sediba is really not like any other hominin."

All 81 other hominid species tested, even "Nutcracker Man" so named for its massive jaws and teeth, ate softer plants like grasses and shrubs.

But A. sediba appared to have avoided the grasses growing in abundance in the open grasslands in the region at the time.

Bark and woody tissues, which contain protein and soluble sugars, had not previously been documented as a dietary component of other African hominids, but are consumed by some modern-day primates.

"The diet of A. sediba may have been similar to that of today's African savanna chimpanzees," said study co-author Paul Sandberg of the University of Colorado's anthropology department.

Henry said it did not appear that A. sediba had teeth specially adapted for eating harder plants, adding that bark "can actually be pretty nutritious".

"All the nutrients of the tree run through the inner bark. For example, maple syrup is just the concentrated version of the sap that runs through the inner bark of maples."

A species' diet is an important indicator for anthropologists of its behaviour, habitat and evolution.

"What fascinates me is that these individuals are oddballs," added co-author Matt Sponheimer of the University of Colorado.

"I had pretty much convinced myself that after four million years ago most of our hominid kin had diets that were different from living apes, but now I am not so sure."