A pre-human relative species ate only forest foods even though they lived near grassy savanna with tasty tubers and even juicy animals, U.S. researchers say.

Australopithecus sediba, an apelike creature with human features living in southern Africa 2 million years ago, exclusively consumed fruits, leaves and other forest-based foods despite the easy availability of savannah alternatives, researchers from Johns Hopkins University reported Wednesday.

"This astonished us," said Benjamin Passey, a Johns Hopkins University geochemist on the international team that conducted the study.

"Most hominin species appear to have been pretty good at eating what was around them and available, but sediba seems to have been unusual in that, like present-day chimpanzees, it ignored available savanna foods."

The findings provide clues about our various pre-human relatives, and why some thrived and continued to evolve while others became extinct, researches said.

"We know that if you are a hominin, in order to get to the rest of the world, at some point you must leave the forests, and our ancestors apparently did so," Passey said.

"The fates of those that did not leave are well-known: They are extinct or, like the chimpanzee and gorilla today, are in enormous peril.

"So the closing chapter in the story of hominin evolution is the story of these 'dids' and 'did nots.'"

"[O]ne thing people probably don't realize is that humans are basically grass eaters," Passey said. "We eat grass in the form of the grains that we use to make breads, noodles, cereals and beers, and we eat animals that eat grass.

"Eating grasses is a hallmark of humanity, and we are simply trying to find out where in the human chain that begins."