New York, Sep 16 (IANS): Our ancestors made the shift from a diet based on trees and shrubs to grass-based foods -- that include not only grasses and their roots, but also insects or animals that ate grass -- roughly 400,000 years earlier than experts previously thought, says new research.

The diet shift is one of an array of changes that took place during the Pliocene era -- 2.6 million to 5.3 million years ago -- when the fossil record indicates human ancestor species were starting to spend more time on the ground walking on two feet, said study lead author Naomi Levin from Johns Hopkins University in Baltimore, US.

"A refined sense for when the dietary changes took place among early humans, in relation to changes in our ability to be bipedal and terrestrial, will help us understand our evolutionary story," Levin said.

The shift toward a grass-based diet marked a significant step toward the diverse eating habits that became a key human characteristic, and would have made these early humans more mobile and adaptable to their environment.

The shift in eating habits would have broadened our ancestors' horizons and improved their species' capacity for survival, Levin said.

Researchers analysed 152 fossil teeth from an array of animals including pigs, antelopes, giraffes and human ancestors gathered from a roughly 100 square-mile area of what is now the Afar region of Ethiopia.

Among the samples were teeth from hominins -- including contemporary humans and our extinct ancestors -- believed to represent 16 different individuals.

The teeth were examined for carbon isotope distribution, a marker that can distinguish the types of foods the animals ate.

The data showed that both human ancestors and members of a now-extinct, large species of baboon were eating large amounts of grass-based foods as early as 3.76 million years ago -- roughly 400,000 years earlier than the date supported by previous research.

The study was published in the journal Proceedings of the National Academy of Sciences.