Our ancestors shifted from a diet based on trees and shrubs to grass-based foods about 3.8 million years ago, 400,000 years earlier than previously thought, an analysis of fossil teeth has found.
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, the researchers said.
Understanding the timing of these events can help show how one change related to another.
“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,” said lead author Naomi E Levin, an assistant professor at the Johns Hopkins University in US.
The study reports on an analysis of fossil teeth found in Ethiopia that shows the shift from a diet based on trees and shrubs to one that included grass-based foods took place about 3.8 million years ago – roughly 400,000 years earlier than the date supported by previous research.
Grass-based foods could include not only grasses and their roots, but also insects or animals that ate grass.
The shift in eating habits would have broadened our ancestors’ horizons and improved their species’ capacity for survival, Levin said.
“The results not only show an earlier start to grass-based food consumption among hominins and baboons but also indicate that form does not always precede function,” said co-author Yohannes Haile-Selassie of the Cleveland Museum of Natural History.
“In the earliest baboons, dietary shift toward grass occurred before its teeth were specialised for grazing,” she said.
Researchers analysed 152 fossil teeth from an array of animals including pigs, antelopes, giraffes and human ancestors gathered from a roughly 259 square-km 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, said Levin.
The teeth were examined for carbon isotope distribution, 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.
Previous research dated the earliest evidence for grass-based foods in early human diets to about 3.4 million years ago.
The researchers attribute the dietary expansion to changes in relations among members of the African primate communities, such as the appearance of new species of primates.
The study was published in the journal PNAS.