Human ancestors' diet switched to grass 3.5 million years ago Human ancestors' taste for grass may have been a major step toward large brains and modern humans. Savanna grassland in Sudan. CC/Timm Guenther Four new studies of carbon isotopes in the fossilized tooth enamel of human ancestors and baboons in Africa shows how a dietary switch to tropical grasses and possibly animals led to the expansion and diversification of early hominins, or human-like species. Before 3.5 million years ago, hominins ate a forest-based diet of fruits and leaves similar to modern primates. Two species, Australopithecus afarensis and Kenyanthropus platyops moved into the widespread grassy savannas and grassy woodlands in East Africa. They began to eat grasses, maize, sorghum and sugar cane, sedges, and possibly grazing animals. Matt Sponheimer, a University of Colorado anthropologist and lead author of one study, notes that changes in diet have been linked to larger brain size and the advent of upright walking roughly 4 million years ago. "If diet has anything to do with the evolution of larger brain size and intelligence, then we are considering a diet that is very different than we were thinking about 15 years ago," said Thure Cerling, principal author of two of the studies. Several hominin species managed to co-exist with each other and the apes, and now it seems there was less competition for food as different species diversified their diets, and non-human primates stuck with fruit. The move into the grasslands "may signal a major and ecological and adaptive divergence from the last common ancestor we shared with African great apes, which occupy closed, wooded habitats," wrote University of South Florida geologist Jonathan Wynn, chief author one study. Spearheaded by the University of Utah, the studies are published together in the journal Proceedings of the National Academy of Sciences.