The research, by University of Utah geochemist Thure Cerling and an international team of scientists, refutes the previously held belief that those early humans shared the diets of forest-dwelling primates.
The studies analyzed the tooth enamel of ancestors of humans and great apes to show early man gained a taste for grasses and sedges, grass-like plants with edged stems.
"It was like the opening of a new restaurant and they didn't have to eat the same old stuff," Cerling told Reuters on Tuesday.
No longer dependent on forests for their supply of food, the change in diet helped pave the way for early man to explore new habitats, Cerling said.
The question of whether those ancestors were pure herbivores or carnivores remains unanswered.
"That is a mystery still to be unraveled," Cerling said.
Earlier studies indicate that early man did not scavenge for meat until 2.5 million years ago and did not begin hunting for game until about 500,000 years ago.
(Editing by Edith Honan and Peter Cooney)