Before the major change hominids ate what can only be considered the real paleo diet: shrubs, leaves, herbs and fruits.
That changed as hominids moved into Africa's open savannahs and began eating grass and possibly animals.
This evidence leads to new theories about evolution – which is partly explained through diet — and the way in which our very early ancestors interacted with the environment.
"Because feeding is the most important factor determining an organism's physiology, behavior and its interaction with the environment, these finds will give us new insight into the evolutionary mechanisms that shaped our evolution," the study says.
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"We don't know exactly what happened," study author Matt Sponheimer told BBC News.
"But we do know that after about 3.5 million years ago, some of these hominids started to eat things that they did not eat before, and it is quite possible that these changes in diet were an important step in becoming human."
The study used the fossilized enamel of 11 species of hominids and other primates.
Researchers analyzed different isotopes (carbon atoms) on the teeth to determine what they ate.
"What we have is chemical information on what our ancestors ate, which in simpler terms is like a piece of food item stuck between their teeth and preserved for millions of years," said study co-author Zeresenay Alemseged, of the California Academy of Sciences.
The findings appeared in four papers in the journal Proceedings of the National Academy of Sciences.