A new report published Monday revealed that early humans' diet started diverging from apes' at about 3.5 million years ago, about 1 million years earlier than previously thought.

Scientists were able deducted the new number studying tooth enamel from fossil of early hominids. The study revealed that about 3.5 million years ago, our ancestors started eating grasses and other low-lying plants, and sometimes even meat - unlike apes of the day.

"The development was an important step in becoming human," said Matt Sponheimer, professor of anthropology at Colorado University-Boulder and author of one of four studies published in Proceedings of the National Academy of Sciences on the subject.

Sponheimer team made use of chemical residues on dental records from approximately 175 fossilized specimens from 11 different hominids between 1.3 and 4.4 million years old.

"What we have is a 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 Zeresenay Alemseged, a researcher at the California Academy of Sciences and author of two of the papers published.

What can we make of this new discovery? Well, Sponheimer says, these findings open up as many questions as they answer.

"There is much here that changes our understanding of various human antecedents," he continues. "For the earliest [human ancestors], the surprise is that the composition of their diets is so similar to that of chimpanzees. As we got later in time, the surprise is how little the diets resemble those of living apes."

Although several theories has been advanced as to what has caused the change in diets - including some pointing to rapid climate change or competition from other hominids - it remains largely unclear what was behind this change.

It's also unclear when early human ancestors began eating meat, a topic that remains hotly debated in the anthropology community.

"Everyone would like the dietary expansion to be linked to climatic and environmental change, and perhaps it was, but our evidence for this is rather weak at present," Sponheimer said.