The skull of Paranthropus boisei, an African hominid that among other hominids began to add grasses and sedges to their diet about 3.5 million years ago.
The skull of Paranthropus boisei, an African hominid that among other hominids began to add grasses and sedges to their diet about 3.5 million years ago. (University of Colorado)

A new study led by the University of Colorado indicates that a "game changer" occurred about 3.5 million years ago which caused some early African hominids to add grasses and sedges to their diet.

CU anthropology professor Matt Sponheimer, lead author on the study, said that prior to about 4 million years ago, African hominids ate, much like chimpanzees, primarily fruits and some leaves. This is despite the fact that grasses and sedges -- rushes or grasslike plants growing in wet environments -- were readily available.

But tests using stable isotope analysis, comparing stable carbon isotopes from ancient hominids' teeth, have shown that eventually, one genus of hominid, Paranthropus boisei, started to diverge to a diet more restricted to grasses and sedges, while the genus Homo -- from which modern man is descended -- maintained a broader diet.

The genus Paranthropus went extinct about 1 million years ago, while the genus Homo included the 3-million-year-old fossil Lucy, considered by many to be the matriarch of modern humans.

"We don't know exactly what happened," Sponheimer said in a news release. "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 were an important step in becoming human."

The study was published Monday online in the Proceedings of the National Academy of Sciences. The research was funded by the National Science Foundation, the CU-Boulder Dean's Fund for Excellence, the national research Foundation in South Africa, and others. There were 10 additional co-authors on the paper.