Four new studies of carbon isotopes in fossilized tooth enamel from early hominins, including Australopithecus afarensis, Paranthropus boisei and Kenyanthropus platyops, also Homo sp. and extinct baboons, show that early human ancestors surprisingly expanded their menu around 3.5 million years ago, adding grasses and sedges to an ape-like diet and setting the stage for our modern diet.

“High-tech tests on tooth enamel by researchers indicate that prior to about 4 million years ago, Africa’s hominids were eating essentially chimpanzee style, likely dining on fruits and some leaves. Despite the fact that grasses and sedges were readily available back then, the hominids seem to have ignored them for an extended period,” explained Prof Matt Sponheimer from the University of Colorado Boulder, lead author of the study “Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia” published online in the Proceedings of the National Academy of Sciences.

“We don’t know exactly what happened. 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,” he said.

Prior to the new PNAS studies, scientists had analyzed teeth from 87 ancient hominid specimens. The new papers provide detailed information on the teeth of 88 additional specimens, including five previously unanalyzed hominid species, doubling the dataset.

“The stable carbon isotopes obtained from ancient hominids helps researchers determine what types of plants they were eating. Carbon signals from hominid teeth are derived from two distinct plant photosynthetic pathways. The C3 signals are from plants like trees and bushes, while the C4 signals are from plants like grasses and sedges,” Prof Sponheimer said.

The researchers also looked at the microscopic wear of hominid teeth, which provides scientists with more information on the foods they were eating

“While the hominids from the genus Homo that evolved from australopithecines like the 3 million-year-old fossil Lucy – considered by many the matriarch of modern humans – were broadening their food choices, a short, upright hominid known as Paranthropus boisei that lived side by side with them in eastern Africa was diverging toward a more specific, C4 diet.”

Scientists initially had dubbed Paranthropus boisei ‘Nutcracker Man’ because of its large, flat teeth and powerful jaws, but recent analyses indicate it might have instead used its back teeth to grind grasses and sedges.

“We now have the first direct evidence that as the cheek teeth on hominids got bigger, their consumption of plants like grasses and sedges increased.”

“We also see niche differentiation between Homo and Paranthropus – it looks probable that Paranthropus boisei had a relatively restricted diet, while members of the genus Homo were eating a wider variety of things. The genus Paranthropus went extinct about 1 million years ago, while the genus Homo that includes us obviously did not,” Prof Sponheimer said.

In one of the studies, scientists found that while human ancestors ate more grasses and other apes stuck with trees and shrubs, two extinct Kenyan baboons (Theropithecus brumpti and T. oswaldi) represent the only primate genus that ate primarily grasses and perhaps sedges throughout its history.

T. brumpti ate a 65 percent tropical grass-and-sedge diet when the baboons lived between 4 million and 2.5 million years ago, contradicting previous claims that they ate forest foods. Later, T. oswaldi ate a 75 percent grass diet by 2 million years ago and a 100 percent grass diet by 1 million years ago. Both species went extinct, perhaps due to competition from hooved grazing animals. Modern Theropithecus gelada baboons live in Ethiopia’s highlands, where they eat only C3 cool-season grasses.

“Primate tropical grass-eaters – Theropithecus baboons and Paranthropus human relatives – went extinct while human ancestors ate an increasingly grass-based diet. Why is an open question,” said Dr Thure Cerling from the University of Utah, lead author of two PNAS studies (paper 2 and paper 3).

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Bibliographic information: Matt Sponheimer et al. Isotopic evidence of early hominin diets. PNAS, published online before print June 3, 2013; doi: 10.1073/pnas.1222579110

Thure E. Cerling et al. Stable isotope-based diet reconstructions of Turkana Basin hominins. PNAS, published online before print June 3, 2013; doi: 10.1073/pnas.1222568110

Thure E. Cerling et al. Diet of Theropithecus from 4 to 1 Ma in Kenya. PNAS, published online before print June 3, 2013; doi: 10.1073/pnas.1222571110

Jonathan G. Wynn et al. Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia. PNAS, published online before print June 3, 2013; doi: 10.1073/pnas.1222559110