Diet Diverged In Earliest Human Ancestors, Researchers Find

Peter Ungar, associate professor of anthropology at the University of Arkansas, and Mark Teaford, professor of cell biology and anatomy at Johns Hopkins University School of Medicine, report their findings in the Dec. 5 issue of the Proceedings of the National Academy of Sciences.

Ungar and Teaford used dental and jaw data from Ardipithecus ramidus, Australopithecus anamensis, Australopithecus afarensis and Australopithecus africanus, Pliocene hominids that date back 2.3 to 4.4 million years ago. They looked at tooth size, tooth shape, tooth enamel structure, dental microwear and mandibular biomechanics of the fossils, most of which date back to a time before stone tools, before culture and before meat was introduced to the diet.

"No one has looked at diet variability within this group," Ungar said. "Until now, we had no idea of what happened from the standpoint of diet in the first half of human evolution."

The researchers found that even the dental fossils of the oldest human ancestor studied -- Ardipithecus ramidus -- showed signs of a generalized diet. A few hundred thousand years later, the fossils show larger teeth with thicker enamel, and a million years later the fossils sport larger teeth and heavier jaws suited for heavy chewing of hard, brittle foods. But microscopic marks on the teeth also indicate that the hominids had not lost the ability to eat soft, tough foods, like fruit.

"You're seeing an ability to broaden the diet," Ungar said.

This generalized diet became crucial 2.5 million years ago, when our human ancestors split from the specialized forms of hominid species that eventually died out. Researchers speculate that the hominids with a more varied diet were able to survive environmental changes, while the specialists could not adapt quickly enough.

Until now, there was no evidence of dietary changes in the first half of human evolution. Ungar and Teaford's research shows that diets were changing throughout the evolution of our human ancestors, from the time soon after they split from the apes.

"The specialists and generalists branch off, but both stem from a trend that started 5 million years ago," Ungar said.