Chew on this: Bits of food stuck in the two-million-year-old teeth of a human ancestor suggest some of our forebears ate tree bark, a new study says.

A first ever find for early human ancestors, the bark evidence hints at a woodsier, more chimplike lifestyle for the Australopithecus sediba species. Other so-called hominins alive at the time are thought to have dined mostly on savanna grasses.

A. sediba was identified from stunningly preserved fossils of a female and a young male discovered in a South African cave in 2008 by scientists led by paleoanthropologist and National Geographic Society grantee Lee Berger. (See "'Key' Human Ancestor Found: Fossils Link Apes, First Humans?")

"We think these two individuals fell down a sinkhole ... and were quickly covered in very fine-grained sediment that created an environment of very little oxygen," explained Amanda Henry, lead author of the new study.

"So there wasn't a lot whole lot of bacteria or decomposition, and there certainly wasn't any interaction with the air," said Henry, a paleobiologist at Germany's Max Planck Institute for Evolutionary Anthropology.

That airless entombment resulted in a rare state of preservation—to the point that even microscopic, fossilized particles of plant tissue remain trapped in dental plaque.

By comparing isotopes and other properties of the ancient particles, called phytoliths, with modern examples, the team was able to identify which plant parts individual specks came from—revealing a diet that included fruit, leaves, and bark.

In some cases the researchers could even pinpoint the type of plant.

For example, "we had a palm [tree] phytolith," Henry said. "We weren't able to tell whether it came from the fruit or the leaf or another part of the palm, but we could definitely identify that it came from that family of plants."

Part Ape, Part Human

The findings indicate that A. sediba preferred to feed in more enclosed, woodland environments, much like modern-day chimpanzees and gorillas—which also happen to eat bark.

"The way I interpret this is that, around two million years ago, our ancestors and relatives were exploring a variety of environments and behaviors within those environments," Henry said.

Like the chimpanzee, A. sediba may have even climbed trees and used simple tools to get its food—ideas supported by the fossils. The ancient ancestor had long, apelike arms; wrists suited to climbing; and a remarkably humanlike "precision grip," necessary for making stone tools.

"I imagine something kind of gorilla-like and something kind of chimpanzee-like," she said, "but also definitely bipedal, and also looking a bit more like us."

Henry speculates that similarly experimental eating may have been key to the success of early members of Homo, the human genus, which includes Homo erectus—the first known human species with long legs and short arms.

"By 1.8 million years ago Homo erectus is able to survive in a whole variety of different environments," she said. "They leave Africa and spread around the world. But just before that, you had a variety of different species that were focusing on smaller habitats and niches."

(Explore a graphic of the possible branches on the human family tree.)

New Tool: Bark Only the Beginning

The A. sediba tooth-trace research was "very well done, and the conclusions are strong and set a very important precedent for future research," said Smithsonian Institution phytolith expert Dolores Piperno, who wasn't part of the project.

The new study marks the first time phytoliths have been used to examine the diet of a creature as old as A. sediba, added Piperno, an archaeobotanist at the National Museum of Natural History in Washington, D.C.

Previously, scientists had used phytoliths to glean clues about the diets of Neanderthals and of modern humans during the beginnings of plant domestication, but these are relatively recent events in the history of human evolution.

Study author Henry added that, now that her team has proven the technique with A. sediba, she wants to use it to study the diets of other early hominins—and she's not wasting any time.

"I'm going to South Africa this summer to start collecting some of that data," she said.