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. 

"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."