"We are, effectively, looking back 2 million years and watching our ancestors chew their food," says Lee Berger.
A palaeontologist at the University of the Witwatersrand in Johannesburg, South Africa, he shot to scientific stardom in 2010 when he discovered Australopithecus sediba, one of the most remarkable fossils of the hominin lineage known to date.
Now he, Amanda Henry of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a team of collaborators have discovered what A. sediba ate. On the menu: bark.
A. sediba is known from two skeletons uncovered in South Africa. The species shares a mix of features seen in earlier australopithecines, modern humans and chimpanzees. It turns out that they had poor dental hygiene. From plaque on the fossils' teeth, the team extracted "phytoliths" – mineral traces of A. sediba's food. They found signs of fruit, bark and woody tissues.
"That blew me away," says Berger. "I had never heard bark associated with what we ate before."
Primatologists were less surprised. "Bark represents a considerable fraction of orang-utan diets," says Madeleine Hardus of the University of Amsterdam in the Netherlands. Other primates also chew on the hard stuff – species from the golden snub-nosed monkey (Rhinopithecus roxellana) to chimpanzees eat bark when times are tough.
"It is really exciting that they were able to extract phytoliths from the dental calculus [plaque]," says Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research in New York. "This is important information about detailed plant consumption in the hominins – something that has been invisible to us."
Greater dietary surprises were in store for Berger and his colleagues though. The team looked at sediment samples and fossilised animal faeces – coprolites – to get an idea of what the environment in which A. sediba lived was like. They found remnants of savannah grasses in the sediment, and pollen and woody fragments in the coprolites suggested that there might have been some woodlands in the vicinity.
The team then looked at the carbon isotopes in A. sediba teeth to see what types of plants they ate. A "C4" signature is typical of savannah plants like grasses and the grains they carry. These plants fix carbon in a four-carbon molecule. "C3" indicates fruits and leaves foraged from a more forested environment.
The team expected a C4 signature – it's what most hominins have and fits the evidence that A. sediba lived in an open savannah. They found the exact opposite. The results are fascinating, says Aiello. "The most important thing is that the diet of A. sediba was different from the diet of other early hominins."
Why A. sediba had such an unusual diet is still a mystery.
Henry suggests that A. sediba may have lived in small woodlands that lined bodies of water in the savannah. Its lifestyle may have been similar to savannah chimps, which travel long distances to forage the limited woodlands available to them.
"They may have been trying new things. We're getting an idea of what comes later when modern humans became so flexible that they could exploit almost any environment," she says.
To Berger one thing is clear: A. sediba was a picky eater – and picky eaters are clever. "They had to be smart enough to select specific foods from their environment," he says.
Could there be other explanations for why early humans consumed bark? Was it an early form of tooth-brushing, for instance? Or were they getting some kind of medicinal advantage from the barks they chose? Many animals are known to self-medicate. So we put these questions to the experts.
A. sediba's teeth are covered in plaque, says Berger. If it brushed, it was really bad at it.
"A. sediba would have to play with its toothpick for several hours a day" for the traces of wood found by Berger's colleagues to accumulate, says Hardus. "The same applies to medication, which is supposedly only consumed sporadically – unless A. sediba chewed on leaves or bark very much like tobacco leaves today."
It looks as if our ancestors were just after a meal then.
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