Curiously, given that, in general, humans can’t eat wood, there is a recent claim that Austrelopithecus sediba ate bark. In “From Tree-Living to Tree-Eating: Human Ancestors Ate Bark” (Discover, July 2, 2012), we learn,
For example, the ratio of different carbon isotopes in the teeth suggested that they preferred to consume forest food rather than the grassland pickings that other early humans enjoyed. The tartar, meanwhile, contained plant particles linked to bark and woody material. But bark may not have been the hominins’ first food choice: modern primates tend to eat it only when tastier food is unavailable. Perhaps, the researchers suggest, these early humans had to deal with drought or a similarly stressful environment.
But how do we know the hominins were eating it for food? Bark can have medicinal, sedative, or analgesic qualities, and it is just as likely, surely, that they were chewing it for that purpose.
Historical note: White cedar bark has been used historically to cure scurvy, a vitamin C deficiency disease.