An international team from the United Kingdom, Portugal, the United States, and Japan authored the paper. These authors include Susana Carvalho, Dora Biro, EugÃ©nia Cunha, Kimberley Hockings, William C. McGrew, Brian G. Richmond, and Tetsuro Matsuzawa.
As seen in the abstract to their paper, these researchers asked the following question to begin their research: 'Why did our earliest hominin ancestors begin to walk bipedally as their main form of terrestrial travel?'
They continue, 'The lack of sufficient fossils and differing interpretations of existing ones leave unresolved the debate about what constitutes the earliest evidence of habitual bipedality.' [Abstract]
However, some clues do exist. They state, 'Compelling evidence shows that this shift coincided with climatic changes that reduced forested areas, probably forcing the earliest hominins to range in more open settings. While environmental shifts may have prompted the origins of bipedality in the hominin clade, it remains unknown exactly which selective pressures led hominins to modify their postural repertoire to include a larger component of bipedality.' [Abstract]
Thus, the open areas were less secure to humans, so they wanted to carry their food to forested areas, which had more hiding places for eating their food. As forested areas became less plentiful, they had to forage in open places more frequently, and bring their food back to the forests to eat.
The researchers performed two experiments on wild chimpanzees. Page two discusses both experiments.