New evidence collected during two expeditions to Guinea in West Africa supports that theory, which has been one of several leading explanations for why humans became bipedal somewhere between three and six million years ago. Researchers from the United States, England, Japan and Portugal spent weeks watching chimpanzees in their natural habitat to see how they would move about if they needed to carry something.
If they had a chance to grab a fistful of tasty treats before someone else snatched it, they stuffed the goodies into their mouths, and their hands, and ran to a safe haven on two feet. In fact, they were four times more likely to assume a human posture if the treats were particularly rare and the competition fierce.
In 1961 anthropologist Gordon W. Hewes postulated that about the time that our lineage branched away from chimps and apes, the environment in Africa changed to more open savannahs, thus making some resources scarce. So to survive, our ancestors probably needed to gather resources when they were available and carry them to their normal habitat.
And there would have been no better way to do that than to stand upright and use their hands to hold whatever they needed to transport, Hewes maintained.
Over the course of many generations, our ancestors slowly developed the right muscles and the right skeletal system to facilitate walking on two legs, and that gave us a very different profile from the quadrupeds we left behind. Unfortunately, it also gave us aching backs and sore knees, but more on that later.
Although Hewes's theory makes sense, some anthropologists argue that carrying stuff was at best only part of the reason our ancestors became bipeds. Walking is also more energy efficient, and it's easier to do a lot of things if you aren't stepping on your own hands.
It isn't possible to turn back the calendar and see what was really happening all those years ago, so an international team of scientists turned to the next best thing -- wild chimps.
"These chimpanzees provide a model of the ecological conditions under which our earliest ancestors might have begun walking on two legs," Brian Richmond of George Washington University said in releasing the study, published in this month's "Current Biology."
"Something as simple as carrying -- an activity we engage in every day -- may have, under the right conditions, led to upright walking and set our ancestors on a path apart from other apes that ultimately led to the origin of our kind," Richmond said.
The researchers spent 14 months watching chimps in one of the most remarkable sanctuaries in the world, Kyoto University's "outdoor laboratory" in Guinea's Bossou Forest, where scientists have intensely studied how chimps use tools, mainly rocks, to crack open nuts and access other foods. The study, conducted by Kimberley Hockings of Oxford Brookes University, focused on wild chimps that routinely raid nearby farms.
Chimps often walked on two feet as they carried papayas and other crops in their hands, and their mouths, and even on their feet, the study notes.