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.
That may make sense, even to a chimp, but it's not all that easy for a chimp to walk upright. Unlike humans, for example, a chimp can't stand on one leg and let its leg bones carry the weight. A chimp has to use muscles for that, because its legs are structured differently, and that can be tiring.
But the gradual evolutionary changes, which had to take millions of years, brought a mixed bag. It made it possible for early humans to roam over vast areas, picking low hanging fruit, and carrying supplies, tools, and kids. It also made them appear larger and more intimidating.
But we still have a backbone left over from the years when our more distant ancestors were chiefly horizontal, both in the water and on the land. It wasn't designed to work in a vertical position, which is why modern humans suffer from sore backs, slipped disks, arthritis, and so forth.
So when did all this begin? According to the Smithsonian Institution's Human Origins Program, it probably started at least six million years ago with changes in the leg bones of one of the earliest hominoids, Sahelanthropus. Next came the knee, a couple of million years later.
By around three million years ago, according to many experts, our ancestors were pretty much like us, at least structurally, and probably moved around mostly on two feet, which had lost the ability to cling to a branch. Last year in the journal Science, researchers reported the discovery of a fossilized foot in Ethiopia that was clearly made for walking, not climbing trees.
That foot was stiff enough to push off from the ground when walking, and flexible enough to absorb the shock of touching down, so it was a monumental change. No more swinging in the trees.