There have been many suggestions for how and why our hominid ancestors got out of the trees and started walking. The latest: a prehistoric shuffle driven by an empty belly.
This proposal relies on a mathematical model to suggest that shuffling emerged as a precursor to walking, between 4 million and 7 million years ago, as a way of saving metabolic energy.
The idea is that some hungry primate was perhaps picking fruit, stood on its hind legs and reached up, and then rather than go back down on all fours, shuffled over to another bit of low-hanging fruit.
"Metabolic energy is produced by what an animal eats, enabling it to move. But it is a limited resource, particularly for young-bearing females which have to take care of and feed their offspring. Finding food is vitally important, and an animal needs to save energy and use it efficiently," said Patricia Kramer, a University of Washington assistant professor of anthropology and co-author of the new study.
Kramer thinks it was an empty belly, along with a need to conserve energy, that prompted that early ancestor to shuffle.
"Hunger. It is always hunger," she said. "There is nothing that will get you to do something you don't want to do other than food. That's why we bribe animals with food to train them."
Because of a gap in the fossil record that hides when humans split off from other primates, Kramer and co-author Adam Sylvester, now a postdoctoral fellow at Johns Hopkins University, used the chimpanzee as a way of looking into the past and testing other researchers' ideas about the origins of bipedalism (walking on two legs).
Chimpanzees are humans' closest relatives (some folks in Europe even want a chimp named Matthew to be legally called a human). They basically walk on all fours, partially resting their weight on the knuckles of their hands.
"A chimp's body plan is very much like that of a primitive ape, and our last common ancestor probably had a body like that of a chimp," Kramer said. "Modern humans are different with long legs and a big head. So chimps are a good place to start."
Using the model they devised, Kramer and Sylvester calculated it would not be metabolically efficient for a chimp to use bipedalism for distances greater than about 50 feet. But it would be efficient for distances less than 30 feet, and that's when most shuffling would occur. In addition, walking on two feet would be used most frequently for distances less than three feet.
"These are predictions other people can test," Kramer said. "You should rarely, if ever, see a chimp walking upright at longer distances. The flip side of this is if a chimp is going a short distance returning to all fours is not going to happen. You can see this in human babies learning to walk. If they are going between a couch and a coffee table, they are up on their feet. But if they are going a longer distance, they go down and crawl."
The study, announced today, was published in the American Journal of Physical Anthropology.