Saving Metabolic Energy Seen As Factor In Switch To Bipedalism

Hmmm. They propose something that's testable.

Somewhere in the murky past, between four and seven million years ago, a
hungry common ancestor of today's primates, including humans, did something
novel. While temporarily standing on its rear feet to reach a piece of fruit,
this protohominid spotted another juicy morsel in a nearby shrub and began
shuffling toward it instead of dropping on all fours, crawling to the shrub
and standing again.

A number of reasons have been proposed for the development of bipedal behavior,
or walking on two feet, and now researchers from the University of Washington
and Johns Hopkins University have developed a mathematical model that suggests
shuffling emerged as a precursor to walking as a way of saving metabolic energy.

"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 UW research assistant professor of anthropology and co-author of a recent

She believes it was an empty belly, along with a need to conserve energy, that
prompted that early ancestor to shuffle.

"Hunger. It is always hunger," said Kramer. "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."
"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. Modern humans are
different with long legs and a big head. So chimps are a good place to start,"
Kramer said.

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 and that most shuffling would occur for
distances less than 30 feet. In addition, walking on two feet would be used most
frequently for distances less than three feet.

"These are predictions other people can test. You should rarely, if ever, see a
chimp walking upright at longer distances. The flipside 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," she said.

"We think metabolic energy is extremely important and we have only touched the
surface of the information we can get with this work. The model allows people to
plug in the body characteristics of any primate so a researcher can change the
parameters for a specific species."

The study was published in the American Journal of Physical Anthropology.