Human Ancestors Shuffled Before Walking, Study Says
for National Geographic News
|June 11, 2008|
The split between primates and humans may have its roots in some common ancestors' simple shuffle, a new study says.
Living between four and seven million years ago, these ancestors are believed to have walked on four limbs.
But the ancestors likely began lifting up their torsos and shuffling about on two feet for short distances when foraging for high-hanging food, according to new research.
Moving on two legs for short distances—between 9 and 16 meters (30 to 53 feet)—required less energy than returning to all fours to move to the next foraging spot, said study co-author Patricia Kramer, an anthropologist at the University of Washington.
Any farther, however, and the awkward bipedal shuffling ceases to be energy efficient for an animal used to traveling great distances on all fours, she added. So the shuffle was saved for series of short trips among closely situated spots.
The logic holds true today.
"If you think about the situation where people don't have enough food to eat, people will adapt ways [so] that they physically move less," Kramer said.
"When gas is very expensive, we line up all of our errands to be more efficient. It's the same kind of idea, conserving in one area so you may spend in another."
The study was published recently in the American Journal of Physical Anthropology.
Shuffle to Walk
Kramer and colleagues constructed a mathematical model by studying chimpanzee movement. Chimps, which have a similar bone and muscle makeup as humans, travel on all fours using their powerful arms.
The apes are also thought to be about the size of the early ancestors.
What these ancestors looked like still remains a mystery, since few fossil records exist, Kramer said.
"There are lots of gaps in the fossil records. We always hope we'll find the record to help fill it in," she said.
The model could also provide more detail about the evolution of modern humans. (Explore a map of the human evolutionary highway.)
Over time, that awkward shuffle became a full-fledged walk, she said. The more the ancestors moved about bipedally for short distances, the more they began to develop variations in their anatomies that encouraged walking upright.
Those able to walk well passed the trait on to their offspring, Kramer said.
The ability to move on two feet also put into motion a new set of behaviors, which became precursors to modern humans' long legs and bigger brains.
"Bipedalism didn't cause [the changes], but maybe bipedalism allowed it to happen," Kramer said.
(Related: "6-Million-Year-Old Human Ancestor 1st to Walk Upright?" [March 20, 2008].)
Karen Steudel is a zoology professor at the University of Wisconsin in Madison and expert in bipedal movement in humans.
Kramer and colleagues' model could be expanded to incorporate new findings, such as what [early ancestors] ate, Steudel said.
"If we were able to find out more about what they were feeding on and if there was some way to extrapolate from modern relatives of these plants, then this model could be used to evaluate [what they ate]."
The theory that foraging spurred protohominds into bipedal movement is one of many. Other theories say they stood on two legs to see predators over tall grasses, or to carry things in their forelimbs. (Related: "Upright Walking Started in Trees, Ape Study Suggests" [May 31, 2007].)
"[The new study has] offered a new way of conceptualizing what had been previously a purely theoretical argument," Steudel said.
"They're putting some quantitative limits to the argument."
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