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.
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