Making a stand: The study compared ground reaction force vectors for humans and chimpanzees to find that the human method of locomotion is highly efficient.
Credit: Cary Wolinsky
SYDNEY: Once they came down from the trees, walking upright required considerably less effort of early humans than 'knucklewalking,' say researchers, explaining why we evolved to walk on two legs.
Their new study, Chimpanzee locomotor energetics and the origin of human bipedalism hypothesizes that walking on two legs, or bipedalism, developed because it required less energy than scampering on all fours, like other primates.
"For decades now researchers have debated the role of energetics and the evolution of bipedalism," said anthropologist David Raichlen of The University of Arizona in Tucson, U.S., co-author of the study published this week in the U.S. journal Proceedings of the National Academy of Sciences.
Chimps on treadmills
To add evidence to conjecture, the researchers used biomechanical modelling to simulate primate gaits. They studied the gaits of four adult humans walking on a treadmill, and also studied five chimpanzees that were trained to walk quadrupedally (on four legs) as well as bipedally (on two legs.)
The researchers found that human walking is around 75 per cent less costly, in terms of energy and caloric expenditure, than both quadrupedal and bipedal walking in chimpanzees.
The modelling also found that – unlike humans – chimpanzees use the same average amount of energy on two legs as on four, but suggested there was variation from animal to animal. Chimps who took longer strides were more efficient when walking upright than those who took shorter steps.
"We were able to tie the energetic cost in chimps to their anatomy," said Raichlen. "We were able to show exactly why certain individuals were able to walk bipedally more cheaply than others," he said.
Defining human feature
The researchers applied their data to the fossil record to see whether the adaptations of human ancestors might have evolved to reduce the amount of energy they expended when walking bipedally, said Raichlen. "We and many others have found these adaptations [such as slight increases in hind limb extension or length] in early hominins, which tells us that energetics played a pretty large role in the evolution of bipedalism."
Those energy savings could have provided early hominids with an evolutionary advantage over other apes, by freeing their hands up and reducing the cost of foraging for food.
"Walking upright on two legs is a defining feature that makes us human," added Herman Pontzer, Raichlen's co-author and anthropologist at the University of Washington in St. Louis. "It distinguishes our entire lineage from all other apes."
with Agençe France-Presse
