A chimp will do almost anything for a juice box - even exercise on a treadmill.
So a team of scientists in the United States relied on juice and other sweet treats in an experiment that compared how much energy adult chimpanzees and humans use when they walk. The researchers collected detailed metabolic and biomechanical data from five chimps and four humans as they used a treadmill.
Humans, it turns out, are far more efficient; we use 75 per cent less energy than chimps, which usually knuckle-walk on all fours.
This suggests that our ancestors started walking upright because it burned fewer calories, said Herman Pontzer, with the department of anthropology at Washington University in St. Louis. He and his colleagues published their findings in this week's online edition of the Proceedings of the National Academy of Sciences, a U.S. journal.
"Walking upright on two legs is a defining feature that makes us human," Dr. Pontzer said. "We have good evidence now, for the first time, that energy efficiency was an important selection factor in the origin of bipedalism."
Both the chimps and the humans in the experiment had to wear a loose-fitting mask to measure how much oxygen they used.
The chimps weren't thrilled about it.
"Chimps are brilliant animals, and they would hit the big red stop button when they were sick of it," said Dave Raichlen of the department of anthropology at the University of Arizona in Tucson. He is a co-author of the paper.
The juice boxes were essential for getting them to stay on for at least three minutes, Dr. Pontzer said.
"I'm not sure they loved doing it. But they love a good juice box."
The experiment bolsters the theory that our two-legged gait evolved because it saves energy. It was first put forward three decades ago without a lot of actual data to support it, Dr. Pontzer said.
The chimps learned how to knuckle-walk on the treadmill, using all fours. But in the wild, they walk on two feet about 10 per cent of the time, Dr. Raichlen said. So the trainer helped the chimps learn how to manage the treadmill on two feet.
Four of them used the same amount of energy walking on two feet as when they also used their hands. But one 33-year-old female had a longer stride, and was more efficient on two feet than when she knuckle-walked.
"She extended her legs more, and took longer steps," Dr. Pontzer said.
The researchers said she made it easy to picture evolution in action seven million years ago, when chimps and human ancestors began to diverge. A group of ape-like creatures started to walk on two feet because it takes less energy; this allowed them to put more resources into finding food and producing offspring.
Dr. Pontzer said he now wants to compare how much energy chimps and humans use in other daily activities, such as gathering food. He would like to see whether chimps in the wild use more energy than humans still living a hunter-gatherer lifestyle.
He said energy efficiency - using fewer calories to get food, water and other essentials - may be key to understanding why the human lineage became so successful and so different from the other apes.
"Humans are the only ape that can live in dry savannahs, Siberia, across the globe. All the other apes are confined to the rain forest where there is food available all the time. It is lush, there is lots of ripe fruit," he said.
"There is something about humans, for most of evolutionary history, that has allowed us to break from that constraint."
Dr. Pontzer said the chimps are not efficient walkers, even though they spend much of their time on the ground. But their bodies have evolved for climbing trees.
"If it turns out humans are a lineage adapted to efficiency, that's interesting," Dr. Pontzer said. "That is different from the other apes, which are built to be safe in trees."