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When chimps walk upright, they use an inefficient bent-legged gait, unlike the straight-legged stride of humans.

Konrad Wothe/Minden Pictures

Our tree-climbing human ancestors could walk upright like us, study of chimps and other primates shows

With their opposable toes and flat feet, early human ancestors have often been portrayed as weird walkers, swaying from side to side or rolling off the outside edges of their feet. Now, a new study finds that this picture of awkward upright locomotion is wrong: Early members of the human family, or hominins, were already walking upright with an efficient, straight-legged gait some 4.4 million years ago. The study helps settle a long-standing debate about how quickly our ancestors developed a humanlike gait, and shows that ancient hominins didn’t have to sacrifice climbing agility to walk upright efficiently.

For years, some paleoanthropologists argued that hominins like the famous 3.1-million-year-old Lucy weren’t graceful on the ground because they retained traits for climbing trees, such as long fingers and toes. In one famous experiment, researchers donned extra-long shoes—one critic called them clown shoes—to mimic walking with longer toes. The scientists stumbled over their long feet and concluded that early hominins would have been just as clumsy. But other researchers argued that natural selection would have quickly favored adaptations for efficient walking given the dangers on the ground, even while hominins were still scurrying up trees.

To test these hypotheses, evolutionary anthropologist Herman Pontzer of the City University of New York (CUNY) in New York City and his team compared how humans, living apes, and monkeys use their hips, leg bones, and muscles when they walk and climb. CUNY graduate student Elaine Kozma filmed chimps, bonobos, gorillas, gibbons, and other primates in zoos so she could measure the precise angles of their legs and hips when they walked upright. She then calculated the stresses on their bones during maximum extension and found that apes put a lot of force on their massive thighs, hamstrings, and knees—forces that also help them power up trees.

By contrast, humans, who have a shorter pelvic bone called an ischium (informally known as a “sit bone”), can hyperextend their legs in a way that generates less force on the hamstrings at the knees. When humans take a stride, the muscles that attach to the ischium—the hip flexors and the hamstrings—swing in a broader arc from front to back than in other living apes so they use less energy to move farther.

The size and orientation of a pelvic bone called the ischium (green) determines the range of motion (yellow angle) in a primate’s stride and how much power the leg has in its hamstring as it walks upright or climbs a tree.

E. Kozma et al., PNAS 10.1073 (2018)

In the new study, Pontzer and Kozma also calculated the range of hip and leg extensions of three species of ancient hominins. Lucy and other members of Australopithecus had the full human range of motion, they report today in the Proceedings of the National Academy of Sciences, even though they still had traits that helped them climb trees (albeit less powerfully than other living apes).

Even older ancestors may have been able to take straight-legged strides. The 4.4-million-year-old hominin known as Ardi (Ardipithecus ramidus) had pelvic bones oriented in such a way that its hip flexors could extend almost as much those of modern humans, despite having a long, apelike ischium. That reveals a flexible adaptation that allowed Ardi to walk upright efficiently but still power up trees, Pontzer says. “Ardi seems to have been able to bridge both worlds,” says paleoanthropologist Carol Ward of the University of Missouri in Columbia, who was not part of the team.

Whereas Pontzer thinks natural selection favored energy-efficient gaits, paleoanthropologist Owen Lovejoy of Kent State University in Ohio suggests Ardi and its descendants may have cared more about avoiding injury. “If you run with a massive thigh and must stop suddenly, it’s much easier to tear a hamstring,” says Lovejoy, who was not involved in the study. “And if you tear one, you’ll be consumed by a pack of wild dogs. Good luck with that.”