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

By Ann Gibbons

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.

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