Our ancient human ancestors traded in the ability to climb trees for the power to walk on two legs, but it is unclear when this happened in evolutionary time.

A new study could help pin down the timing of this exchange, revealing that human ancestors as far back as 4 million years ago didn't have quite the climbing skills of modern chimpanzees, so climbing was phasing out by this time.

The evidence: Early humans lacked the ankle structure that assists chimps in climbing, according to anthropologist Jeremy DeSilva of Worcester State College in Massachusetts.

Wild comparison

DeSilva videotaped wild chimpanzees — our closest living animal relatives — in Uganda to study their bodies while climbing. He measured the angle of dorsiflexion, or how far the ankle could rotate so that the toes point upward, and found that chimps can make much more extreme ankle rotations than modern humans.

To investigate whether early hominins were more like modern humans or chimpanzees, DeSilva analyzed the ankle bones in fossils of human ancestors at various times from 1.5 million to 4 million years ago. He discovered that early humans during this span have dorsiflexion ranges similar to those of modern humans, and couldn't have climbed trees in quite the same way as chimps do, if they climbed at all.

"Frankly, I thought I was going to find that early humans would be quite capable, but their ankle morphology was decidedly maladaptive for the kind of climbing I was seeing in chimps," DeSilva told LiveScience. "It kind of reinvented in my mind what they were doing and how they could have survived in an African savannah without the ability to go up in the trees."

Since tree climbing is useful both for foraging food and for hiding from predators, the benefits of walking upright must have been great to make humans give up their ankles more suited for climbing.

Other research suggests that early humans at this time had limb proportions similar to other mammal species that are particularly aggressive. Perhaps early humans used aggression to discourage predators from targeting them.

Plus, walking on two feet not only enabled travelling long distances, but also escaping more quickly on the ground from predators.

Energy required

Bipedalism, or the ability to walk upright, is thought to demand sacrificing climbing skills because the body proportions required for both are different.

Specifically, walking on two legs requires a lot of energy to lift the foot and ankle, so minimizing their weight is important. But climbing requires bones in different places on the foot than walking does, and keeping both sets of bones would be too heavy, DeSilva said.

"I think by 3 [million] to 4 million years ago that tradeoff was occurring," he said. "Our ancestors were becoming very capable upright walkers, and it came at the expense to our ability to climb trees."

Will Harcourt-Smith, an anthropologist at the American Museum of Natural History in New York, said the  DeSilva study was unique in comparing ancient fossils to observations of living animals' movements.

"More studies doing that will help us," he said. "This is a very nice finding, but I think it's only part of the story. And it doesn’t preclude these fossil creatures from being able to climb at all. But if they did climb, they were climbing in different ways than chimps do."

Harcourt-Smith said DeSilva's ankle data could be compared to measurements of other body parts from other studies to form more of a complete picture.

More to learn

DeSilva said other research found that many early humans' toes were not likely able to grasp as well as chimpanzees' do, which would be a useful attribute for climbing. And measurements of early human knees and hips also indicates that these bones weren't well adapted for climbing.

However, fossils of upper body parts, such as the strong arms and curved fingers on some early human specimens, suggest they retained some of their climbing skills.

Ultimately, to get to the bottom of the issue, more fossil discoveries are essential.

"The thing we really need is to find more fossil hominins between 5 and 7 million years old," Harcourt-Smith said. "We need to find more creatures that would have been closely related from roughly the time of this last common ancestor between chimps and humans."

The study is detailed in this week's issue of the journal Proceedings of the National Academy of Sciences.