photo credit: George H. Perry
photo credit: George H. Perry
caption: Twa man from Uganda ascending a vine in pursuit of honey

Several million years ago, human ancestors began striding around on two legs, in what was a key moment in the evolution of modern humans. But the question of whether each species of prehuman left treetops behind and made their home in grasslands has not always been easy to discern from fossils, which can provide conflicting evidence.

For example, Lucy, a member of the prehuman species Australopithecus afarensis, has the long fingers and longish arms that suggest she might have been a skilled climber. But the lower half, especially her ankle and foot, are confusingly human-like, suggesting they may have been adapted for an upright existence. Now, a team of scientists from Dartmouth College that has been studying a hunter-gatherer population in Uganda has found clues among living humans that further muddies the picture, making it clear that even a modern human foot can be useful in impressive feats of climbing.

The researchers started out studying the short, pygmy stature that can be advantageous among hunter-gatherers who dwell in the rainforest, when they realized they also had an opportunity to use living humans to weigh in on a raging debate in paleoanthropology.

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During the right season, hunter-gatherers dwelling in rainforests consume as much as two pounds of honey a day, receiving 80 percent of their calories from honey. To get the honey, they need to reach the bees’ nests, typically found more than 60 feet up in trees.

The group of people the researchers focused on, the Twa of Uganda, are former hunter-gatherers who in the last 15 years have had to leave the forest. The Twa are remarkable climbers, able without ropes and harnesses to clamber high into treetops by gripping trees with their hands and feet. (A cool video of a man climbing a narrow tree seeking honey is available here.)

The researchers took videos of the men to see whether they could understand the biomechanics of their climbing abilities and found that the Twa were able to bend their ankles to an extreme degree, bringing their shins close to the tree. In normal people, such flexing in the ankle would cause injury or broken bones.

“It is similar to what you see in chimpanzees, the angle we measured,” said Vivek Venkataraman, a graduate student at Dartmouth who led the research. “This allows the climber to get closer to the tree, reduces energy expenditure, and makes them safer.”

Then, the researchers used ultrasound to analyze the muscles in the legs of Twa men and found that they had specially developed, unusual calf muscles that seemed to allow them to perform this feat.

The study, published in the Proceedings of the National Academy of Sciences, does not prove that Lucy or any other prehuman ancestor with a humanlike foot was spending a lot of time in the trees or was primarily a climber. Unfortunately, fossils do not provide any clues about their muscles. But the study suggests that the urge to neatly divide prehuman species into buckets—some that dwelled in the grasslands and others that lived in the trees like apes—may be too simplistic a way of thinking about life.

Daniel Lieberman, a professor of human evolutionary biology at Harvard University who was not involved in the work, said the study’s findings make sense.

“I have always thought that there has been too much of an effort to dichotomize human locomotion, and that the australopiths such as Lucy were probably able to both climb and walk well,” Lieberman wrote in an e-mail.

He added, however, that just because a group of modern humans have this unusual ability, their climbing abilities are still far from that of chimpanzees. The question of when those abilities were lost remains an important one to unravel.

Even a non-scientist can easily see the agile Twa do not have the climbing abilities of a chimpanzee. And the modern human foot has obviously made even that climbing a specialized skill: Venkataraman observed that the older men, who probably had the most experience, were the best climbers. It also wasn’t a matter of just emulating a technique, which Venkataraman and his colleagues experienced firsthand.

“We have documentation of us trying to climb in the same fashion, and we were just really bad at it, it’s a lot harder than it looks,” Venkatarman said. “They would go up a tree, and we’d say, ‘That looks easy and let’s try it.’ And of course it was impossible.”