The traditional idea that our ancestors descended from the trees and gradually—and exclusively—began walking upright might be a gross over simplification. Fossil evidence from early hominins suggests that adaptations for tree climbing, such as long arms and fingers, coexisted with adaptations for upright walking, such as an arched foot and humanlike hips. Eventually, these upper-body climbing adaptations vanished and we became the adept striders that we are today. But just because our ancestors seemed to be adapting to bipedal walking, does that mean they left behind a path into the trees?

Some researchers have tried to answer this question not by looking at fossils but rather by looking at modern human hunter-gatherers. Indigenous groups often climb trees to gather food without relying on chimp-like branch-climbing or supportive equipment. And though they’re not as good at climbing as chimpanzees, neither are they all that much worse—deaths from falls are only marginally higher (6.6 percent compared to 4 percent) in some studies of frequent tree climbers.

New research on hunter-gatherer groups in Uganda and the Philippines, conducted by a team based at Dartmouth, found that these people are using their two feet to ascend straight up a small tree's trunk. These findings suggest that other earlier hominins that were adapted for upright walking might also have used their upright anatomy to ascend into the trees—perhaps much more often that we would have previously expected.

The researchers, led by Vivek Venkataraman, a graduate student at Dartmouth, studied two Ugandan groups—the Twa, who are hunter-gatherers, and the nearby Bakiga, who are farmers—and two Philippine groups—the Agta, who are hunter-gatherers, and the Manobo, who are farmers. Both groups of hunter-gatherers consume locally collected honey as an important part of their diets. Both groups climb trees to gather the honey, and many individuals start climbing at a young age. To ascend the trees, the climbers wrap their arms around the tree trunk at head-level, then, placing one foot in front of the other, the climbers advance upward to the honey source; in a sense, they "walk" up trees.

This tree-based foraging appears to alters these individuals' feet, ankles and legs to be much more adept at this form of locomotion. Using ultrasound imaging, the researchers found that the muscle fibers of the people who regularly climbed trees were drastically different than the fibers of those who did not. Thanks to longer calf muscle fibers, climbers could flex their ankles more than 45 degrees toward their shins—much farther forward than most non-climbing humans can, and closer to that of a chimpanzee foot flexion. If individuals began climbing trees as children, it would give them years to develop this soft-tissue trait.

The skeletal features of the climbers' feet looked no different from a foot from someone that has spent their life walking the plains—or the sidewalks of New York City (and in fact, people who spend decades wearing high heels experience a shortening of the calf muscle fibers).

This functional, soft-tissue shift suggests that upright-walking human ancestors, such as Lucy (Australopithecus afarensis), may have been quite capable of ascending into the trees in this fashion. And she would have had plenty of incentive to do so, the researchers noted, including foraging, escape and perhaps even just finding a safe place to rest. The findings were published online December 31 in Proceedings of the National Academy of Sciences. "Au. afarensis might be expected to climb on tree trunks and near the central core of trees, rather than within a fine-branch niche," as other extant primates do today, the researchers noted.

"Our findings challenge the persistent arboreal-terrestrial dichotomy that has informed behavioral reconstructions of fossil hominins," the researchers wrote. The finds, they suggested, also "highlight the value of using modern humans as models for inferring the limits of hominin arboreality."