If you're quickly trying to define what sets humans apart from other primates, you might first point to our greater intelligence and our capacity for complex language. The next big difference is that we walk on two feet and stay out of the trees... except, as this video explains, it's not quite that simple.

Part of the reason why the 1974 discovery of the remains of the hominid Lucy was such a big deal — beyond the fact that you don't find a 40% complete skeleton of a 3.2-million-year-old Australopithecus afarensis skeleton just any day — was that it rewrote a key part of our evolutionary story. Before the discovery of Lucy, anthropologists and paleontologists assumed the expansion of brain size was the first key evolutionary leap that moved the evolutionary ancestors of humans away from the other primates. But Lucy was relatively small-brained, and yet she was bipedal, meaning the shift out of the trees predated the growth in intelligence.

Ancient hominids may have left the trees, but we never abandoned themS

Since then, bipedalism has been recognized as one of the most crucial milestones in our evolution, and with it the abandonment of an arboreal life for an existence spent firmly on the ground. But as a team of researchers at the University of Dartmouth argue, the idea that a bipedal existence automatically puts trees out of reach of humans and our ancestors isn't necessarily true. As co-author Nathaniel Dominy explains in the video up top, plenty of humans still spend lots of time in the trees today, even if we're seemingly no longer properly adapted for climbing trees.

Specifically, he and his two co-authors Vivek Venkataraman and Thomas Kraft compared sets of modern hunter-gatherers and farmers both in Uganda and in the Philippines. The Twa of Uganda and the Agta of the Philippines are hunter-gatherers that climb trees regularly to get honey, which forms a key part of their diet, a behavior that sets them apart from their respective agricultural counterparts, the Bakiga and the Manobo.

Dartmouth Now explains just how the Twa and Agta climb trees:

They climb in a fashion that has been described as "walking" up small-diameter trees. The climbers apply the soles of their feet directly to the trunk and "walk" upward, with their arms and legs advancing alternately. Among the climbers, Dominy and his team documented extreme dorsiflexion-bending the foot upward toward the shin to an extraordinary degree- beyond the range of modern "industrialized" humans. Assuming their leg bones and ankle joints were normal, "we hypothesized that a soft-tissue mechanism might enable such extreme dorsiflexion," the authors write.

The researchers subsequently found out that the Twa and Agta had much longer muscle fibers in their calves, revealing how they were able to keep climbing trees so easily even when the rest of their anatomy was no longer well-suited for it. This, according to the researchers, is a good reminder that there are few absolutes in our evolution, and indeed Lucy and her Australopithecus afarensis contemporaries could have possessed similarly expanded calf muscles so that they could keep up some of their species's old arboreal habits even after the shift to bipedalism.

For more, check out the video up top, in which Dominy offers a good overview of the finding. Also, the complete paper is currently available at PNAS.

Photo of Twa man climbing by Nathaniel Dominy.