The predominant image of human evolution is one that involves our early hominid ancestors leaving the trees and taking their first steps — on two legs — on land. But a new study challenges this widely held notion about how bipedalism — walking upright on two legs — began. A team of researchers from the University College London, the University of Kent, and Duke University, contend that early humans began to walk upright in the trees instead.

Exploring the behavior of wild chimpanzees in the Issa valley of Tanzania, the study published in Science Advances observes that despite the region containing larger expanses of ground than tree cover — known as the savanna-mosaic — chimpanzees moved upright in the trees more than they did on the ground. They specifically chose wild chimpanzees, moreover, because they’re our closest relatives, evolutionarily speaking.

The savanna-mosaic is a landscape characterized by openness, with a few patches of grass and forest cover. The researchers hypothesized that openness encouraged early hominids to spend more time on the ground, and thus evolve into bipedal beings. “We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimpanzees live, we would see individuals more often on the ground than in the trees. Moreover, because so many of the traditional drivers of bipedalism (such as carrying objects or seeing over tall grass, for example) are associated with being on the ground, we thought we’d naturally see more bipedalism here as well. However, this is not what we found,” said study co-author Alex Piel, an anthropologist from UCL.

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It may well be that trees were essential to our evolution — not, contrary to popular belief, open ground without tree cover. We still don’t know why this is, however. But the idea is significant because it challenges the idea that land-dwelling is synonymous with fewer trees and more open spaces. The dominant idea about how we came to walk on two legs came from Peter Rodman and Henry McHenry, paleontologists at the University of California, Davis, in 1980 — who suggested that shrinking forest cover compelled our ancestors to navigate the open grounds between forests. Bipedal movement was the most energy-efficient way, they said.

This isn’t a theory that’s entirely disproved with the new suggestion. In 2007, researchers from the University of Birmingham observing Sumatran orangutans found that they used “hand-assisted bipedalism” to navigate tree canopies themselves. The ancestors of chimps and gorillas may have evolved to cross trees from above the ground using this method, whereas human ancestors may have approached the forest floor.

But this doesn’t mean that our ancestors left tree-climbing behind entirely. Fossil evidence suggests that tree-climbing traits and adaptations for walking upright coexisted. Moreover, modern hunter-gatherer human communities, too, retain this ability — complicating the dichotomy between arboreality and terrestriality, and the story of our evolution as we’ve understood it so far.