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One longstanding theory of human bipedalism is that it first developed in our ancient hominid ancestors as a way to more easily move around the African savannah—an ecosystem created by rising temperatures after the last Ice Age.
Now, a new study suggests that bipedalism likely emerged as primates navigated woodland trees in search of food, standing on hind legs and balancing with branches to reach the best seeds and fruits.
Scientists supported this conclusion by studying a population of chimpanzees in the Issa Valley in Tanzania—an ecosystem similar to the one that likely existed millions of years ago when bipedalism first began to take shape.
Although we are a member of the Great Ape primate family, we sport an array of differences from our closest genetic cousins. Our most obvious distinguishing feature is our big brain, which is some three times larger than a chimpanzee’s, whose genetic similarity to Homo sapiens is somewhere in the high 90 percent range. However, the human brain may not have grown to such an astonishing size if it wasn’t for our second most distinguishing feature—bipedalism.
Millions of years before the arrival of the genus Homo, some early hominids had the bright idea to free up their hands and start walking upright. Of course, this was a slow process—one that scientists believe was likely kickstarted by climate change, as a warming world turned the forests of equatorial Africa into dry woodland called “savannah-mosaic.” As trees became more sparse, distant human ancestors began walking upright as a more efficient way to move in this new environment.
Or, at least, that’s what we thought.
Now a new study from scientists at University College London, the Max Planck Institute, and Greater Mahale Ecosystem Research and Conservation in Tanzania shows that the story of human bipedalism isn’t so clear-cut. To get a better understanding of what may have taken place six to seven million years ago, scientists studied a population of chimpanzees in Issa Valley along Lake Tanganykia in western Tanzania. The area is perfect for studying bipedalism’s origins, as the area is made up of gallery forests and woodland savannah.
They found that chimpanzees often adopted bipedalism to effectively feed on the sparse woodland trees. This could have been a key driver for ancient hominids developing their upright gait. The results were published in the journal Frontiers, and support the team’s previous 2022 findings that first began to piece together this arboreal bipedalism origin story.
“For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah,” Rhianna Drummond-Clarke, lead author of the study, said in a press statement. “Here we show that safely and effectively navigating the canopy can remain very important for a large, semi-arboreal ape, even in open habitat. Adaptations to arboreal, rather than terrestrial, living may have been key in shaping the early evolution of the human lineage.”
During the dry season, modern chimpanzees drift from the forested riverbanks to the savannah-mosaic in search of food. The research team monitoring the adults in the Issa Valley population, taking note of what types and trees and fruits they munched on in the woodlands, and found that they spend the most time in larger trees. They also used specialized techniques—such as standing on branches—to reach highly prized food in the tree’s large, open crown.
“We suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat,” Drummond-Clarke said in a press statement. “Observational studies of great apes demonstrate they can walk on the ground for a few steps, but most often use bipedalism in the trees. It’s logical that our early hominin relatives also engaged in this kind of bipedalism, where they can hold onto branches for extra balance.”
The authors admit to some limitations of their theory. For instance, these conclusions only apply to the Issa Valley chimpanzees (so more data would be needed to apply this idea to chimps more broadly), and the feeding patterns were only analyzed during the dry season. The fossil record is also incomplete around the time when African forests initially transitioned to savannah-mosaic.
Although we walk on to legs to navigate our terrestrial world today, our upright gait may actually be an evolutionary relic of our tree-climbing past.
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