Long ago, Earth’s climate started reshaping lush forests into dry, scattered woodlands, a savannah-mosaic. Think fewer trees, more open land. Scientists have long puzzled over how this shift nudged our ancestors toward walking on two legs. But there’s a catch: fossil records are sketchy for that time, and many early humans could climb and walk upright. So, when did the big leap happen, and why?
Enter the Issa Valley chimpanzees. Living in similar savannah conditions today, they still climb trees a lot, especially when searching for precious food. This suggests that ancient hominins may have retained their tree-climbing skills even as the world beneath them changed, walking on two legs when needed, but not ready to let go of the branches fully.
When ancient forests began thinning into open savannahs, early humans faced a big decision: stay in the trees or take their chances on the ground. While it made sense to walk upright for faster travel and survival, a new study shows that our tree-loving instincts didn’t vanish overnight.
Modern chimpanzees living in similar open environments still spend most of their time foraging in trees, especially for the good stuff. This hints that our ancestors, even as they learned to walk, held tight to their climbing habits.
Chimpanzees spontaneously dance to music
Far from being a relic, tree skills may have helped fuel the transition to upright walking by offering survival advantages when food was high and danger was low. In other words, bipedalism wasn’t a sudden evolutionary flip; it was a slow climb down, with plenty of swings along the way.
Dr Rhianna Drummond-Clarke of the Max Planck Institute for Evolutionary Anthropology said, “For decades, it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah. 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.”
In Tanzania’s Issa Valley, the landscape is a mix of thick forest near rivers and open woodlands. During the dry season, chimpanzees head into the woodland to find more food. Their habitat and diet are surprisingly similar to those of ancient early humans, so their behavior might offer clues to how our ancestors lived.
Researchers were curious: if trees are spread out in this open terrain, why do these chimps still spend so much time climbing? Turns out, foraging in trees is still worth the effort, especially when the tastiest food like fruits, flowers, and seeds grows way out on the skinny ends of branches. The chimps have to be nimble climbers to reach those treats safely.
The sounds of chimpanzees are combined to convey new meanings
They spent the most time in trees that were big, tall, and packed with food. So even in an environment where trees are far apart, these chimps showed specialized climbing strategies to get high-value meals. It’s a smart trade-off; the harder the climb, the richer the reward.
This behavior might help explain why early humans held onto climbing skills long after they started walking on two legs. Evolution wasn’t just about getting around; it was about staying fed, too.
Chimpanzees are big animals, so moving around in trees is a bit of a balancing act. Instead of scrambling along thin branches, they hang below them or stand upright and grab nearby limbs for support.
These behaviors are typically linked with deep forests, but in Issa Valley’s patchy, savannah-like habitat, they’re surprisingly helpful too.
Wild chimpanzees and the healing power of medicinal plants
Researchers believe that walking upright, our bipedal gait, may have continued evolving in the trees, not just on the ground. Why? Because apes often walk upright while holding branches in trees for stability. If early humans behaved like Issa’s chimpanzees, then climbing and walking upright in the trees may have been key to surviving in a landscape full of scattered, food-rich trees.
That said, the scientists stress the need for more proof as the study only covered the dry season; wet season behaviors could be different. Also, a deeper look at the nutritional value of chimp foods could reveal whether the effort of climbing is worth the energy gained.
And because Issa is just one chimp community, more studies in similar habitats are needed to know if this is a wider pattern or a local quirk.
Journal Reference:
- Rhianna C. Drummond-Clarke, Susan Chege Reuben et al. Foraging strategy and tree structure as drivers of arboreality and suspensory behaviour in savannah-dwelling chimpanzees. Frontiers in Ecology and Evolution. DOI: 10.3389/fevo.2025.1561078