Walking upright may have started in the trees, not on land

Why did our earliest ancestors begin walking upright? For decades, the textbook answer pointed to shrinking trees and widening grasslands.

As East Africa dried out, the story went, apes were forced onto open ground, and two-legged strides proved more efficient than knuckle-walking over the hot savanna.

Yet fossil ankles and finger bones tell a more complicated tale. Some of the first hominins able to stand tall – species such as Ardipithecus and Australopithecus – still carried shoulders, curved fingers, and flexible feet ideal for climbing. If the trees were gone, why keep the tree gear?

Human ancestors in patchy forests

A recent field study of chimpanzees in western Tanzania offers a fresh piece of the puzzle.

Working in the remote Issa Valley, a team led by Rhianna Drummond-Clarke from the Max Planck Institute for Evolutionary Anthropology followed a community of wild chimps.

These chimpanzees live in a habitat that is strikingly similar to the patchy woodland thought to be inhabited by early hominins six to eight million years ago.

Feeding keeps chimps in trees

The results show that even when the landscape looks open and the next trunk stands many feet away, large apes still spend huge chunks of their day aloft. They balance, hang, and sometimes stand bipedally on branches to reach fruit, leaves, and seeds.

“For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savanna,” Drummond-Clarke said.

“We show 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.”

Chimps mapped branch by branch

The Issa Valley lies on a high plateau northwest of Lake Tanganyika. Rainfall is seasonal. Along streams, ribbon-like galleries of evergreen forest persist, but the hills between are covered by “savanna-mosaic” – tall grasses studded with single trees or small clumps.

Previous GPS and accelerometer work by the same team revealed that Issa chimpanzees spend just as much time climbing as their cousins in dense equatorial jungle. The new study set out to ask why.

Observers shadowed adult males and females through the 2022 dry season, recording every feeding bout in detail. They measured each tree’s height, trunk diameter, crown spread, and the thickness of branches that chimps ventured onto.

Back at camp, the researchers sorted photographs and notes into a vast spreadsheet. They recorded what food item was taken, how long the ape stayed, and which postures – hanging, clambering, upright balancing – were used to reach it.

Fruit drives upright tree behavior

Patterns soon emerged. The animals are, first and foremost, fruit connoisseurs. Soft pulp accounted for most mouthfuls, followed by young leaves and occasional flowers or seeds. Those items grow at the very tips of branches, where supports taper from wrist-thick to finger-thin.

To exploit them safely, chimps often adopt a suspensory style. They hang or stand bipedally, using nearby limbs or the trunk for support while reaching for food.

The longer a feeding session lasted, the larger and more food-laden the tree tended to be. Large fruit-laden trees, full of ripe figs or drupes, tempted chimps to invest extra time and more acrobatic maneuvers.

“We suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat,” Drummond-Clarke explained. “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.”

Trees offered food and safety

Classic depictions of human origins show early humans standing upright on an empty plain, scanning for predators. The Issa observations point to a more nuanced scenario: mixed woodland offered both risk and reward.

Staying in trees gave access to rich food, making traits like long arms, grasping toes, and flexible spines advantageous. Bipedal balance on a branch could be a stepping-stone, literally and figuratively, toward efficient walking on the ground.

The study also underscores the role of seasonality. During the dry months the Issa chimps actually used the open miombo woodland more than the riverine forest, because fruiting peaks there at that time. Whether wet-season patterns look the same remains to be tested.

“This study only looked at foraging behavior during the dry season,” Drummond-Clarke cautioned.

She and her colleagues plan to replicate their methods during rains, add nutritional analysis of foods, and compare Issa chimps with other savannah-dwelling populations in Uganda and Senegal.

Habitats with evolutionary insights

Understanding how today’s apes solve the challenges of a drying world carries modern as well as paleoanthropological relevance.

East Africa is warming; woodlands are being fragmented by farms and roads. If chimps rely heavily on large fruiting trees even in sparse habitat, conserving those keystone species becomes critical.

Protecting corridor forests could safeguard not only an endangered ape but also a living analogue for our own distant past.

“Importantly, this is also only one community of chimpanzees,” Drummond-Clarke noted. “Future studies of other chimpanzees living in such dry, open habitats will be vital to see if these patterns are truly a savanna-mosaic signal or unique to Issa.”

Whatever the outcome, the Issa data remind us that the path to humanity did not leap instantly from branch to prairie. It meandered through a landscape where climbing and walking, hanging and balancing, all mattered – where survival meant mastering both the ground beneath and the canopy above.

The study is published in the journal Frontiers in Ecology and Evolution.

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