Our Ancestors May Have Walked Upright Long Before Leaving The Trees

Early human ancestors may have first mastered walking upright on two legs in the trees, rather than on the hard ground of Africa’s ancient savannah.

Issa Valley chimpanzees and the origins of bipedalism

A first-of-its-kind study led by scientists at the Max Planck Institute for Evolutionary Anthropology in Germany has found an association between the foraging strategies of Tanzania’s Issa Valley chimpanzees in woodland habitats and how the apes move. The findings raise the possibility that our own bipedalism may initially have helped us reach upwards, not simply travel across open terrain.

Issa Valley chimpanzees live in savannah-mosaics: dry, open woodlands that resemble the palaeohabitats crossed by early hominins (a ‘tribe’ that includes modern humans and extinct ancestors). These are among the driest chimpanzee-inhabited regions on Earth, where grass fires burn more than 75 per cent of the landscape during the May-October dry season.

Rethinking the classic savannah narrative

The proposal runs against a long-standing picture of human evolution: as climates shifted and landscapes opened up, our ancestors were compelled to come down from the trees and trek across the savannah-exposed to large cats and unfamiliar conditions-in search of food and refuge.

"For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah," saysMax Planck Institute anthropologist Rhianna Drummond-Clarke.

So when, exactly, did early human ancestors begin walking, and what drove that change?

These remain two of the most compelling questions in palaeoanthropology. By examining how chimpanzees (Pan troglodytes schweinfurthii) forage in trees in relation to different tree types, the researchers behind this latest investigation have teased out fresh hints.

Tree foraging, food “processing”, and upright movement

Unexpectedly, the Issa Valley chimpanzees in savannah-mosaics spent a surprising amount of time in trees-comparable to chimpanzees living in densely vegetated forests. One reason is that their foods demand more “processing”: seeds often need to be extracted from pods first, and unripe fruit is more fibrous, requiring greater effort to consume.

The striking detail is how they get around aloft. Because chimpanzees are relatively large, they move through the canopy either by hanging beneath branches or by standing and walking upright while using nearby branches for stability.

The notably high levels of arboreality and branch-walking seen in these chimpanzees-also recorded previously in other populations-support hypotheses proposing that ancient apes and human ancestors could have inched towards habitual bipedalism (regular upright walking) within an arboreal setting.

What the fossil record does-and doesn’t-show

This view differs from the widely held explanation for why two-legged walking became so prominent.

Towards the end of the Miocene Epoch (23 to 5.3 million years ago), forests gave way to savannahs and, in theory, pushed hominins towards orthograde locomotion (upright walking) as a better way to move through a more open landscape where food resources were spaced further apart.

Unfortunately there is a shortage of hominid fossils dating around the end of the Miocene and beginning of the Pliocene circa 4 to 7 million years ago; a key period that may have seen a more widespread emergence of habitual bipedalism in response to this ecological change.

If anything, late Miocene fossil evidence indicates that several extinct hominins still retained adaptations for living in trees, such as relatively long upper limbs and curved fingers.

In addition, analyses of dental wear and carbon isotopes indicate that some hominins continued to depend heavily on tree-derived foods even while inhabiting open environments. Elements of their diets may have resembled those of living chimpanzees, making chimpanzees a particularly useful analogue for comparison.

A “training wheels” route to upright walking

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

Much like learning with stabilisers on a bicycle, human ancestors may have rehearsed their gait in the trees, where branches could be grasped for balance. Over time, this may have helped them develop the upright movement abilities needed to persist in increasingly open habitats with more limited food resources and, later on, to disperse to nearly every part of the world.

This research is published in Frontiers in Ecology and Evolution.