Savannah-living chimpanzees suggest human bipedalism evolved in the trees

A new study of wild chimpanzees in Tanzania led by Kent PhD student, Rhianna Drummond-Clarke, suggests that an open, dry savannah environment did not encourage our fossil human relatives to walk upright.

Bipedalism, or walking on two feet, is a defining feature of humans when compared to other apes like chimpanzees, gorillas and orangutans, who walk on all fours. It is also a defining trait of our earliest fossil ancestors, scientifically called hominins.

However, why we alone amongst the apes began to walk on two feet remains a mystery. Numerous hypotheses as to why our ancestors started moving bipedally have been proposed, including to save energy when traveling, to carry objects, to reach for fruit, or to see over tall grass. All of these hypotheses have one thing in common – they are based on our ancestors coming down from the trees to first walk upright on the ground as their habitats became more open and dry and trees retreated.

To explore ‘why’ bipedalism may have first evolved on the ground, Rhianna Drummond-Clarke and colleagues from the University of Kent’s School of Anthropology and Conservation as well as University College London, UK and Duke University, USA studied the behaviours of wild chimpanzees living in a similar environment to that of our earliest human ancestors – a drier habitat with fewer trees mixed with denser forest, called a ‘savannah-mosaic’.

This study was the first to explore if a savannah-mosaic habitat would account for increased time spent on the ground by our closest living relative, the chimpanzee. While similar studies on how chimpanzees move within their habitats have been conducted by other researchers, all of these studies are on chimpanzees living in dense forests only.

The team investigated the behaviour of wild chimpanzees living in the savannah-mosaic habitat in the Issa Valley of western Tanzania, a habitat very similar to the habitats of early hominins.

It was expected that the Issa chimpanzees would spend more time on the ground and walk upright on two feet more in open savannah vegetation where they cannot easily travel via the tree canopy, like they can in the forests. Moreover, when compared to their forest-dwelling cousins in other parts of Africa, it was expected that the Issa chimpanzees would be more terrestrial overall.

Instead, compared to chimpanzees living in forest sites, Issa chimpanzees did not spend more time on the ground. The Issa chimpanzees spent just as much time, if not more, in the trees as the forest-dwelling chimpanzees. Moreover, when they used bipedalism, it was almost always in the trees, rather than on the ground, as predicted.

The research reveals that it is not a simple rule of fewer trees leads to more time on the ground. While it remains unclear why Issa chimpanzees spend so much time in the trees, despite their more open habitat, the study suggests that the earliest hominins may have started walking on two feet first in the trees and that a more open, dry habitat was not necessary for bipedalism to evolve in the human evolutionary story.

Rhianna Drummond-Clarke is a PhD student in Social Anthropology and Conservation, working with Professor Tracy Kivell – a palae0anthropologist who studies the evolution of the postcranial skeleton in living and fossil primates, including our human ancestors – hominins.

The paper ‘Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism‘ is published via Science Advances.