Early humans may have begun to stand upright in trees

One of the features that sets humans apart from all other apes is the way they move around on the ground. They walk upright, placing one foot after the other and holding the torso erect. Other apes may adopt an upright stance at times, and may walk on two back legs for a short distance, but none of them has committed fully to bipedal locomotion. Since this was the first major adaptive change from the apelike ancestors of modern humans, it would be helpful to know why bipedalism evolved. 

The savanna-based theory is one that is commonly used to suggest why early hominids began to walk upright. It postulates that climate change led to a reduction in forests and a proliferation of more open, savanna habitats. In savannas, a bipedal posture and locomotion would have allowed hominins (human ancestors) to watch over tall grasses, carry things, hunt effectively and be more aware of predators. This theory has had a lot of support over the years, but has also had its detractors. 

In a new study, researchers from the University College London, the University of Kent, and Duke University, USA, the behaviors of wild chimpanzees living in the Issa Valley of western Tanzania, have been observed over a 15-month period. Chimpanzees are our closest living relatives, but are not on the same evolutionary line as modern humans. Instead, chimpanzees and humans share a common ancestor that lived somewhere around 7 million years ago. 

The Issa Valley falls within the East African Rift Valley region and has a vegetation type known as “savanna-mosaic.” This a mix of dry open land with few trees, interspersed with patches of dense forest, and is considered very similar to the habitat in which our earliest human ancestors lived. The researchers chose to study chimpanzees in this particular habitat to explore whether the openness of this type of landscape could have encouraged bipedalism in hominins.

The study is the first of its kind to explore if savanna-mosaic habitats would account for increased time spent on the ground by the Issa chimpanzees, and compares their behavior to other studies on forest-dwelling chimpanzees in other parts of Africa.

The researchers recorded more than 13,700 instantaneous observations of positional behavior from 13 chimpanzee adults (six females and seven males), including almost 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging). In particular, they noted each time a chimpanzee stood upright on its back legs and recorded whether this was associated with being on the ground or in the trees. 

The results of the study, published in the journal Science Advances, showed that the Issa chimpanzees did not spend more time on the ground, as the researchers had expected. In fact, they spent as much time in the trees as other chimpanzees that live in dense forests, despite the fact that open savanna habitat was available.  Furthermore, although the researchers expected the Issa chimpanzees to walk upright more often in the open savanna vegetation, where they cannot easily travel via the tree canopy, more than 85 percent of instances of bipedalism took place in the trees, when chimpanzees reached up to pick food or balanced themselves against upright trunks. 

The authors say that their findings contradict the widely accepted theory that it was an open, dry savanna environment that encouraged our prehistoric human relatives to walk upright. Instead, their results suggest that early hominins may actually have evolved to walk on two feet in order to move around in the trees.

“We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimpanzees live, we would see individuals more often on the ground than in the trees. Moreover, because so many of the traditional drivers of bipedalism (such as carrying objects or seeing over tall grass, for example) are associated with being on the ground, we thought we’d naturally see more bipedalism here as well. However, this is not what we found,” said study co-author Dr. Alex Piel.

“Our study suggests that the retreat of forests in the late Miocene-Pliocene era around five million years ago, and the more open savanna habitats, were in fact not a catalyst for the evolution of bipedalism. Instead, trees probably remained essential to its evolution – with the search for food-producing trees a likely a driver of this trait,” he added.

“To date, the numerous hypotheses for the evolution of bipedalism share the idea that hominins came down from the trees and walked upright on the ground, especially in more arid, open habitats that lacked tree cover. Our data do not support that at all,” said study co-author Dr. Fiona Stewart.

“Unfortunately, the traditional idea of fewer trees equals more terrestriality (land dwelling) just isn’t borne out with the Issa data. What we need to focus on now is how and why these chimpanzees spend so much time in the trees – and that is what we’ll focus on next on our way to piecing together this complex evolutionary puzzle.”

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By Alison Bosman, Earth.com Staff Writer