According to a study, human bipedalism may have evolved in trees rather than on the ground.

In the Issa Valley in western Tanzania, which is part of the East African Rift Valley, scientists investigated the behaviours of wild chimpanzees—our closest living relatives. (Illustrative Image)

Recent research suggests that human bipedalism, or standing upright on two legs, may have evolved in high trees rather than in more open landscapes as previously believed.

Researchers from UCL, the University of Kent, and Duke University in the United States examined the behaviours of wild chimpanzees—our closest living relatives—dwelling in the Issa Valley of western Tanzania, a region of the East African Rift Valley, for their study, which was published in the journal Science Advances. Chimpanzees are our closest living relatives.

The chimpanzees’ habitat, known as “savanna-mosaic,” is very similar to that of our earliest human ancestors and was chosen to allow the scientists to investigate whether the openness of this type of landscape may have encouraged bipedalism in hominins. Savanna-mosaic is a mixture of dry open land with few trees and patches of dense forest.

The study, which compares the behaviour of the Issa chimpanzees to that of studies on their only forest-dwelling cousins in other parts of Africa, is the first of its kind to investigate whether savanna-mosaic habitats would account for the increased time spent on the ground by the Issa chimpanzees.

Overall, the study discovered that, contrary to expectations, the Issa chimpanzees were not more terrestrial (land-based), but rather spent just as much time in the trees as other chimpanzees living in dense forests.

Additionally, even though the researchers anticipated that the Issa chimpanzees would walk more uprightly in open savanna vegetation, where they are unable to easily pass through the tree canopy, more than 85% of bipedalism incidents occurred in the trees.

According to the authors, their research defies generally accepted theories that contend that an open, dry savanna environment encouraged our ancestors to walk upright and instead suggests that they may have evolved to walk on two feet in order to navigate the trees.

“We naturally believed that since there are fewer trees in Issa than in the typical tropical forests where the majority of chimpanzees reside, we would encounter individuals more frequently on the ground than in the trees. Furthermore, we anticipated naturally seeing more bipedalism in this area as well because so many of the traditional drivers of bipedalism (like carrying objects or seeing over tall grass, for example) are connected with being on the ground. But this is not what we discovered.

“According to our research, bipedalism did not develop as a result of forests disappearing during the late Miocene–Pliocene era, around five million years ago, or of savanna habitats becoming more open. The search for food-producing trees is likely what drove this trait, as trees likely remained crucial to its evolution.”

Over the course of the 15-month study, the researchers collected more than 13,700 instantaneous observations of positional behaviour from 13 adult chimpanzees (six females and seven males). This data included nearly 2,850 observations of individual locomotor events (such as climbing, walking, hanging, etc.).

They then used the relationship between behaviour on trees and other land features and vegetation (forest vs. woodland) to look into association patterns. They also recorded every instance of bipedalism and whether it occurred on the ground or in trees.

The authors point out that humans’ ability to “walk on two feet” distinguishes them from other great apes that “knuckle walk.”Despite their research, scientists claim that it is still unclear why humans, not the other apes, were the first to start walking on two feet.

Study co-author Dr Fiona Stewart (UCL Anthropology) said: “To date, the numerous hypotheses for the evolution of bipedalism share the idea that hominins (human ancestors) came down from the trees and walked upright on the ground, especially in more arid, open habitats that lacked tree cover. That is not at all supported by our data.

“Unfortunately, the Issa data don’t support the conventional wisdom that more terrestriality (dwelling on land) is correlated with fewer trees. We will now turn our attention to how and why these chimpanzees spend so much time in the trees in order to start putting this intricate evolutionary puzzle together.