Early humans may have first walked upright in trees

Human bipedalism – walking upright on two legs – may have evolved in trees, not on the ground as previously thought, according to a new study involving researchers from UCL.

In the study, published today in the journal scientific advances, researchers from UCL, the University of Kent and Duke University in the United States explored the behaviors of wild animals chimpanzees—our closest living relative—from the Issa Valley in western Tanzania, in the East African Rift Valley region.

Known as the “savannah mosaic” – a mixture of dry open land with little trees and patches of dense forest – the habitat of chimpanzees is very similar to that of our earliest human ancestors and was chosen to allow scientists to explore whether the openness of this type of landscape might have encouraged bipedalism in hominins.

The study is the first of its kind to explore whether savanna-mosaic habitats would explain the increased time spent on the ground by Issa chimpanzees, and compares their behavior to other studies of their forest-only cousins ​​in other parts of Africa.

Overall, the study found that Issa’s chimpanzees spent as much time in trees as other chimpanzees living in dense forests, despite their more open habitat, and were no more terrestrial (terrestrial) than intended.

Additionally, although researchers expected Issa’s chimpanzees to walk more upright in open savannah vegetation, where they cannot easily move via the tree canopyover 85% of bipedalism occurrences occurred in trees.

The authors say their findings contradict widely accepted theories that suggest it was an open, dry savannah environment that encouraged our prehistoric human relatives to walk upright – and instead suggest they may have evolved to walk on two feet to move around the trees.

Study co-author Dr Alex Piel (UCL Anthropology) said: “We naturally assumed that because Issa has fewer trees than typical rainforests, where most chimpanzees live, we would see individuals more often on the ground than in the trees. Because so many of the traditional drivers of bipedalism (like carrying objects or seeing over tall grass, for example) are associated with being on the ground, we figured we’d naturally see more bipedalism here too. we have found.

“Our study suggests that late Miocene-Pliocene forest retreat about five million years ago and more open savannah habitats were not in fact a catalyst for the evolution of bipedalism. Instead of this, trees likely remained essential to its evolution – with the search for food-producing trees likely driving this trait.”

To reach their conclusions, the researchers recorded more than 13,700 instantaneous observations of the positional behavior of 13 adult chimpanzees (six females and seven males), including almost 2,850 observations of individual locomotor events (for example, climbing, walking, suspend, etc.), over the course of the 15 month study. They then used the relationship between arboreal/terrestrial behavior and vegetation (forest vs woodland) to investigate patterns of association. Likewise, they noted each instance of bipedalism and whether it was associated with being on the ground or in trees.

The authors note that walking on two feet is a defining characteristic of humans compared to other great apes, which “walk on joints.” Yet despite their study, the researchers say why the only humans among the apes began to walk on two feet still remains a mystery.

Study co-author Dr Fiona Stewart (UCL Anthropology) said: “To date, the many hypotheses about the evolution of bipedalism share the idea that hominids (human ancestors) descended from trees and walked upright on the ground, especially in more arid, open habitats lacking tree cover, which our data does not support at all.

“Unfortunately, the traditional idea that fewer trees equals more land (land dwelling) is simply not supported by Issa’s data. What we need to focus on now is how and why these chimpanzees spend so much time in the trees – and that’s what we’ll focus on next as we piece together this complex evolutionary puzzle.”