Human bipedalism – walking upright on two legs – may have evolved in trees, and not on the ground as previously thought, according to a new study involving researchers from London’s Global University (UCL) in the Britain.
In the study, researchers from UCL, the University of Kent and Duke University, USA, explored the behaviors of wild chimpanzees – our closest living relative – living in the Issa Valley of western Tanzania, within the East African Rift Valley, he writes EurekAlert.
Known as a ‘mosaic savanna’ – a mixture of dry open land with few trees and patches of dense forest – the chimps’ habitat 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 may have encouraged bipedalism in hominins.
The study, published in the journal Science Advancesis the first of its kind to look at whether savanna-mosaic habitats could explain the greater time spent on the ground by Issa’s chimpanzees, and compares their behavior with other studies of their exclusively forest-dwelling cousins in other parts of Africa.
What did chimpanzee behavior reveal?
Overall, the study found that Issa chimpanzees spent as much time in trees as other chimpanzees living in dense forests, despite their more open habitat, and were not more terrestrial, as expected.
Furthermore, although researchers expected chimpanzees to walk more upright in open savanna vegetation, where they cannot easily move through the tree canopy, more than 85% of bipedal occurrences occurred in trees.
The authors say their findings contradict widely accepted theories that suggest it was the open, dry savannah environment that encouraged our prehistoric human relatives to walk upright – and suggest instead that they may have evolved to walks on two legs through the trees.
Walking on two legs, a defining characteristic of humans
To establish their conclusions, the researchers recorded more than 13,700 observations of positional behavior from 13 adult chimpanzees (six females and seven males), including nearly 2,850 observations of individual locomotor events (eg, climbing, walking, clinging, etc. ), during the 15 months of study.
They then used the relationship between tree/terrain-based behavior and vegetation to investigate patterns of association. Similarly, they noted each instance of bipedalism and whether it was associated with being on the ground or in trees.
The authors note that bipedal walking is a defining characteristic of humans compared to other great apes. However, despite their study, the researchers say why humans, the only apes, first started walking on two legs remains a mystery.