According to a new study, humans walking on two legs use only a quarter of the energy that chimps use while “knuckle-walking” on all fours. According to a new study led by UCL researchers, human bipedalism – walking upright on two legs – may have evolved in trees rather than on the ground as previously thought.
Researchers from UCL, the University of Kent, and Duke University in the United States investigated the behaviors of wild chimps – our closest living relative – living in the Issa Valley of western Tanzania, within the region of the East African Rift Valley. The chimps’ habitat, known as’savanna-mosaic’ – a mix of dry open land with few trees and patches of dense forest – is very similar to that of our earliest human ancestors and was chosen to allow scientists to investigate whether the openness of this type of landscape may 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 behaviour to other studies on their solely forest-dwelling cousins in other parts of Africa.
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.
Dr. Alex Piel
Overall, despite their more open habitat, the Issa chimps spent the same amount of time in the trees as other chimps living in dense forests, and were not more terrestrial (land-based) as expected. Furthermore, despite the researchers’ expectations that the Issa chimps would walk upright more in open savanna vegetation, where they could not easily travel through the tree canopy, more than 85% of bipedal occurrences occurred in the trees.
According to the authors, their findings contradict widely held beliefs that an open, dry savanna environment encouraged our ancestors to walk upright, and instead suggest that they evolved to walk on two feet to move around the trees.
“We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimps live, we would see individuals more frequently on the ground than in the trees,” said study co-author Dr. Alex Piel (UCL Anthropology). Furthermore, because 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 expected to see more bipedalism here as well. However, this is not what we discovered.
“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.”
Over the course of the 15-month study, the researchers recorded more than 13,700 instantaneous observations of positional behavior from 13 chimp adults (six females and seven males), including nearly 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging, etc.). They then investigated patterns of association by looking at the relationship between tree/land-based behavior and vegetation (forest vs woodland). Similarly, they recorded each instance of bipedalism and whether it was associated with being on the ground or in the trees.
The authors point out that walking on two feet distinguishes humans from other great apes, who “knuckle walk.” Despite their research, researchers say it is still unknown why humans, alone among the apes, began to walk on two feet.
“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,” said study co-author Dr Fiona Stewart (UCL Anthropology). That is not supported by our data.
“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.”
