Humans may have started walking upright on two legs in trees, and not on the ground, a new study has found. Bipedalism is a major type of locomotion, involving movement on two feet. Human bipedalism refers to walking upright on two legs.
Researchers from University College London, University of Kent, and Duke University explored the behaviours of wild chimpanzees, humans' closest living relatives, from the Issa Valley of western Tanzania, as part of the study. The chimpanzees' habitat within the region of the East African Rift Valley is known as "savanna-mosaic", and is very similar to that of earliest human ancestors. The habitat is called savanna-mosaic because it is a mix of dry open land with few trees and patches of dense forest. The researchers chose this region to explore whether the openness of this type of landscape could have encouraged bipedalism in hominins, a statement released by University College London said. Hominin is a term used to describe humans and extinct members of the human lineage.
The study describing the findings was recently published in the journal Science Advances.
Issa chimpanzees spent significant amounts of time in trees despite open savannas
The researchers explored if savanna-mosaic habitats would account for increased time spent on the ground by the Issa chimpanzees, as part of the study. They also compared the behaviour of Issa chimpanzees to other studies on their solely forest-dwelling cousins in other parts of Africa.
Issa chimpanzees spent as much time in the trees as other chimpanzees living in dense forests, the study found. The chimpanzees exhibited this behaviour, despite their more open habitat.
The researchers expected the Issa chimpanzees to walk upright more in open savanna vegetation. However, more than 85 per cent of occurrences of bipedalism took place in trees.
Prehistoric humans may have evolved to walk on two feet to move around trees
The findings contradict widely accepted theories that suggest that an open, dry savanna environment encouraged prehistoric human relatives to walk upright, the authors noted. Instead, the study suggests that prehistoric human relatives may have evolved to walk on two feet to move around the trees.
In the statement, study co-author Dr Alex Piel said researchers naturally assumed that bipedalism would be more prevalent on the ground than on trees in Issa Valley because the region has fewer trees than typical tropical forests, where most chimpanzees live. Also, many traditional drivers of bipedalism, such as carrying objects or seeing over tall grass, are associated with being on the ground. Hence, the researchers thought they would naturally see more bipedalism on the ground in Issa Valley as well. However, this is not what they found.
Piel added that the study suggests that trees probably remained essential to the evolution of bipedalism due to the need to search for food-producing trees.
How the study was conducted
The researchers recorded more than 13,700 instantaneous observations of positional behaviour from 13 chimpanzees, in order to establish their findings. They analysed six female and seven male chimpanzees over 15 months.
In order to investigate patterns of association, the researchers used the relationship between tree or land-based behaviour and vegetation. The researchers noted each instance of bipedalism and tried to determine if it was associated with being on the ground or in the trees.
Walking on two feet is a defining feature of humans when compared to other great apes, who "knuckle walk", the authors noted. The reason why humans alone amongst the apes first began to walk on two feet still remains a mystery, the authors further said.