The ability to walk on two legs, or bipedalism, is the distinguishing trait of the human species.
Paleontologists have long held the theory known as the Savannah hypothesis, that the evolution of bipedalism started with the fall of knuckle-walking primates from trees to the ground.
While chimpanzees and gorillas still walk on all fours, our ancestors ultimately learned to walk upright. Bipedalism is considered to have evolved at least 4.5 million years ago.
The study, which was published today in the journal Science Advances, examined the habits of wild chimpanzees — our closest living cousin — in the Issa Valley of western Tanzania, near the East African Rift Valley.
The chimpanzees’ 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 so that scientists could investigate whether the openness of this type of landscape may have encouraged bipedalism in hominins.
This study is the first to investigate whether savanna-mosaic habitats may lead to increased time spent on the ground by Issa chimpanzees. It compares their behavior to that of other chimpanzees in Africa that only live in forest habitats.
The research concluded that the Issa chimpanzees were not more terrestrial (land-based) than chimpanzees living in thick forests, despite spending as much time in the trees as their forest-dwelling counterparts.
Additionally, more than 85% of cases of bipedalism took occurred in the trees, despite the fact that the researchers anticipated the Issa chimpanzees to walk upright more in open savanna vegetation, where they cannot readily traverse through the tree canopy.
The authors say that their research goes against the widely held belief that our prehistoric ancestors learned to walk upright because they lived in an open, dry savanna. Instead, their research suggests that they may have learned to walk on two feet so they could get around the trees.
“We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimpanzees live, we would see individuals more often on the ground than in the trees,” says co-author Dr. Alex Piel. “Moreover, because so 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 thought we’d naturally see more bipedalism here as well. However, this is not what we found.
The findings of the study, according to the co-author suggest “ 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.”
More than 13,700 instantaneous observations of positional behavior were collected from 13 adult chimpanzees (6 females and 7 males) during the duration of the 15-month study, including over 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging, etc.). After that, they investigated patterns of association by using the connection between tree/land-based behavior and the kind of flora present (forest vs. woodland). In the same way, they wrote down each time a creature walked on two legs and whether it was on the ground or in the trees.
The authors point out that humans’ ability to “walk on two feet” distinguishes them from other great apes that “knuckle walk.” Despite their studies, scientists claim that it is still unclear why humans, not the other apes, were the first to start walking 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,” says co-autho Dr. Fiona Stewart.
“Our data do not support that at all.
“Unfortunately, the traditional idea of fewer trees equals more terrestriality (land-dwelling) just isn’t borne out with the Issa data,” adds the co-author.
“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.”
Source: 10.1126/sciadv.add9752
Image Credit: Getty