LONDON — Plenty of people prefer keeping their feet planted firmly on the ground, but it turns out early humans may have actually began walking upright on two legs in the trees, according to a new study. Scientists at University College London say their findings suggest human bipedalism may not have evolved on solid ground after all.
In collaboration with researchers from the University of Kent, and Duke University, the team at UCL observed and analyzed the behaviors of wild chimpanzees, our closest living relatives. The chimps were living in the Issa Valley of western Tanzania, within the region of the East African Rift Valley. This area, known as “savanna-mosaic,” is a mix of dry open land with few trees and areas of dense forest. This location was chosen because it is quite similar to the surroundings our early ancestors lived in. Researchers intended to determine if the openness of this type of landscape may promote bipedalism in early humans.
This is the first project of its kind to assess specifically if savanna-mosaic habitats may explain increased time spent on the ground by the Issa chimpanzees. Researchers also compared the chimps’ behaviors to other studies focusing on their solely forest-dwelling cousins in other parts of Africa. Their shows that Issa chimpanzees spend just as much time in the trees as other chimps that live in dense forests. So, despite a more open habitat, the Issa chimps were not more terrestrial (land-based).
While the research team predicted the Issa chimpanzees would tend to walk upright more in open savanna vegetation, where traveling via tree canopy isn’t as easy, in actuality over 85 percent of occurrences of bipedalism took place in the trees.
‘Search for food-producing trees’ may have led to early humans walking upright
These findings are noteworthy for a number of reasons. Study authors explain this work contradicts widely accepted theories stating that it was an open, dry savanna environment that ultimately led to the prehistoric relatives of humans beginning to walk upright. Instead, this study suggests our ancestors may have started standing upright to move 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. 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,” says study co-author Dr. Alex Piel (UCL Anthropology) in a media release.
“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,” he continues.
To facilitate these findings, over 13,700 instantaneous observations of positional behavior from 13 chimpanzee adults (six females and seven males) were recorded. Over the course of 15 months, those recordings covered almost 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging, etc.). The relationship between tree/land-based behaviors and vegetation (forest vs woodland) was used by researchers to investigate association patterns. Each instance of bipedalism was also recorded, as well as if it was associated with being on the ground or in the trees.
Walking upright on our two feet, of course, is a defining feature of humans, even in comparison to great apes that tend to “knuckle walk”. Researchers note that exactly why only humans evolved to walk upright on two feet is still a mystery.
“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. Our data do not support that at all,” adds study co-author Dr Fiona Stewart (UCL Anthropology). “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,” she concludes.
The study is published in Science Advances.