Climbing is for the forest, and walking is for the savannah. That’s the basic premise behind one of the biggest theories of human evolution, which holds that our ancient ancestors transitioned from climbing in the trees to walking on the ground when climate change transformed their environment from a forest to a savannah.
But a new study in Frontiers in Ecology and Evolution suggests that things may not have been so simple, as many early hominins maintained their arboreal adaptations once they developed their terrestrial ones.
Looking at the chimpanzees in the Issa Valley in Tanzania, the study confirms that chimpanzees climb trees to find food in the savannahs — a behavior that may explain why many early hominins were climbers as well as walkers. The study also suggests that chimpanzees need to make the most of the trees that they climb in the savannahs (since trees are few and far between), encouraging a method of movement that may have encouraged the bipedalism of our ancient ancestors.
“For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across an open savannah,” said Rhianna Drummond-Clarke, a study author and a researcher at the Max Planck Institute of Evolutionary Anthropology, according to a press release. “Here we show that safely and effectively navigating the canopy can remain very important for a large, semi-arboreal ape, even in open habitat. Adaptations to arboreal, rather than terrestrial, living may have been key in shaping the early evolution of the human lineage.”
Read More: Chimpanzees Could Answer Why Humans Evolved to Walk Upright
Chimpanzee Clues into Hominin Locomotion
Of course, why the early hominins stopped climbing trees and started walking on the ground is difficult to explain. But modern chimpanzees in the Issa Valley in Tanzania can help. During the dry seasons, these chimpanzees live in a savannah mosaic that’s a lot like the environment that our ancient ancestors probably lived in, meaning that their lives may help us understand our own evolutionary past.
So, to learn more about the movement of the early hominins, Drummond-Clarke and her colleagues monitored the Issa Valley chimpanzees, measuring where and how they found their food. The results confirmed their findings from previous studies, which showed that chimpanzees spent a significant amount of time foraging in the trees.
In addition, the results revealed that the chimpanzees tended to forage in larger and richer trees — those with an abundance of food. They also revealed that the chimpanzees adapted their movements to travel over the thin branches of these trees safely, allowing them to eat the trees’ fruits, leaves, and flowers. Indeed, to get to these foods, the chimpanzees didn’t climb. Instead, they hung under or stood over the branches, holding onto other branches for support.
Read more: Sahelanthropus Was The First Hominin To Walk On 2 Feet
Upright in the Trees
Though these types of upright movements are seen in forests, too, they might have been especially beneficial in a savannah, where trees are scarce. “Savannah-mosaics are characterized by more sparsely distributed trees,” Drummond-Clarke said in the release, “so we hypothesized that adapting behavior to forage efficiently in a tree would be especially beneficial when the next tree is further away.”
According to the team, if a similar type of upright movement was developed by early hominins when they first transitioned into the savannahs, it may have helped them develop their bipedalism.
“We suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat,” Drummond-Clarke said in the release. “Observational studies of great apes demonstrate they can walk on the ground for a few steps, but most often use bipedalism in the trees. It’s logical that our early hominin relatives also engaged in this kind of bipedalism, where they can hold onto branches for extra balance.”
Drummond-Clarke adds that more research is needed to test this idea, both from a paleontological and an observational perspective.
“This is also only one community of chimpanzees,” she said in the release. “Future studies of other chimpanzees living in such dry, open habitats will be vital to see if these patterns are truly a savannah-mosaic signal or unique to Issa.”
Read More: Ancient Ape’s Inner Ears May Hold Key to Evolution of Walking on Two Feet
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
