A new study suggests that early humans may have evolved to walk upright in trees, not on the ground

According to the authors, their findings "contradict widely accepted theories" that suggest an open environment encouraged prehistoric humans to walk upright – and instead suggest that they may have evolved to walk on two feet to move around the trees.

Issa chimpanzees did not prefer walking on land over trees

The study found that the Issa chimpanzees spent the same time in trees as other chimpanzees living in forests, despite their open habitat. They did not prefer the land over trees.

Interestingly, though the researchers expected the chimpanzees to walk on two legs more in the open landscape, where they cannot easily travel via the tree canopy, more than 85 percent of occurrences of bipedalism took place in the trees, the release said.

"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," study co-author Dr. Alex Piel (UCL Anthropology) said in a statement.

Why do the chimpanzees spend so much time in the trees?

The researchers recorded more than 13,700 instantaneous observations of positional behavior from 13 chimpanzee adults (six females and seven males), including almost 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging, etc.), throughout the 15-month study. 

The relationship between tree/land-based behavior and vegetation was used to explore the patterns of association. Each instance of bipedalism was also noted, and it was associated with being on the ground or in the trees.

Study co-author Dr. Fiona Stewart (UCL Anthropology) said: “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."

However, the study does not answer all questions. "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," said Stewart.

Study Abstract:

Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees (Pan troglodytes schweinfurthii) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy.