How did humans learn to walk upright?

Figuring out exactly how humanity's early ancestors learned to walk has been one of the great mysteries of biology. But new research published at the end of 2022 might have just revealed how our predecessors learned to move throughout their world on two feet.

Evolutionary biologists have always hypothesized that our capacity to walk upright preceded the development of larger brains in primates since bipedalism seemed like the obvious precursor to freeing our ancestor's hands to use and carry tools. 

The Father of today’s modern evolutionary biology Charles Darwin was the first to postulate about the link between bipedalism, brains, and tool use—and he wasn’t the last. 

Karl Ernst von Baer—founder of embryology and an all-around interesting early 19th-century scientist— confidently wrote in 1828 that “upright posture is only the consequence of the higher development of the brain.”

However, after studying wild chimpanzees in Tanzania, the authors of a new study published in Science Advances in 2022 concluded that early human ancestors may have begun walking upright as a means to move amongst tree branches rather than to carry tools. 

This new thesis completely throws out the old theory of bipedalism known as the Savanna Hypothesis—namely, that man’s early ancestors only developed bipedalism after they left the safety of their tree-top habitats to roam amongst the grassy savannas of prehistoric Africa. 

“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,” wrote study co-author Alexander Piel in a statement to the public. 

“Instead,” Piel added, “trees probably remained essential to its evolution—with the search for food-producing trees likely a driver of this trait.”

Piel noted in his statement to the public that his study, co-authored with a number of other leading anthropologists, was the first of its kind to actually study and test our understanding of the Savanna Hypothesis. 

In order to make their conclusions, the authors of the study spent time observing a specific group of savanna-oriented chimpanzees known as the Issa and compared their behavior to other studies that logged the behavior of other chimpanzee groups who lived in densely packed African forests. 

Issa chimpanzees live in the Issa Valley of western Tanzania and are special because of the habitat in which they live. The Issa Valley mimics the landscape that our early human ancestors would have faced millions of years ago, which included both open savanna landscapes as well as some densely packed forested areas. 

“Overall, the study found that the Issa chimpanzees spent as much time in the trees as other chimpanzees living in dense forests,” Piel noted. 

“Despite their more open habitat,” Piel added, the Issa chimpanzees “were not more terrestrial (land-based) as expected.”

During the 15-month experiment, the authors of the study observed the behavior of 13 adult chimpanzees and noted thousands of instances of movement that they compared against other research. In the end, they concluded that no real difference existed between Issa chimps and other chimpanzees.  

“Issa chimpanzees were no more terrestrial in woodland vegetation than chimpanzees living in forest habitats, suggesting that it is not a simple rule of less trees means more time on the ground,” the researchers write in their paper.

While the new theory still needs more rigorous testing, it does offer an interesting new insight into how early human ancestors may have developed, and it opens a new chapter in the exploration of our collective past.