Fossil study reveals when early humans stopped climbing trees and started walking

Archaic humans shifted from regularly climbing trees to walking upright on the ground 1.8 million years ago, according to a new study. Scientists from the University of the Witwatersrand in South Africa and the University of Southern California arrived at this finding by examining the internal strength of limb bones from fossil skeletons.

Bone Strength

The research looked at the bone shafts of each species to understand the physical demands placed on them and how much time they spent in trees. Researchers compared the bone strength ratios of arms and legs in fossils with those of modern humans, chimpanzees, gorillas, and orangutans. While a 3.67-million-year-old Australopithecus individual known as Little Foot had limb proportions like African tree-dwelling apes, a 1.8-million-year-old archaic human specimen from Georgia showed limb strengths closer to modern humans. Another famous fossil, the 3.2-million-year-old Australopithecus afarensis named Lucy from Ethiopia, showed limb strength proportions closer to chimpanzees, indicating heavier loads on forelimbs and more time spent in trees.

Reasons and Past Studies

Researchers suggest the shift to ground walking might be linked to changes in range, foraging behavior, and a greater need for lower limbs adapted to life on the ground. The paper also noted a probable shift toward using forelimbs for making stone tools and high-speed throwing for hunting. A previous 2020 study published in the Proceedings of the National Academy of Sciences had suggested that human ancestors could still have been regularly climbing trees two million years ago.