EARLY humans may not have walked on their knuckles before they stood up on two feet. Instead, our ancestors walked upright in the trees and then set foot on the ground, according to a study comparing the wrist bones of our closest relatives.
Sceptics of this 鈥渢rees to two feet鈥� idea argue that the ancestor of all African apes, including gorillas, chimps and humans, must have been a , because both gorillas and chimps knuckle-walk. They also have specialisations such as ridges and concavities on the wrist bones that keep the palm of the hand from collapsing onto the ground when bearing weight. It seems reasonable to think that bipedal walking in humans evolved from this intermediate stage, they argue.
Not so, says , a palaeoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. She and her colleague Daniel Schmitt of Duke University in Durham, North Carolina, compared the wrist bones of chimps, bonobos, gorillas and other more distantly related monkey species. They found that many of the supposed knuckle-walking adaptations in chimp and bonobo wrists are missing in gorillas but present in non-knuckle-walking monkeys (Proceedings of the National Academy of Sciences, ).
Advertisement
Since chimps and gorillas use different bones for knuckle-walking, the activity could well have evolved separately in the two apes, Kivell says. If that is the case, it means the ancestral African ape need not have been a knuckle-walker and neither did the ancestral human.
Kivell thinks the wrist bones of chimpanzees may instead have adapted to stabilise the wrist while standing on one tree branch and holding onto another, with knees and elbows bent (see diagram). 鈥淲hen you鈥檙e walking on ice, you bend your elbows and knees to make yourself more stable,鈥� says Kivell. 鈥淵ou do the same thing when you鈥檙e walking on a branch.鈥�
Indeed, modern chimps and bonobos do exactly that. The posture may put more bending strain on the wrist, leading to the kinds of adaptations visible today, Kivell says. She and her colleagues hope to test this idea in the future.
Advocates of the knuckle-walking theory are not convinced. Early Homo species show knuckle-walking adaptations on several wrist bones that were not included in Kivell鈥檚 study, says , a palaeoanthropologist at the State University of New York at Albany. The best explanation for this, he says, even considering the bones Kivell studied, is that early humans, like all other African apes, walked on their knuckles.
