Our ancestors walked on their knuckles, just as chimpanzees and gorillas do today, new research suggests. Henry Gee reports.
History is always written by the victors. Because humans are bipeds (two-legged creatures) we think that bipedalism is a perfectly normal way of getting around. Indeed we assume that evolution has directed us to the bipedal state, so that our hands are free for picking our noses, throwing rocks at one another and all those other tasks that mark us out as sophisticated human beings.
Taking a wider view, bipedalism is just one of several weird styles of locomotion that apes and humans have specialized in. Orangutans (Pongo) use all their limbs as arms and hands, whereas gibbons (Hylobates) swing through the trees using their arms as pendulums.
The African great apes -- the gorilla (Gorilla) and chimpanzee (Pan) -- climb trees, and may even do a little swinging, but on the ground they are usually quadrupeds, walking on the soles of their feet and the second joints of their fingers. This style of locomotion, known as 'knuckle-walking', is unique to the African apes.
Now Brian G. Richmond and David S. Strait of George Washington University, Washington DC, show that our ancestors also walked on their knuckles before they rose up on their hindlimbs to move around much as we do now.
Knuckle-walkers' wrist bones have several distinctive features. The radius (one of the bones in the forearm) and the wrist bones lock together during the weight bearing phase of knuckle-walking to form a solid supporting structure. Gorillas and chimps have these features, whereas humans do not.
Given today's consensus view that humans, chimps and gorillas share a common ancestor, the presence of the knuckle-walking habit in gorillas and chimps and its absence in humans poses a riddle. Either the common ancestor of all three species was not a knuckle-walker, and chimps and gorillas acquired the habit independently; or the common ancestor was a knuckle-walker, and the human lineage simply lost the knack of knuckles.
Richmond and Strait have looked at the wrist bones of two extinct members of the human family, Australopithecus anamensis from Kenya and Australopithecus afarensis (the famous 'Lucy' skeleton) from Ethiopia, as they report in Nature1. These species lived between around 3 and 4.1 million years ago. Although they are both extinct, they were more closely related to us than either the chimp or the gorilla are. Both, for example, were bipeds -- they walked as upright as you or I, and probably not on their knuckles.
But their wrist bones show signs of the same knuckle-walking features seen in present-day apes -- relics of a still earlier, knuckle-walking past. This, the researchers say, is a sign that both African apes and humans descended from a knuckle-walking ancestor.
As usual, this solves one riddle only to raise more in its place. Recent research has suggested that Lucy, for example, descended from an animal capable of climbing trees. This hints that our ancestors climbed out of trees and immediately walked upright, not that they walked on their knuckles for a while first, as Richmond and Strait's work implies. What, then, prompted our ancestors to get up on their hind legs?
Richmond,B. G., & Strait, D. S. Evidence that humans evolved from a knuckle-walking ancestor. Nature 404, 382 2000
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Gee, H. These fists were made for walking. Nature (2000). https://doi.org/10.1038/news000323-7