Humans evolved from an ancestor that walked on its knuckles, like several modern-day apes, according to a new analysis of casts of 3-million- to 4-million-year-old hominid bones. "For the first time we are able to say early hominids bear the echo of a knuckle-walking ancestry that they shared with gorillas and chimps," says paleoanthropologist Rick Potts of the Smithsonian Institution in Washington, D.C.
When chimps and gorillas scamper on the ground, they curl their long fingers, plant the second segment of their finger bones on the ground, and shift their weight. Over time, this creates a bony ridge near the base of the finger. Humans don't have this ridge, nor did early hominids like Lucy, a member of the species Australopithecus afarensis, because she, like us, walked upright. This lack of evidence for knuckle walking among human ancestors implied that chimps are closer to gorillas than to humans, yet strong molecular data and some subtle anatomical data suggests the opposite.
Then in 1998, anthropologist Brian Richmond, a predoctoral fellow at the Smithsonian Institution at the time, read some old papers on the evolution of the primate hand. He noted that early descriptions of knuckle walking in modern chimps and gorillas reported not only the ridges on the finger bones, but also ledges and notches in the wrist joint that help keep the arm rigid. He and David Strait of George Washington University decided to check the collection of casts across the hall at the Smithsonian's National Museum of Natural History. "I walked over to the cabinet, pulled out Lucy, and--shazam!--she had the morphology that was classic for knuckle walkers." Lucy herself wasn't a knuckle walker, notes Richmond; rather, these wrist traits are a leftover from her knuckle-walking ancestors.
Richmond and Strait report in the 23 March Nature that chimps, gorillas, and two early hominids--A. afarensis and A. anamensis--had similar specialized wrist features that are lacking in modern humans and other primates. And these common traits imply that the common ancestor of australopithecines, chimps, and gorillas was a knuckle walker. The knuckle-walking traits were lost in the human line--by about 2.5 million to 3.0 million years ago, according to specimens of A. africanus, Richmond says.
But the finding raises other questions, such as why a climbing creature already adapted for traveling on the ground would evolve the ability to stand on two feet as well. That question, as well as the issue of knuckle walking in our ancestors, probably won't be answered until anthropologists have more bones to study. "The big problem is that we don't have a fossil record of the chimp-human-gorilla ancestor," says Carol Ward, a paleoanthropologist at the University of Missouri, Columbia.