Lucy on the ground with knuckles

Anthropologists generally regard an upright gait as essential for membership in the human evolutionary family. However, some of our earliest ancestors may have favored knuckle-walking on all fours, much as chimpanzees and gorillas do, according to a study in the March 23 Nature.

Brian G. Richmond and David S. Strait, both anthropologists at George Washington University in Washington, D.C., examined previously found wrist bones from several Australopithecus species. A. anamensis and A. afarensis—the latter represented by the famous skeleton known as Lucy—had wrists capable of locking the hands in place during knuckle-walking, the scientists say. A. anamensis lived just prior to 4 million years ago; A. afarensis existed from 4 million to 3 million years ago.

Later human ancestors, such as A. africanus, had flexible wrists unsuited to knuckle-walking, Richmond and Strait hold. These findings support genetic evidence for a close evolutionary linkage of humans to chimps and gorillas, they contend.