(AP) - A new fossil analysis is rattling the family tree with evidence humans evolved directly from an ancestor that walked around on its knuckles like gorillas and chimpanzees. For decades, anthropologists have considered upright walking, or bipedalism, a defining characteristic of the human lineage.

Knuckle-walking was thought to have evolved uniquely in apes after humans had taken a separate evolutionary path. But in an article in Thursday's issue of the journal Nature, researchers said they found fossil evidence that two species of early humans descended from knuckle-walkers. "Instead of coming down out of the trees and walking upright, the ancestors of early upright walkers were already adapted to a life on the ground," said Brian G. Richmond, an anthropologist at George Washington University in Washington, D.C., and a co-author of the study.