Once thought to be a biped, Sahelanthropus is instead the earliest known knuckle-walking ape, says study

In a new paper in the Journal of Human Evolution, researchers from Chaffey College, New York University, and California State University San Bernardino find the distinctive forelimb morphology of the African knuckle-walking apes in the forelimb of the roughly 7-million-year-old Sahelanthropus tchadensis. Only chimpanzees and gorillas exhibit robust and forward-curving ulna shafts, which are thought to serve as an adaptation to knuckle-walking. This bone in all other primates is straight or curves backward and contrasts with humans and other fossil hominins (bipeds).

The researchers also find clear signs of knuckle-walking in OH 36, an isolated fossil dated to 1.2 million years ago, once classified as Paranthropus boisei. Previous research has noted that this fossil was extremely chimp-like and different from hominins. The new analysis clarifies that this fossil was indeed a knuckle-walking African ape.

With respect to Sahelanthropus, the team proposes two hypotheses: (1) that this species represents the earliest known knuckle-walking African ape and was not a hominin, or (2) that if this species was facultatively bipedal, like chimps and gorillas it was a habitual knuckle-walker.

Supporting the first hypothesis is a recent analysis of the Sahelanthropus tchadensis femur which finds no evidence for bipedality. While new research from the leg and arm bones of this species strongly controverts the idea that Sahelanthropus was an early biped, the new evidence indicating that it was a knuckle-walking Miocene ape shifts it to a uniquely privileged evolutionary position.