Ancient 'Ardi' Skull Linked to the Origins of Humans

The origins of humans has remained a hotly contested debate for years. Part of this debate is the 4.4 million-year-old African species Ardipithecus ramidus, an unusual primate that might have connections to humans. Now, scientists have discovered that "Ardi" has a close evolutionary relationship to humans, which may bring researchers one step closer to finding out where we came from.

Ardi possessed a tiny brain and a grasping big toe used for clambering in trees. In addition, it had small, humanlike canine teeth and an upper pelvis that was modified for bipedal walking on the ground. Because of these unusual features, researchers often wondered whether the species was an ape with a few humanlike features, or a true relative to the human line? In order to find that out, the scientists examined fossil remains of Ardi.

"Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human's is astonishing," said William Kimbel, one of the researchers, in a news release.

The cranial base is a valuable tool when studying natural evolutionary relationships. It's anatomically complex and associated with the brain, posture and chewing systems. Because the human cranial base differs profoundly from that of apes and other primates, researchers can use it for evolutionary comparisons.

In humans, the structures marking the articulation of the spine with the skull are more forwardly located than in apes, where the base is shorter from front to back and the openings on each side for passage of blood vessels and nerves are more widely separated. These differences affect the way bones are arranged on the skull base, which makes it fairly easy to tell species apart.

So what did the scientists find as far as Ardi goes? Ardi's cranial base shows the distinguishing features that separate humans from the apes. More specifically, it shows that Ardi possesses human peculiarities. Because the human cranial base pattern is at least a million years older than Lucy's species, A. afarensis, researchers were able to place Ardi on the tree of life that links humans to this species.

The findings reveal a bit more about this species and shows a bit more about the origins of humans. Currently, though, scientists need to conduct more research before any definitive conclusions are drawn. Yet it does seem that Ardi is more closely related to humans than to chimpanzees.

"The Ardi cranial base fills some important gaps in our understanding of human evolution above the neck," said Kimbel in a news release. "But it also opens up a host of new questions...just as it should!"

The findings are published in the journal Proceedings of the National Academy of Sciences.