4.4 Million-year-old 'Ardi' Skull Reveals Close Ties to Humans

Through a careful look at the base of a hominid species known as Ardipithecus ramidus, or "Ardi," an international team of researchers has affirmed a close evolutionary relationship between the group and humans.

Ardi roamed Eastern Africa as early as 4.4 million years ago, more than 1 million years before Lucy, the partial female Australopithecus afarensis skeleton believed to date back some 3.2 million years. Ardi's discovery is relatively new, its first thorough description not arriving until 2009. Published in the journal Science, the paper detailed a female skeleton who weighed 110 pounds and stood nearly 4 feet high.

At the time the study was published, co-author Tim White of the University of California, Berkeley, described Ardi as "a mosaic creature, that is neither chimpanzee, nor is it human. It is Ardipithecus."

Its brain was tiny and its grasping big toe tailored for climbing around in trees. However, its teeth were small and human-like and the upper pelvis fashioned for bipedalism. Such eclecticism has since given way to intense debate regarding Ardi's relationship to humans. Now White is back with a team of researchers in a report outlining new evidence for an evolutionary link between Ardi and both Lucy and modern humans.

The study, published in the Proceedings of the National Academy of Sciences, hinges on a well-preserved partial Ardi cranium.

By flipping the skull over and focusing on its base, the researchers were able to tap into a wealth of evolutionary insights. This is because the cranial base is an anatomically complex part of the skeleton with clues as to how a species chewed and what their posture was like, among other things. In the past, focusing on this area revealed a range of similarities to humans, including a short cranial base. The new study adds to the list, identifying a number of traits Ardi possesses that are present in Lucy and humans and absent in apes, such as a lateral shift in the holes for the carotid arteries.

"Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human's is astonishing," said William Kimbel, a paleoanthropologist from Arizona State University who led the new research.

According to Kimbel, "the evidence overwhelmingly shows that Ardipithecus ramidus is more closely related to humans than to chimpanzees," he told Nature World News in an email. That doesn't necessarily mean, he noted, that Ardi is a direct human ancestor, but instead could belong to a "sister-lineage."

Besides weighing in on Ardi's relation to humans, the skull may offer insight to another debate: What caused the shift in the head's positioning relative to the neck in humans? Some paleoanthropologists argue that bipedality and the adoption of a more upright posture triggered the change, in which case Ardi's humanlike cranial base may offer evidence for partial bipedality in the species. Another possibility is that the changes may offer clues as to the shape of the brain, possibly indicating an early stage of brain reorganization in the human lineage.

Sergio Almecija is a researcher from Stony Brook University School of Medicine's Department of Anatomical Sciences whose recent analysis of a femur belonging to one of the oldest human ancestors revealed it was a bipedal who lived in the trees.

Though not involved in the new study, Almecija argued that its most striking feature is the inclusion of the relative length of Ardi's basicranium, or bones that form the skull's base. Modern humans' short basicranium has partly to do with the anterior location of the opening for the spinal cord and is tied to bipedalism. The specimen used was highly fragmented, but of the six estimated length measurements the team came up with, four match "exclusively with modern humans while two fall within the lower range of common chimpanzees," he said in an email. "Therefore, although their results provide cranial evidence that Ardi was a biped (evidence complemented by pelvic and foot morphology), there is still some uncertainty."

The only way to be sure, Almecija said, is through the discovery of a skull complete enough to bury any doubt regarding Ardi's basicranial length and location of its spinal opening.

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