Humanlike Features Discovered In 4.4M-Year-Old Ardipithecus Skull
January 7, 2014

Humanlike Features Discovered In 4.4M-Year-Old Ardipithecus Skull

April Flowers for - Your Universe Online

The 4.4-million-year-old African species Ardipithecus ramidus, or "Ardi," is the focus of one of the most hotly debated issues in current human origins research. Scientists want to know how Ardi, an unusual primate, is related to the human lineage.

The study, published in the online edition of Proceedings of the National Academy of Sciences (PNAS), shows that although Ardi possessed a tiny brain and a grasping big toe for climbing trees – the primate had small, humanlike canine teeth and an upper pelvis modified for bipedal walking on the ground.

Where this mixture of features places Ardi on the tree of human and ape relations remains an uncertainty. Scientists question whether Ardi was an ape with a few humanlike features retained from an ancestor near in time -- between 6 and 8 million years ago, according to DNA evidence -- to the split between the chimpanzee and human lines, or perhaps a true relative of the human line that had yet to shed many signs of its remote tree-dwelling ancestry.

Arizona State University (ASU) paleoanthropologist William Kimbel, director of the ASU Institute of Human Origins, collaborated with Gen Suwa (University of Tokyo Museum), Berhane Asfaw (Rift Valley Research Service, Addis Ababa), Yoel Rak (Tel Aviv University), and Tim White (University of California at Berkeley).

The team's findings confirm Ardi's close evolutionary relationship to humans using the underside, or base, of a beautifully preserved partial Ardi cranium. A pattern of similarity was confirmed that reveals links to Ardi from Australopithecus and modern humans but not to apes.

Tim White led a field team that has been recovering fossil remains of Ardipithecus ramidus in the Middle Awash Research area, Ethiopia, since the 1990s. Suwa led the most recent examination of the Ardi skull in 2008, which revealed humanlike aspects of its base.Kimbel was co-lead investigator for the team that recovered the earliest known Australopithecus skulls from Ethiopia's Hadar site, home of the famous "Lucy" skeleton.

"Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human's is astonishing," says Kimbel.

Scientists find the cranial base to be a valuable resource for studying phylogenetic, or natural evolutionary relationships, because of its anatomical complexity. The base's association with the brain, posture, and chewing system have provided numerous opportunities for adaptive evolution over time. Accordingly, the human cranial base differs significantly from that of apes or other primates.

Structures marking the articulation of the spine with the skull are more forwardly located in humans rather than in apes, while the base is shorter front to back. On each side there are openings that allow for the passage of blood vessels and nerves – which in humans are spaced more widely apart.

The arrangement of the bones on the skull base are affected by the shape differences, so much so that it is fairly easy to tell apart even isolated fragments of ape and human basicrania.

The cranial base of Ardi shows the distinguishing features that separate humans and Australopithecus from the apes. An earlier study by Kimbel's team revealed that these human peculiarities were present in the earliest known Australopithecus skulls by 3.4 million years ago.

The current study builds the catalogue of anatomical similarities linking humans, Australopithecus and Ardipithecus on the tree of life and shows that the human cranial base pattern is at least a million years older than Lucy's species, A. afarensis.

When debating the cause of evolutionary changes in the human cranial base, paleoanthropologists usually fall into one of two main camps.

The first theory questions whether it was the adoption of upright posture and bipedality causing a shift in the poise of the head on the vertebral column. If so, does the humanlike cranial base of A. ramidus confirm postcranial evidence for partial bipedality in this species?

The second theory asks if the changes tell us about the shape of the brain (and of the base on which it sits). If so, perhaps this is an early sign of brain reorganization in the human lineage. The discovery of Ardi, and finding that Ardi apears more closely related to humans than the chimpanzees, means that both theories need to be reevaluated.

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