When the remains of Ardi, a 4.4 million-year-old female member of Ardipithecus ramidus, were revealed in 2009, the scientific community was buzzing in hopes of trying to understand how or if she fit in to early human lineage. New research led by William Kimbel of Arizona State University reveals that Ardi’s skull indicates that her species was more closely related to early humans, not other apes. The results were published in the Proceedings of the National Academy of Sciences.
Humans and modern chimpanzees diverged about 6-8 million years ago. While very clear differences can be seen between us and our closest cousins now, this was not the case at the onset of the split. Many of the up and coming species were not clear representations of one descendant or another, which can make identification and extrapolation of traits somewhat difficult sometimes.
Controversy about Ardi’s place in human evolution began because she was such a mixed bag of features. She had several ape-like features, including a small brain, feet suited for an arboreal quadruped. She also had plenty of similarities to humans, including the length and shape of her canine teeth, and a pelvis that indicated bipedality on the ground.
The research team has now announced evidence that Ardi's skull indicates at least partial bipedality, making A. ramidus more human than ape. In bipedal animals, the spine articulates with the skull toward the bottom. This allows us to look forward while standing upright. Skulls of quadrupeds are shaped much differently, allowing them to comfortably eat and see while on all fours. The openings on Ardi’s skull that allow for connection with the spine, nerves, and blood vessels are organized more like a human. This means that A. ramidus is now the oldest known hominin to have this type of skull patterning.
The human-like skull morphology is also present in the 3.4 million year old Australopithecus genus, of which Lucy represents. What is not clear, however, is what caused skulls to evolve this shape. It is possible that the emergence of hominin bipedality caused this shift in skulls, but changes in brain patterning may be responsible.
The skull was found in the Middle Awash Research area in Ethiopia. Because modern humans migrated out of East Africa, it is no surprise that remains of our earliest ancestors can be found in these fossil hotbeds.
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