Ardipithecus ramidus (Ardi) is back. In 2009, the skeleton discovered by Tim White’s team in the mid-90s was published in full. Dated to approximately 4.4 million years old based on volcanic stratigraphy, ramidus was found in the Middle Awash river valley in Ethiopia. The most complete individual, over 40% of a female skeleton, had the brain approximately the size of a chimp and exhibited a similar level of facial prognathism. Hand and foot morphology pointed to an arboreal creature, most notably a divergent big toe that would make many primates proud.
However, ramidus exhibits a bunch of other traits which seemed derived in the direction of Homo, which is what the authors of the study are pushing. These traits include reduced canines, and aspects of the foot and pelvis which could indicate a level of at least facultative bipedalism. Critics argue that it is possible that these traits were present in many apes of the time period around the human-chimp split, and that chimps rather than humans moved away from these adaptations.
The new study published in PNAS furthers the original argument made by the discoverers that Ardi is in fact a hominin. In particular, the reconstructed basicranium was observed in the study, led by William Kimbel of Arizona State University. Kimbel and his team demonstrate that like Homo and Australopithecus, Ardi had a short basicranium, and a relatively anteriorly placed foramen magnum. The placement of the foramen magnum, in particular, is particularly important in determining whether or not a species was bipedal. Along with the broadening of the cranial base, came notable modifications/shifting of tympanic elements and other foramena. In short, the authors put forward the argument that because of the age of ramidus, it is possible to say that these types of changes in the cranial base were some of the earliest derivations towards Homo.
Paleoanthropologists are in a tough place. Inferring descent through morphology alone is tough, but it is all fossils really give us right now. This was just demonstrated in an even more recent case where H. heidelbergensis was demonstrated to be more closely related to Denisovans on its matriline than Neandertals, with whom they shared many morphological affinities. Going back millions of years doesn’t make it any easier, and we’re not going to be recovering 4.4 million year old DNA any time soon to help give answers. Till then here’s to the many personalities of paleoanthropology, who direct our understanding of human evolution with biology, anthropology, and sometimes a sprinkling of ego.