La diaphyse fémorale AL 333-61 : preuve d’une bipédie comme unique mode locomoteur il y a 3,2 Ma en Ethiopie ? AL 333-61 femoral diaphysis: evidence of obligate bipedalism 3.2 million years ago in Ethiopia?

The Pliocene femoral diaphysis AL 333-61, from Hadar (Ethiopia), attributed to Australopithecus afarensis found in a stratigraphic level estimated to be 3.2 million years old, has only been briefly described in previous studies, with no consideration given to its biomechanical or cross-sectional properties. Our new and exhaustive analyses show an overall morphology similar to Homo. AL 333-61 has a real linea aspera and a high pilastric index compared to Early and Mid Pleistocene Homo. Moreover, hypertrophy is notable, even though the cross-sectional properties have been scaled to biomechanical length only, and not to biomechanical length and body mass as is usually done. AL 33361 is singular because it is similar to Early and Mid Pleistocene hominins in some features but similar to Neanderthals and Middle Palaeolithic Modern Humans in others. Biomechanically, it shows the same typical levels of loading as those observed in Homo. Possibilities for interpreting the loading patterns are limited due to the lack of information about the body proportions of AL333-61, but its internal and external morphology suggests that it may have belonged to an individual that was an obligate biped. However, two other hypotheses can be put forward to explain this Homo-like morphology. particular body proportions and/or activities that are unusual, for environmental reasons, in this type of population could also account for the Homo-like body of an individual initially adapted to bipedalism and treedwelling. But none of these three hypotheses can be given more weight than the others due to the lack of exhaustive biomechanical studies of this rare Pliocene diaphysis.