Evolutionary Relationships Among Robust and Gracile Australopiths: An Evo-devo Perspective

Hominin fossils of gracile and robust australopith groups were found both in East and in South Africa. It is unclear, however, whether all robusts belong to a monophyletic Paranthropus clade, as the craniofacial resemblance among robust australopiths might only be a superficial correlate of similar masticatory adaptations and not evidence of shared ancestry. It has been suggested that the East African Australopithecus/Paranthropus boisei and the South African A./P. robustus might be convergent allometric variants of their gracile geographical neighbors A. afarensis and A. africanus. Here we approach the phylogenetic questions about robust and gracile australopiths from an evo-devo perspective, examining how simple alterations of development could contribute to the shape differences among hominin species. Using geometric morphometrics we compare gracile and robust australopith crania in the context of the allometric scaling patterns of Pan troglodytes, P. paniscus, and Gorilla gorilla. We examine support for two alternative evolutionary scenarios based on predictions derived from quantitative genetics models: either (1) A./P. robustus evolved in South Africa from the gracile A. africanus, or (2) A./P. robustus is a local variant of the eastern African A./P. boisei. We use developmental simulations to demonstrate that some robust characteristics (wide faces, anteriorly placed zygomatics, and facial dishing) can be predicted by allometric scaling along the ontogenetic trajectory of the gracile A. africanus. We find, however, that the facial differences between A. africanus specimens (Taung, Sts 5, Sts 71, and Stw 505) and A./P. robustus specimen SK 48 cannot be explained by allometric scaling alone. Facial shape differences between A./P. robustus SK 48 and A./P. boisei (KNM-ER 732, KNM-ER 406, OH 5) and the A./P. aethiopicus specimen KNM-WT 17000, on the other hand, can largely be explained by allometric scaling. This is consistent with a close evolutionary relationship of these robust taxa.