Postcanine EDJ morphology in Homo habilis and its implications for the evolution of early Homo

A large portion of the Homo habilis hypodigm consists of dental remains, including the key specimens from Olduvai Gorge in Tanzania. The type specimen of the species, OH 7, includes the mandible with a nearly complete tooth row, and specimens such as OH 13 and OH 16 preserve mandibular and maxillary tooth rows. Teeth are useful in studying hominin systematics, and the outer enamel surface (OES) morphology of the H. habilis dentition has been studied extensively. Relatively few features of the postcanine dentition are considered distinctive in H. habilis, but the mandibular premolars and molars are frequently described as being

buccolingually narrow [1]. Based on new microCT scans we investigate the enamel-dentine junction (EDJ) morphology of H. habilis postcanine teeth from Olduvai Gorge, early Homo specimens from Koobi Fora (Kenya), Australopithecus afarensis, A. africanus, Homo erectus s.l., and later Homo (modern humans, Neanderthals, and middle-Pleistocene Homo). We use geometric morphometrics to quantify EDJ shape of every postcanine tooth position for these taxa.

We find that for a number of H. habilis specimens, the postcanine dental morphology is very primitive. Type specimen OH 7, as well as OH 24 and several Koobi Fora specimens (KNM-ER 1802, KNM-ER 1813), show an EDJ morphology that is largely within the range of variation for Australopithecus and retain several primitive features. However, there are a number of features of the postcanine EDJ morphology that reliably distinguish later Homo from Australopithecus, including an increase in the height of the dentine body (defined as the distance between the cervix and marginal ridges) in all tooth positions, reduction of the talon or talonid in the premolars, and reduction in the distal aspect of the crown of the upper molars. Our findings are consistent with previous work emphasizing the generalized postcanine dentition of H. habilis [1]. Contrary to previous work [1,2], our EDJ shape analyses do not suggest that H. habilis is characterized by buccolingually narrow mandibular molars and premolars, in line with observations on the outer enamel surface [3]. However, there are some aspects of the postcanine dentition that we find to be more distinctive; for example, the maxillary third molars of H. habilis are distinguished from those of Australopithecus by the presence of a further mesially placed metacone and a mesiodistally shorter crown.

We also find variation within the H. habilis hypodigm. While Bed I Olduvai specimens such as OH 7 and OH 24 show a largely primitive morphology, some Bed II specimens show a more derived morphology. This is most notable in OH 16, which displays a suite of derived features, particularly in the mandibular and maxillary premolars, which show a tall dentine body and a reduced talon/talonid region; in these respects the specimen is more derived than the early African H. erectus sample. OH 13 is the youngest specimen assigned to H. habilis, and although it shows some derived aspects of postcanine morphology, other key features are more similar to Bed I specimens such as OH 7, which is consistent with the mandibular dental arcade shape [4].

References: [1] Wood, B.A., 1992. Origin and evolution of the genus Homo. Nature 355, 783-790. [2] Leakey, L.S.B., Tobias P.V., Napier J.R., 1964. A New Species of The Genus Homo From Olduvai Gorge. Nature 202, 7-9. [3] White, T.D., Johanson D.C., Kimbel W.H., 1983. Australopithecus africanus: Its Phyletic Position Reconsidered. In: Ciochon, R.L., Corruccini, R.S. (Eds.), New Interpretations of Ape and Human Ancestry, Springer US, Boston, MA. pp. 721-780. [4] Spoor, F., Gunz, P., Neubauer, S. et al., 2015. Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature 519, 83-86.