Craniofacial morphology of 'Homo floresiensis': Description, taxonomic affinities, and evolutionary implication

Abstract

This paper describes in detail the external morphology of LB1/1, the nearly complete and only known cranium of 'Homo floresiensis'. Comparisons were made with a large sample of early groups of the genus 'Homo' to assess primitive, derived, and unique craniofacial traits of LB1 and discuss its evolution. Principal cranial shape differences between 'H. floresiensis' and 'Homo sapiens' are also explored metrically. The LB1 specimen exhibits a marked reductive trend in its facial skeleton, which is comparable to the 'H. sapiens' condition and is probably associated with reduced masticatory stresses. However, LB1 is craniometrically different from 'H. sapiens' showing an extremely small overall cranial size, and the combination of a primitive low and anteriorly narrow vault shape, a relatively prognathic face, a rounded oval foramen that is greatly separated anteriorly from the carotid canal/jugular foramen, and a unique, tall orbital shape. Whereas the neurocranium of LB1 is as small as that of some 'Homo habilis' specimens, it exhibits laterally expanded parietals, a weak suprameatal crest, a moderately flexed occipital, a marked facial reduction, and many other derived features that characterize post-'habilis Homo'. Other craniofacial characteristics of LB1 include, for example, a relatively narrow frontal squama with flattened right and left sides, a marked frontal keel, posteriorly divergent temporal lines, a posteriorly flexed anteromedial corner of the mandibular fossa, a bulbous lateral end of the supraorbital torus, and a forward protruding maxillary body with a distinct infraorbital sulcus. LB1 is most similar to early Javanese 'Homo erectus' from Sangiran and Trinil in these and other aspects. We conclude that the craniofacial morphology of LB1 is consistent with the hypothesis that 'H. floresiensis' evolved from early Javanese 'H. erectu's with dramatic island dwarfism. However, further field discoveries of early hominin skeletal remains from Flores and detailed analyses of the finds are needed to understand the evolutionary history of this endemic hominin species.