Taxonomic identification of Lower Pleistocene fossil hominins based on distal humeral diaphyseal cross-sectional shape.

Michael R Lague

doi:10.7717/peerj.1084

Abstract

The coexistence of multiple hominin species during the Lower Pleistocene has long presented a challenge for taxonomic attribution of isolated postcrania. Although fossil humeri are well-suited for studies of hominin postcranial variation due to their relative abundance, humeral articular morphology has thus far been of limited value for differentiating Paranthropus from Homo. On the other hand, distal humeral diaphyseal shape has been used to justify such generic distinctions at Swartkrans. The potential utility of humeral diaphyseal shape merits larger-scale quantitative analysis, particularly as it permits the inclusion of fragmentary specimens lacking articular morphology. This study analyzes shape variation of the distal humeral diaphysis among fossil hominins (c. 2-1 Ma) to test the hypothesis that specimens can be divided into distinct morphotypes. Coordinate landmarks were placed on 3D laser scans to quantify cross-sectional shape at a standardized location of the humeral diaphysis (proximal to the olecranon fossa) for a variety of fossil hominins and extant hominids. The fossil sample includes specimens attributed to species based on associated craniodental remains. Mantel tests of matrix correlation were used to assess hypotheses about morphometric relationships among the fossils by comparing empirically-derived Procrustes distance matrices to hypothetical model matrices. Diaphyseal shape variation is consistent with the hypothesis of three distinct morphotypes (Paranthropus, Homo erectus, non-erectus early Homo) in both eastern and southern Africa during the observed time period. Specimens attributed to non-erectus early Homo are unique among hominids with respect to the degree of relative anteroposterior flattening, while H. erectus humeri exhibit morphology more similar to that of modern humans. In both geographic regions, Paranthropus is characterized by a morphology that is intermediate with respect to those morphological features that differentiate the two forms of early Homo. This study demonstrates the utility of the humeral diaphysis for taxonomic identification of isolated postcranial remains and further documents a high degree of postcranial diversity in early Homo.

Highlights

Our ability to reconstruct hominin evolution is predicated upon appropriate taxonomic allocation of fossil specimens
This study analyzes cross-sectional shape variation of the distal humeral diaphysis among fossil hominins (c. 2-1 Ma) attributable to Paranthropus and Homo to test the hypothesis that specimens can be divided into distinct morphotypes
This study finds no morphological basis for differentiating the South African form of non-erectus early Homo from contemporaneous H. habilis in eastern Africa

Summary

Introduction

Our ability to reconstruct hominin evolution is predicated upon appropriate taxonomic allocation of fossil specimens. The diversity of Lower Pleistocene hominins confounds our ability to identify isolated postcrania to species (or even genus), as taxonomic diagnoses are typically based on craniodental remains. While some of the postcranial remains from Swartkrans, South Africa, indicate two distinct morphotypes (presumably representing Homo and Paranthropus; Grine & Susman, 1991; Susman, de Ruiter & Brain, 2001), taxonomic allocation of isolated bones is, in most cases, controversial and/or uncertain (Grine, 2005). At least one form of non-erectus early Homo exhibits aspects of “Australopithecus-like” postcranial morphology that had traditionally been assumed to characterize Paranthropus (Wood & Constantino, 2007). Isolated postcrania cannot be securely referred to P. boisei based solely on their morphological similarity to homologous elements among other australopiths

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