Humeral Length Allometry in African Hominids ( sensu lato ) with Special Reference to A . L . 288-1 and

We have previously argued that the femora of A.L. 288-1 (“Lucy,” referred to Australopithecus afarensis) and Liang Bua (LB) 1 (holotype of Homo floresiensis) have lengths near those expected for modern humans of their diminutive size. Jungers has argued that these same fossil hominins have humeri that are close in length to those expected for modern humans of their diminutive size. Yet the humero : femoral indices of these two fossil hominins are radically different (5 to 6 standard deviations) from the modern human mean. These findings are seemingly contradictory. We investigate bivariate humeral length allometry for A.L. 288-1, LB1, and the three extant African hominid (sensu lato) genera. Results show considerable overlap in bivariate space among Homo, Pan, and (female) Gorilla individuals in humeral length relative to body mass. However, despite this overlap, humeral length allometry varies greatly among the taxa, showing slight positive allometry to isometry in Homo, while showing strong negative allometry in Pan and Gorilla. Ordinary least-squares and reduced major axis regression lines of human humeral length on body mass1/3 lie near, but more often fall below, the observed humeral lengths of A.L. 288-1 and LB1, indicating that these fossil hominins have slightly longer humeri than would be expected for humans of their small size. In terms of femoral length, A.L. 288-1 and LB1 had somewhat shorter femora than expected for a human of their small size. This combination of slightly longer humeri and somewhat shorter femora than modern humans accounts for these specimens’ high humero : femoral indices. INTRODUCTION I terms of its length, the humerus is perhaps the most conservative of the limb bones among both humans and the African apes. Aiello (1981), for example, found that among the hominoids, only the highly suspensory Hylobatidae had significantly longer humeri relative to body mass; the other apes and humans showed considerable overlap in bivariate space. In addition, Jungers (1991a, 1994) has found that humeral length : body mass1/3 ratios are not statistically significantly different across the African hominids (sensu lato), suggesting that the differences in humero-femoral indices between humans and African apes are due entirely, or nearly entirely, to the marked elongation of the lower limb in the former. Likewise, Drapeau and Ward (2007) found that humeral length did not discriminate among the large-bodied hominoids when compared to a multivariate surrogate for body size, nor did variation in humeral length correlate well with locomotor adaptation. Within recent human adults, Holliday and Ruff (2001) found that humeral length showed lower intrinsic (i.e., size-independent) variability than any other limb segment, with the possible exception of the femur. In this analysis, we examine bivariate length allometry of the humerus in three extant hominid genera—Homo, Pan, and Gorilla—and the diminutive specimens A.L. 2881 (the Australopithecus afarensis specimen popularly known as “Lucy”) and Liang Bua (LB) 1 (the holotype of Homo floresiensis). We focus on these individuals because both have significantly higher humero-femoral indices than do modern humans (Brown et al. 2004; Johanson et al. 1982). In previous publications we have examined femoral length to body mass allometry of these two fossils, and determined that these specimens have femoral lengths close to those expected for hominins of their diminutive size (Franciscus and Holliday 1992; Holliday and Franciscus 2009). Thus, we investigate humeral length allometry to determine whether these two specimens’ high humero-femoral index values reflect a relatively shorter femur (which would contradict our earlier analyses), a comparatively long upper limb (a hypothesis rejected by Jungers, 1982, 1994, 2009), or some combination of the two. MATERIALS AND METHODS The recent human skeletal comparative data used in this analysis include a large global sample of 1,308 recent humans (Table 1). The majority of these specimens (772) were measured by the first author (TWH), with the remainder provided courtesy of Drs. C.B. Ruff and C.E. Hilton, along with supplementary data culled from Münter (1936) and Slome (1929). Note that within the global sample not all PaleoAnthropology 2012: 1−12 . © 2012 PaleoAnthropology Society. All rights reserved. ISSN 1545-0031 doi:10.4207/PA.2012. ART68 TRENTON W. HOLLIDAY Department of Anthropology, 101 Dinwiddie Hall, 6823 St. Charles Avenue, Tulane University, New Orleans, LA 70118, USA; thollid@tulane.edu ROBERT G. FRANCISCUS Department of Anthropology, University of Iowa, Iowa City, IA 52242-1322, USA; robert-franciscus@uiowa.edu 2 • PaleoAnthropology 2012 1) includes 35 chimpanzees (Pan troglodytes) and 36 gorillas (Gorilla gorilla) from the Cleveland Museum of Natural History, the Smithsonian Institution, the University of New Mexico, Tulane University, and the University of the Witwatersrand, all measured by the authors, with supplemental data kindly provided by Dr. B. Latimer. As was the case with the human data, not all measurements are preserved in all individuals, and therefore sample size varies from specimens preserve all measurements, and therefore sample size changes from analysis to analysis (minimum sample size=894). Note, too, that in light of the small body size of A.L. 288-1 and LB1, our sample includes several small adult (as determined via epiphyseal closure) individuals, including 10 Central African “Pygmies,” 36 San, and 12 Andaman Islanders. The African ape sample used in this study (see Table TABLE 1. COMPARATIVE SAMPLES.