Body size and body shape in early hominins – implications of the Gona Pelvis

Abstract

Discovery of the first complete Early Pleistocene hominin pelvis, Gona BSN49/P27, attributed to Homo erectus, raises a number of issues regarding early hominin body size and shape variation. Here, acetabular breadth, femoral head breadth, and body mass calculated from femoral head breadth are compared in 37 early hominin (6.0–0.26 Ma) specimens, including BSN49/P27. Acetabular and estimated femoral head sizes in the Gona specimen fall close to the means for non-Homo specimens (Orrorin tugenesis, Australopithecus africanus, Paranthropus robustus), and well below the ranges of all previously described Early and Middle Pleistocene Homo specimens. The Gona specimen has an estimated body mass of 33.2 kg, close to the mean for the non-Homo sample (34.1 kg, range 24–51.5 kg, n = 19) and far outside the range for any previously known Homo specimen (mean = 70.5 kg; range 52–82 kg, n = 17). Inclusion of the Gona specimen within H. erectus increases inferred sexual dimorphism in body mass in this taxon to a level greater than that observed here for any other hominin taxon, and increases variation in body mass within H. erectus females to a level much greater than that observed for any living primate species. This raises questions regarding the taxonomic attribution of the Gona specimen. When considered within the context of overall variation in body breadth among early hominins, the mediolaterally very wide Gona pelvis fits within the distribution of other lower latitude Early and Middle Pleistocene specimens, and below that of higher latitude specimens. Thus, ecogeographic variation in body breadth was present among earlier hominins as it is in living humans. The increased M-L pelvic breadth in all earlier hominins relative to modern humans is related to an increase in ellipticity of the birth canal, possibly as a result of a non-rotational birth mechanism that was common to both australopithecines and archaic Homo.

Introduction

The recently reported early Pleistocene (0.9–1.4 Ma) hominin pelvis from Gona, Ethiopia, BSN49/P27, attributed to Homo erectus (Simpson et al., 2008), is significant for a number of reasons. It is the only largely complete and undistorted hominin pelvis temporally intermediate between two australopithecine specimens, A.L. 288-1 (Tague and Lovejoy, 1986) at 3.2 Ma (Walter, 1994), and Sts 14 (Robinson, 1972) at about 2.5 Ma (Schwarcz et al., 1994), and the Homo Pelvis 1 from Atapuerca (Arsuaga et al., 1999) at 0.6 Ma (Bischoff et al., 2007). This was a time period during which major changes in body form were occurring within the hominin lineage (McHenry and Coffing, 2000). Because pelvic morphology is critical for interpreting obstetric and locomotor adaptations, as well as reconstructing overall body size and shape, the Gona pelvis provides important new information for readdressing these issues within a time period where little direct evidence of this kind has been available.

Previously known early Pleistocene hominin pelvic specimens include the Paranthropus robustus adult partial os coxae SK 50 and subadult partial os coxae SK 3155 (Robinson, 1972, McHenry, 1975a), the adult early Homo partial os coxae KNM-ER 3228 (Rose, 1984), the juvenile Homo erectus (or ergaster) partial pelvis of KNM-WT 15000, and the adult H. erectus partial os coxae OH 28 (Day, 1971). Other pelvic specimens from this time period are more fragmentary and consequently less informative. Of the above specimens, a full pelvic reconstruction was attempted only for KNM-WT 15000 (Walker and Ruff, 1993). However, this specimen did not preserve the pubic portion of the os coxae and preserved only portions of the sacrum; thus, the full reconstruction was only approximate (e.g., see Ruff, 1995). Also, adult morphology of the specimen can only be estimated through extrapolation. Therefore, estimated adult dimensions for this specimen are subject to relatively wide error ranges. Although incomplete, the above specimens have nevertheless proven useful in assessing hip biomechanical function as well as some general obstetric and body shape parameters in australopithecines and early Homo (Day, 1971, McHenry, 1975b, Rose, 1984, Ruff, 1995, Ruff et al., 1999).

The present study focuses primarily on temporal and geographic variation in body size and body shape among early hominins, incorporating the new evidence from the Gona pelvis as well as a reassessment of previously described specimens (e.g., Ruff, 1994, Ruff et al., 1997, Rosenberg et al., 2006). A number of other recent studies have also provided new data and the possibility of new interpretations relevant to a reconsideration of these issues (Lordkipanidze et al., 2007, Spoor et al., 2007, Trinkaus, 2009, Weaver and Hublin, 2009).

The BSN49/P27 pelvis was reconstructed from several fragments using high resolution plaster casts, which also allowed correction of distortions in the original specimens (Simpson et al., 2008). Substantial portions of the iliac blades are missing; however, enough is preserved from each side that, with mirror imaging, a realistic reconstruction of this region is possible (Simpson et al., 2008, and Simpson, pers. comm.). The pubic and ischial portions are largely intact, and the right acetabulum is complete and undistorted. Simpson and colleagues used several morphological similarities to modern human females (e.g., sciatic notch and subpubic shape) to argue that the fossil was female.

In the present context, a major feature of interest of the Gona pelvis is its great total mediolateral (bi-iliac) breadth, measured as 288 mm (Simpson et al., 2008). This is considerably larger than the 266 mm originally estimated for the adult bi-iliac breadth of KNM-WT 15000 (Ruff and Walker, 1993). The relatively narrow body of KNM-WT 15000 was interpreted as an adaptation in early Homo to tropical environments, i.e., maximizing of surface area to body mass (Ruff, 1991, Ruff and Walker, 1993). In this respect, KNM-WT 15000 conformed to both modern human geographic variation in body form as well as patterns of variation among later Pleistocene Homo (Ruff, 1994, Ruff, 2002b, Trinkaus et al., 1999, Rosenberg et al., 2006; although also see Trinkaus, 2009). In contrast, the much wider pelvis of the Gona specimen suggested that “early Homo … did not exhibit the same ecogeographic patterns of body form as seen in modern humans” (Simpson et al., 2008: 1091).

Another notable feature of the Gona pelvis is its very small articular surfaces. Specifically, small dimensions include acetabular breadth, sacral auricular surface, and S1 body surface areas (Simpson et al., 2008). No body mass estimate was attempted by the original authors, but stature estimates of 120-146 cm were derived from a variety of regressions based on joint size and estimated femoral length. This is considerably shorter than statures estimated previously for any specimens attributed to H. erectus, which range between about 150 and 185 cm (Feldesman and Lundy, 1988, McHenry, 1991, Ruff and Walker, 1993). Even considering smaller, very early Pleistocene Homo specimens probably closely related to H. erectus, including KNM-ER 1472 and 1481, and the Dmanisi adult specimens, estimated stature ranges extend only down to the largest estimated stature for the Gona specimen, i.e., about 145 cm (Ruff and Walker, 1993, Lordkipanidze et al., 2007). The very small body size of the Gona specimen implies both a greater than previously recognized range of body size in H. erectus, as well as very marked sexual dimorphism in body size (Simpson et al., 2008). This issue is further considered in the present study in the context of body mass estimations for the Gona and other early hominin specimens.