The Omo-Kibish I pelvis
Introduction
Omo-Kibish I (Omo I) is a partial skeleton that is considered by nearly all researchers to represent anatomically modern Homo sapiens (Day, 1969, Rightmire, 1976, Rightmire, 1978, Stringer, 1978, Day and Stringer, 1982, Pearson, 2000, Pearson, 2008, Trinkaus, 2005, Trinkaus et al., 2006, Pearson et al., 2008). During a 1967 expedition led by Richard Leakey, the Omo I skeleton was found in situ by Kamoya Kimeu at KHS (Kamoya's Hominid Site) in Member I sediments of the Kibish Formation (Butzer, 1969, Butzer et al., 1969, Leakey, 1969). Subsequent recovery efforts by the Kibish Paleoanthropological Project in 2001–2003 revealed additional pieces of this same skeleton (Fleagle et al., 2008).
The recovered Omo I skeleton now consists of significant portions of the cranium and mandible, dentition, and much of the postcranium. The majority of features that distinguish anatomically modern humans from earlier hominins are craniodental features, including neurocranial globularity, maximum cranial breadth across the parietals, a divided supraorbital torus, face positioned below the forepart of the brain, a bony chin (even in infants), presence of a canine fossa, a gracile tympanic bone, and simplified and reduced tooth crowns (Lieberman, 1998, Trinkaus et al., 2006, Stringer, 2012, Stringer and Buck, 2014). The cranial morphology of Omo I is consistent with that of modern humans (Day, 1969, Rightmire, 1978, Stringer, 1978, Day and Stringer, 1982, Trinkaus, 2005, Trinkaus et al., 2006), although it may not display each of the H. sapiens apomorphies (Pearson, 2008). Some of the important markers of anatomical modernity in Omo I include a large and globular neurocranium, a bony chin, and a face that appears to be positioned below the forepart of the brain (Day, 1969, Rightmire, 1978, Stringer, 1978, Day and Stringer, 1982, Day and Stringer, 1991, Trinkaus, 2005, Trinkaus et al., 2006).
A partial hipbone (os coxae) was recovered in 2001 by JGF from the KHS site that had previously yielded the Omo I partial skeleton (Pearson et al., 2008). This hipbone has been discussed and figured in previous works by some of the authors (Royer et al., 2007, Pearson et al., 2008, Royer, 2009, Hammond et al., 2017). Modern human pelves are distinct from other Pleistocene Homo spp. pelves because they are mediolaterally narrower at the level of the anterior superior iliac spine (i.e., narrower across the bispinous breadth), have ilia that are more vertically oriented, lack a well-developed iliac pillar (i.e., the acetabulocristal buttress), and have a short and thick superior pubic ramus that is circular in cross-section (Stringer, 1986, Stringer, 2012, Rak, 1990, Arsuaga et al., 1999, Pearson, 2000, Bonmatí et al., 2010, Gruss and Schmitt, 2015, Ward et al., 2015). Pelves have the potential to be particularly informative about hominin paleobiology because modern human pelves provide indicators of sex (Singh and Potturi, 1978, Rogers and Saunders, 1994, Bruzek, 2002, Royer, 2009), age-at-death (Lovejoy et al., 1985, Miranker, 2016), body size (Jungers, 1988, Plavcan et al., 2014a, Plavcan et al., 2014b), obstetrics and parturition (Ruff, 1995, Rosenberg and Trevathan, 2002, Brown, 2011, Grabowski et al., 2011, Ubelaker and De La Paz, 2012), trunk morphology (Ruff, 1991, Ruff, 2010, Simpson et al., 2008, Betti et al., 2014, Middleton, 2015), physiology (Dunsworth et al., 2012, Wall-Scheffler, 2012, Wall-Scheffler and Myers, 2013), and locomotion (Robinson, 1972, O'Neill et al., 2015). The pelvis of Omo I provides an opportunity to test whether the earliest modern humans had the pelvic morphology characteristic of modern humans today (and if so, similar to which living population) and to shed light onto the paleobiology of the earliest humans.
The major goals of this paper are to provide a formal description of the preservation and morphology of the Omo I pelvic remains, and to quantitatively and qualitatively compare the Omo I hipbone to recent humans and relevant fossil hominins.
Section snippets
Geological context and recovery of fossils
Omo I was recovered from KHS in the uppermost part of Member I of the Kibish Formation (Butzer, 1969, Brown and Fuller, 2008). More detailed explanations of the stratigraphy and dating methods are available elsewhere (McDougall et al., 2005, Brown and Fuller, 2008, Feibel, 2008, McDougall et al., 2008, Brown et al., 2012), but are briefly described here. The KHS fossil locality lies in the upper part of Member 1, between the Nakaa'kire Tuff in Member 1 (below) and the KHS Tuff (just above the
Materials and methods
Omo I hipbone descriptions are based on the original fossils (Fig. 2), which were studied at the Ethiopian Authority for Research and Conservation of the Cultural Heritage (ARCCH) at the Ethiopian National Museum in Addis Ababa. The fossils were measured, photographed, and laser scanned with a Next Engine Desktop 3D Scanner (Next Engine, Malibu, CA).
KHS 1-60A: a partial hipbone
Preservation The partial hipbone preserves most of the ischium, most of the posterior ilium, and a small portion of the pubis (Fig. 2). The pubic ramus is entirely missing, along with the anterior portion of the ilium. The maximum preserved dimensions of the hipbone are approximately 188 mm from the superior ilium to the base of the ischial ramus and approximately 139 mm anteroposteriorly from the pubis to the posterior ilium.
The external (lateral) and internal (medial) surfaces of the hipbone
Body size and stature estimates
Body mass was estimated following Grine et al. (1995; body mass = 2.268 * femoral head diameter −36.5). Using the estimated femoral head diameter (48.7 mm), we predict a body mass of about 73.9 kg.
Previously, stature had been estimated by Pearson et al. (2008) to be 178–182 cm (5′8″–5′10″) based on the proximal humerus, or quite a bit shorter, perhaps only 162–173 cm (5′3″–5′7″) based on the length of the first metatarsal. Pearson et al.'s (2008) stature estimate based on the humerus was
Discussion
The Omo I pelvis is particularly relevant for understanding the pelvic shape of the earliest modern humans prior to human global expansion, regional adaptation, and admixture. Multiple lines of evidence indicate that H. sapiens evolved in sub-Saharan Africa in a tropical climactic zone and then migrated globally after 100 ka (Mellars, 2006a, Mellars, 2006b, Eriksson et al., 2012, Mallick et al., 2016, Pagani et al., 2016). Modern human pelvic morphology varies as a result of neutral
Acknowledgments
The authors thank the Ethiopian Authority for Research and Conservation of the Cultural Heritage (ARCCH) for permission and support of this research. The L.S.B. Leakey Foundation, the Wenner Gren Foundation (6407, 9151), National Science Foundation (BCS-9817950, BCS-0097112, BCS-1232393, BCS-0726115), and National Geographic Society generously supported this project. Fieldwork in the Kibish Formation was conducted through the efforts of Zelalem Assefa, Frank Brown, Ian McDougall, Craig Feibel,
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