Research Letters South African Journal of Science 103, September/October 2007 409 bipedalism’, and, on this basis, concluded that Homo habilis Re-interpreting the evidence should be excluded from the genus. More recent analyses of the approximately 6-million-year-old Orrorin tugenensis fossils have, for bipedality in Homo however, found this species’ femur to be morphologically more floresiensis like that of modern humans than australopithecines,4 which led Pickford et al.5 to conclude that Orrorin was a habitual biped. If this deduction is correct, the story of the evolution of bipedalism ‡ Maria B. Blaszczyk* and Christopher L. Vaughan* and its place in classificatory schemes is likely to change. The unveiling in October 2004 of the remains of a pygmy-sized hominin recovered from a cave on the island of Flores, Indonesia, has sparked an intense series of debates, and the issue of how to 6–9 The unveiling in October 2004 of the remains of a pygmy-sized classify the specimen is one of the major controversies. In 10 hominin recovered from a cave on the island of Flores, Indonesia, describing the skeleton, catalogued as LB1, Brown et al. diag- sparked an intense series of debates within the palaeoanthro- nosed it to be a new species of Homo, and thus gave it the name pology community. The discoverers diagnosed it to be a new Homo floresiensis. Although many experts support the taxonomic species of Homo, which they called Homo floresiensis, and they decision of Brown and his colleagues, others have challenged 8 interpreted the postcranial morphology as being ‘consistent with their interpretation and offered alternative theories. These human-like obligate bipedalism’. We have examined the morphology include the view that the primitive characteristics of LB1’s anat- with the aim of determining whether biomechanical evidence omy suggest an ancestor such as Homo habilis, or that it might 6 supports the claim that this hominin—known as LB1—was indeed perhaps be an offshoot of Australopithecus. habitually bipedal. LB1’s innominate bone differs from that of So what led Brown and colleagues to place LB1 in the genus modern humans through the marked lateral flaring of the ilium, Homo? Although they acknowledged that the LB1 skeleton while her femur has a small head and a relatively long neck. showed a mosaic of primitive and derived traits, and were Although these features are also found in australopithecines and impressed by the characteristics LB1 shared with early hominins are commonly regarded as ‘primitive’ traits, we concluded that such as the australopithecines, they concluded that ‘the facial none would have prevented her from exhibiting an efficient, bipedal and dental proportions, postcranial anatomy consistent with gait. Having established that LB1 walked on two legs, we employed human-like obligate bipedalism, and a masticatory apparatus the principle of dynamic similarity to speculate how she might have most similar in relative size and function to modern humans all 10 walked. Assuming the gait of LB1 was dynamically similar to that of support assignment to the genus Homo.’ The recreation of the 11 modern Homo sapiens, we used known dimensionless parameters, LB1 skeleton in the illustrative pose in National Geographic is cer- together with her leg length (0.55 m), to estimate her fundamental tainly suggestive of a bipedal gait (see Fig. 1). The interpretation of gait parameters: step length = 0.45 m, step frequency = 2.48 the postcranial morphology as being ‘consistent with human- steps/second and speed = 1.11 m/s. Our review has provided like obligate bipedalism’ is thus one of the major pillars upon 10 insights regarding the way in which LB1 and her fellow diminutive which the claim by Brown et al. rests. hominins walked about the island of Flores over 18 000 years ago. Features of the postcranial skeleton of Homo floresiensis The recovery of the LB1 skeleton took place during an archaeo- logical excavation at Liang Bua, Flores, in September 2003.12 The Introduction recovered elements of the postcranial skeleton included a fairly One of the key traits that separate hominins from all other complete right leg and left innominate bone of the pelvis and primates is the habitual use of a bipedal gait. Humans are the less complete left leg, hands and feet. The vertebral column, only extant species in the hominin group, and our bipedality is sacrum, scapulae, clavicles and ribs were represented only by the most distinctive adaptation from our closest living relatives, fragments, while the arms were reported more recently.13 1, 2 the anthropoid apes. This single characteristic is seen to be Describing the postcranial elements, Brown et al.10 noted that, such a defining feature that skeletal adaptations to bipedalism ‘in common with all bipedal hominins, the iliac blade is short and are frequently used to identify our extinct hominin ancestors wide.’ That is where their discussion of LB1’s bipedality began and relatives, and attempts at classification of these extinct and ended. They went on to describe the marked lateral flare species are often made on the basis of these adaptations. Wood of the ilium, whose blade would have projected more laterally and Collard,3 for example, have proposed that the fossil relative to the plane of the acetabulum than in modern humans hominins be divided into two groups on the basis of locomotor (Fig. 2), before going on to discuss the anatomy of the femur inferences that have been made from their postcranial morphol- (Fig. 3). They asserted that the femur’s overall anatomy was ogies. The first group displayed a form of terrestrial bipedalism most consistent with the broad range of variation found in Homo combined with an ability to climb proficiently, thus employing a sapiens, with, for example, strong development of the inter- mixed strategy, and include Praeanthropus, Australopithecus, and trochanteric crest as is characteristically found in our species.10 Homo habilis. The second group, consisting of Homo erectus, Homo However, they also described the biomechanical neck length of ergaster, Homo heidelbergensis and Homo neanderthalensis, was com- the femur (55.5 mm) as being long relative to the femoral head mitted to modern human-like terrestrial bipedalism, with a very diameter (31.5 mm), a feature of the australopithecines and early 3 limited arboreal facility. Wood and Collard furthermore argued Homo (Table 1). They noted that several indices of femoral size that a fossil species should be included in the Homo genus only if and shape—the relationship between femoral head size and it met the criteria of ‘a postcranial skeleton whose functional midshaft circumference (66 mm), and femoral length (280 mm) morphology is consistent with modern human-like obligate and sub-trochanteric shaft size (525 mm2)—fell within the chimpanzee and australopithecine range of variation.10,14 Their *Department of Human Biology, Faculty of Health Sciences, University of Cape Town, Observatory 7925, South Africa. description of the tibia showed that the relationship between the ‡Author for correspondence. E-mail: [email protected] midshaft cross section (347 mm2) and the length of the tibia 410 South African Journal of Science 103, September/October 2007 Research Letters 100 locomotor behaviours. Apes are, however, largely arboreal and thus the basic mechanical conditions of climbing in trees can be related to their skeletal anatomy.1 The most obvious require- ment for a bipedal gait is an upright posture, which in turn requires that the centre of mass (CoM) of the body remain directly over the rectangular area formed by the supporting feet. This explains, for example, why chimpanzees, when walk- ing bipedally, do so with a bent-hip/bent-knee (BHBK) gait.16 A BHBK gait is fatiguing if significant distances are walked,17 whereas the ‘stiff-legged’ gait of humans allows for an effective exchange of gravitational potential and kinetic energy, which minimizes energy consumption.18,19 Morphological features that allow extension of the lower limb joints during locomotion, such as the lumbar lordosis, are consequently seen to be indicators of a bipedal gait. The ape pelvis displays several features common to other quadrupeds, such as ischial corpora that are elongated, and ilial 50 cm blades that lie mostly in the coronal plane.20 The ape’s long ischium is adapted to power the hip in extension by giving the hamstrings a long moment arm, while the lengthened ilium provides gluteus medius and minimus with great fibre length, which is mechanically advantageous for quadrupedal locomo- tion and during climbing.1 In contrast, the human pelvis is short, squat and basin-shaped. The human ilium is unique among primates by virtue of the fact that it is wider than it is high,15 thus eliminating any restrictive contact between the ilia and the lower lumbar vertebrae20 and facilitating bipedal posture. Another exclusively human feature is the greater sciatic notch on the sacral margin of the ilium, created by retroflexion of the ilium, and resulting in a far greater angle between the ilium and the ischium than that seen in apes. This allows the trunk to be held upright without compromising the biomechanical relation- ship between the ischium and the femur.15 0 Differences in the morphology of the femur in humans and apes include the degree of obliquity of the femoral shaft, its Fig. 1. The recovered bones of Homo floresiensis have been assembled in this mediolateral curvature and cortical thickness, and the relative locomotor pose by an artist, clearly suggesting that LB1 displayed a human-like size of the femoral condyles. The latter three parameters reflect bipedal gait. This figure has been redrawn from the original colour illustration in National Geographic.11 the difference in weight transfer during quadrupedal versus bipedal locomotion.15 In humans the knee joint is closer to the (235 mm) was in the chimpanzee range of variation, and distinct mid-line of the body, and placement of the feet facilitates a from Homo.