1 CHAPTER 10 Origin and Evolution of Human Postcranial Anatomy Brian G. Richmond and Kevin G. Hatala INTRODUCTION One of the oldest questions in paleoanthropology concerns what stages, or body shapes and adaptations, characterized our ancestors over the course of human evolution. In the early half of the 20th century, leading scholars debated whether humans evolved from an ancestor that was a large ape, a small ape, a monkey-like arboreal or terrestrial quadruped that walked on all fours, or a succession of several of these models. These models diverged widely because they were primarily based on comparative anatomy of primates and hypotheses about which modern primates were most closely related to modern humans (see review and references in Richmond et al. 2001). For example, Charles Darwin s colleague Thomas Henry Huxley (1863) concluded that humans and great apes must be closely related because of the anatomical similarities they share. Sir Arthur Keith and William King Gregory shared this view and, in the 1920s, were the first to explicitly argue that early human bipedalism evolved from a large-bodied ancestor with an orthograde (i.e., upright-bodied) troglodytian (chimpanzee-like) climbing mode of locomotion, which itself evolved from a hylobatian (i.e., gibbon- or siamang-like) brachiating ancestor. Gregory (1927, p. 3) commented that in human anatomy he saw a veritable museum of relics of a former arboreal condition. At the other extreme, Wood Jones (Keith s student) argued that the human lineage had a much deeper split from the lineages leading to other modern primates, and concluded that humans had evolved from a small, monkey-like arboreal creature or perhaps even a prosimian-like vertical clinger and leaper. At the heart of A Companion to Paleoanthropology, First Edition. Edited by David R. Begun Blackwell Publishing Ltd. Published 2013 by Blackwell Publishing Ltd.
2 184 BRIAN G. RICHMOND AND KEVIN G. HATALA these models were debates about the phylogenetic (i.e., evolutionary relatedness among groups of organisms) relationships of humans to other primates. The middle of the 20th century witnessed major advances in genetic methods that have essentially resolved the phylogenetic relationships of modern primates, and major discoveries of the ape and human fossil record that provided the first clear evidence of the anatomy of extinct ape and human ancestors and relatives. The fossil record now provides a picture that is beginning to take shape about the kinds of ancestors from which we evolved. However, gaps in the fossil record still leave unanswered some of the most important questions about our origins and evolution. This chapter reviews the main scenarios about the origin of body form in hominoids (= ape and human) and hominins (= species more closely related to modern humans than to any other living species), and what the evidence from the fossil record suggests about the evolution of the postcranial skeleton of apes and humans. ORIGIN AND EVOLUTION OF HOMINOID FORM Modern apes all share a suite of postcranial adaptations, most of them attributed to their distinctive way of climbing with frequent orthograde postures and below-branch suspensory movement through trees. Of course, each modern ape species is unique in some aspects of its anatomy and locomotor (movement) behavior. Notably, the various species of gibbons are the smallest (about 5 8 kg; Smith and Jungers 1997), brachiate with a distinctive pendulum-like arm-swinging movement under branches, and have exceptionally long, gracile forelimbs. Their larger cousins, the siamangs (about kg), who are arguably more primitive postcranially, also brachiate but spend more of their time climbing and clambering through canopies. Orangutans (about kg) practice a more deliberate form of clambering in which they hold onto multiple supports at a time with exceptionally mobile limb joints attached to a stiff trunk. African apes (gorillas, about kg; common chimpanzees, about kg; bonobos, about kg) engage in varying degrees of climbing, clambering, and suspension, especially when young. Within species, tree-climbing decreases with age. Among species, tree-climbing decreases with adult body size. When terrestrial, African apes knuckle-walk, a unique form of locomotion among primates that involves flexing the fingers to bear weight on the backs of the middle phalanges and is associated with a suite of adaptations in the fingers, hands, and wrists. Modern apes share a suite of anatomical features (Figure 10.1a) associated with orthograde arboreal climbing and suspension including: long forelimbs; broad, shallow rib cage; tall, wide pelves (gibbons represent an exception with perhaps secondarily narrow pelves); relatively stiff back with fewer lumbar vertebrae than typical of other primates; long, narrow, and laterally-facing scapula; mobile shoulder and hip joints; a spool-shaped humeral trochlea providing a stable hinge-like elbow joint; no tail; long, curved fingers and toes (least expressed in the most terrestrial apes); a short thumb; and robust hallux. Early scholars proposed the parsimonious hypothesis that these anatomical features were homologous; that is, the last common ancestor (LCA) of modern hominoids had this suite of anatomical features. Similarly, parsimony predicts that the great ape LCA was a large-bodied climber and clamberer with anatomy shared by the large-bodied modern great apes, and that the LCA of African
3 (a) Round humeral head Very long forelimbs Broad thorax Short lumbar region No tail Reduced ulnar styloid Moblie hip Spool-shaped trochlea Robust hallux (b) Broad shallow rib cage Spine deep in truck Long forelimbs Reduced ulna-wrist articulation Mobile wrist Mobile hip Long, curved fingers (c) Hominoid synapomorphies No tail Long lumbar region Narrow thorax Subequal limb lengths Moderately mobile hip Spool-shaped trochlea Robust grasping hallux Fingers with moderate length and curvature Long, grasping thumb Figure 10.1 Characteristic postcranial skeletal features of (a) modern apes (from Fleagle 1999), and hypothesized characteristic features of (b) the hominid (great ape and human) last common ancestor (LCA) based primarily on evidence from Eurasian hominid fossils, and (c) the hominoid LCA (adapted from Fleagle 1999). Derived features of the LCAs are denoted with asterisks.
4 186 BRIAN G. RICHMOND AND KEVIN G. HATALA apes and humans was an arboreal climber and knuckle-walker bearing the adaptations to those behaviors. However, the fossil record has raised questions about all three hypotheses, and shows that at least some independent evolution, or homoplasy, must have occurred in the evolution of ape and human postcranial anatomy. Early Miocene The hominoids of the Early Miocene (23 16 Ma) include some species that are probably true hominoids within the hominoid clade, as well as wide array of primitive apes that are probably sister taxa to the true hominoid clade (Figure 10.2) (see Chapter 20 by Harrison and Chapter 21 by Begun, this volume). These fossil apes are diverse and display a range of postcranial adaptations. However, no known taxon from this time has all or even most of the anatomical features shared by modern apes and expected to be present in the hominoid LCA. Mike Rose, a leading scholar in the field, famously asked whether the postcranial morphology of Early Miocene hominoids was monkey-like, ape-like, neither, or both? (Rose 1983:405) Based on the fossils available then, it was already apparent that apes in the Early Miocene showed unexpected combinations of primitive, derived, and unique morphologies. The Early Miocene apes can be separated into three groups of species with similar postcranial anatomies. The first group consists of small apes (7 15 kg), including species of the genera Dendropithecus (D. maccinesi), Simiolus (S. injiessi, S. leakeyorum), Rangwapithecus (R. gordoni), Limnopithecus, and others (Fleagle 1999; Harrison, this volume Chapter 20). Their overall morphology - including an oval radial head associated with pronated hand postures, and other features that resemble quadrupedal New World monkeys suggests that pronograde (i.e., with the body facing down and positioned horizontally) arboreal quadrupedalism was their primary mode of locomotion. However, these species also possessed long and slender limbs, reminiscent of modern spider monkeys and suggesting at least some degree of suspensory behavior. The second group is one of medium- to large-bodied (about kg) apes. Among these is the genus Proconsul (P. africanus, P. heseloni, P. major, P. nyanzae), whose postcranial morphology is perhaps the best known of all Miocene hominoids (Begun, this volume Chapter 21). The overall limb morphology of Proconsul, including limb proportions with roughly equivalent forelimb and hindlimb lengths, is similar to that of extant New and Old World arboreal quadrupedal monkeys. Proconsul had a long and flexible monkey-like torso with probably six lumbar vertebrae, and the circular shape of their femoral condyles suggests their knee could have been mechanically loaded in a variety of positions (Ward 1997). The Proconsul thumb has a short base depth and is long and slender, indicating a thumb adapted to above-branch arboreal grasping. The wrist was well-suited for weight-bearing in a wide variety of postures but especially an extended wrist posture typical of above-branch pronograde qudarupedalism. The postcrania of Afropithecus (A. turkanensis, A. leakeyi) and Turkanapithecus (T. kalakolensis) are similar to Proconsul (Ward 1997) despite marked cranio-dental differences (Begun, this volume Chapter 21), implying that these large-bodied hominoids probably used similar forms of locomotion. While all three of these genera were similar in body size to modern great apes, their postcranial anatomy showed a different suite of adaptations, one more compatible with pronograde arboreal quadrupedalism. A third group consists of one genus whose postcranial body form appears quite different from any others known from the Early Miocene, and possibly most similar
5 Millions of years BP Simiolus Dendropithecus Micropithecus Nyanzapithecus Chororapithecus Mabokopithecus Rangwapithecus Samburupithecus Otavipithecus Turkanapithecus Equatorius Nakalipithecus Afropithecus Nacholapithecus Ardipithecus Proconsul Sahelanthropus Orrorin Morotopithecus Heliopithecus Kenyapithecus Hispanopithecus Oreopithecus Griphopithecus Dryopithecus Graecopithecus Rudapithecus Ouranopithecus Udabnopithecus Africa Europe Asia Ankarapithecus Lufengpithecus Sivapithecus Khoratpithecus Gigantopithecus KEY Hominoid, postcranial fossils Hominoid, no postcranial fossils Potential hominoid, postcranial fossils Potential hominoid, no postcranial fossils Figure 10.2 Phylogram showing known potential hominoid and true hominoid (= clade uniting modern hominoids) taxa from Africa, Europe, and Asia. Taxa that have associated postcranial evidence are denoted in grey.
6 188 BRIAN G. RICHMOND AND KEVIN G. HATALA to living hominoids. This genus is Morotopithecus (M. bishopi). Characteristics of the distal femur, specifically the morphology of the popliteal fossa, suggest a locking and unlocking rotation of the knee similar to that seen in extant great apes. This has been interpreted as evidence that Morotopithecus was not an efficient quadruped (MacLatchy et al. 2000). Furthermore, the broad, evenly-curved glenoid fossa of the scapula suggests a mobile shoulder joint like those of brachiating or arm-hanging hominoids. The lumbar vertebrae are somewhat more like those of modern hominoids compared with those of other Early Miocene apes, with short bodies and transverse processes that originate relatively posteriorly. This morphology suggests that Morotopithecus had a more orthograde posture with a spine situated more deeply within the rib cage. This combination of postcranial morphology, which bears close resemblance to the morphological pattern shared by the extant great apes, has led some to propose Morotopithecus as the sister taxon to all living hominoids (MacLatchy et al. 2000). However, the anatomy of multiple vertebral fragments suggests that M. bishopi retained a long lumbar region with six or seven segments, like P. nyanzae and unlike modern apes (Nakatsukasa 2008), and raises questions about whether the orthograde adaptations in Morotopithecus arose independently or are homologous with those of modern apes (Begun, this volume Chapter 21). Middle Miocene The Middle Miocene (16 12 Ma) saw greater postcranial diversity despite lower taxonomic diversity compared with that seen in the Early Miocene, with genera in Europe and Africa showing disparate morphologies (Figure 10.2). Hominoid postcranial specimens from the Middle Miocene of Europe have been attributed to Dryopithecus and Pierolapithecus. Pierolapithecus had a relatively broad and shallow thorax that would permit a wide range of motion at the shoulder joint, similar to the morphological pattern of extant great apes (Moyà-Solà et al. 2004). Pierolapithecus has moderately curved manual phalanges that are shorter and less curved than the fingers of some Late Miocene and extant hominoids, as well as other anatomical features that suggest that Pierolapithecus used a mixture of climbing and quadrupedalism without much below-branch suspension. The only known postcranial fossils attributed to Dryopithecus are a humeral shaft and a femur. The humeral shaft is straight and gracile, as in extant apes and unlike the condition in monkeys (Begun, this volume Chapter 21). The femur also preserves features of the hip joint shared with great apes. In sum, these European hominoids show varying degrees of adaptations for orthograde and suspensory posture and locomotion, making their postcranial morphology appear similar to, but somewhat less derived than, the morphologies of extant great apes. In Africa, the postcrania of three genera are known from the Middle Miocene. While there are hints of orthogrady, none of these taxa show the same degree of orthogrady seen in the European taxa described above. The first of these, Nacholapithecus (N. kerioi), shows some postcranial similarities to Proconsul but with longer forearms and longer toes, a pattern that suggests a greater emphasis on climbing and grasping than in Proconsul. Some features suggest adaptations for a stiff back and orthograde posture (Nakatsukasa et al. 2007). However, Nacholapithecus lacks the full suite of specializations for suspension that are seen in extant hominoids. While they may have been capable of performing this behavior, it was probably not their
7 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 189 dominant, habitual mode of locomotion. Rather, Nacholapithecus probably engaged in some unique combination of pronograde quadrupedalism, climbing, and suspension. Importantly, the discovery of a terminal caudal vertebra provides the oldest undisputed evidence of a hominoid lacking a tail (Nakatsukasa et al. 2003). A humeral fragment is attributed to Kenyapithecus (K. wickeri). It exhibits a posteriorly inclined medial epicondyle, a feature of today s terrestrial monkeys that orients the origin of hand and finger flexor muscles in a mechanically advantageous position for locomotion on the ground. This implies that Kenyapithecus may have been one of the first terrestrial hominoids. On the other hand, it has a capitulum (for the radius) separated from the trochlea (for the ulna) by a gap (the zona conoidea), which, while less well developed than in modern apes, more closely resembles apes than terrestrial monkeys. The genus Equatorius (E. africanus) is well-represented in terms of postcranial specimens. Equatorius is broadly similar to Proconsul, although a few characters suggest the possibility that it may have been somewhat more terrestrial than Proconsul. However, proportional lengths of manual phalanges fall in between fully terrestrial and arboreal species when compared with extant hominoids, implying that some arboreal locomotion was practiced (Patel et al. 2009). Late Miocene There is a comparatively rich record of Miocene ape fossils from Europe, including a number of partial skeletons. Hispanopithecus laietanus, from Spain, is known from one of these partial skeletons (Moyà-Solà et al. 1996). The morphology of the lumbar vertebrae includes posteriorly oriented transverse processes, indicating that the spine was set deep within the trunk. This suggests that these species were adapted for habitual orthograde posture, a positional behavior many have viewed as a precursor to adaptations for bipedal locomotion. Hispanopithecus and Rudapithecus, the latter from Hungary, both have long, curved fingers that suggest a type of suspensory locomotion similar to that of extant hominoids. Rudapithecus also has a mobile wrist joint, suggesting that climbing and suspension dominated the locomotor repertoire of this genus as it does in Hispanopithecus. The hip joints of Rudapithecus are large and the neck of the femur is long, suggesting increased mobility relative to monkeys, another general similarity with extant apes (Begun, this volume Chapter 21). One hominoid with associated postcrania is known from the Late Miocene of Turkey, Ankarapithecus (Begun, this volume Chapter 21). Although only a few postcranial fossils are known, they show evidence of adaptations for pronograde quadrupedalism, not the orthograde body shape of extant apes. For example, the radius has a short neck and angled, asymmetric head suggesting a particularly stable elbow joint with the forearm in a pronated position. The finger fragments are moderately curved, suggesting quadrupedalism in an arboreal setting. The Late Miocene fossil record of Asia is best represented by the genus Sivapithecus (S. sivalensis, S. indicus, S. parvada). This genus shares distinctive, derived craniodental features with modern orang utans, supporting the hypothesis that Sivapithecus is an early member of the subfamily Ponginae (orang utans and their extinct relatives), yet in a number of features their postcrania are distinct. Sivapithecus shows ape-like features of the wrist and elbow and more pronograde characters of the shoulder, indicating a unique combination of postcranial attributes. It has been called a pronograde
8 190 BRIAN G. RICHMOND AND KEVIN G. HATALA quadruped, a suspensory taxon or even a possible knuckle-walker (Rose 1993; Ward 1997; Begun, this volume Chapter 21). Oreopithecus (O. bambolii), is well-represented from sites in Italy, in Tuscany and Sardinia when both were islands separated from the European mainland. This species had a peculiar combination of apparently primitive craniodental morphology along with postcranial anatomy similar, perhaps superficially, to that of extant hominoids (Harrison 1986; Begun, this volume Chapter 21). This postcranium included a broad and shallow torso that would permit a wide range of movements at the shoulder joint, and forelimbs with flexible joints and long, highly curved fingers. These anatomical features, combined with long forelimbs and short hindlimbs relative to African apes, suggest that below-branch climbing and suspension were the primary forms of locomotion in this genus. Despite postcranial similarities to extant apes, the primitive craniodental anatomy suggests that Oreopithecus is a derived, insular descendant of a Proconsul or Dryopithecus-like ancestor that is likely a terminal descendant of earlier apes rather than a close relative of any modern ape (Begun, this volume Chapter 21). It is particularly notable that major gaps exist in the fossil record of modern apes during the Late Miocene to the Late Pleistocene. With the exception of a few relatively recent fossils, the fossil records of hylotabid, orang utan, gorilla, and chimpanzee and bonobo lineages are virtually unknown. These major gaps leave unanswered key questions about the origins and evolution of postcranial anatomy unique to each group. Origin of the hominoid body plan Evidence to date suggests that the hominoid LCA was a pronograde quadruped unlike any living primate today, lacking the derived anatomy of modern Old World monkeys. The anatomy of early hominoids and parsimony suggest that the hominoid LCA had long, grasping thumbs and first toes, considerable hip and wrist mobility, an incipient spool-shaped trochlea of the elbow joint, and no tail (Figure 10.1c). Some of these traits have been hypothesized to be adaptations to climbing. However, all of these features can potentially be explained as an anatomical complex adapted to the problem of maintaining balance walking atop branches at fairly large size. Most primates are relatively small and use their tails to help maintain balance atop branches. However, as body mass increases, the tail must disproportionately increase in length to maintain its effectiveness as a balance mechanism. This scenario argues that the hominoid LCA had a moderate body size that was large enough that the tail was not effective in maintaining balance. At moderate large body sizes, branches are relatively smaller, making the hands and feet more effective at grasping around them. This can explain the relatively long thumbs and halluces of fossil (not modern) apes that would have been effective in powerfully grasping branches and maintaining balance. This would also explain the unique spool-shaped elbow anatomy that separates the ulnar and radial articulation in order to maintain a stable elbow joint during flexion/extension while permitting mobility in pronation/supination. The moderate body size is also consistent with a behavioral repertoire that involved greater amounts of clambering and climbing than typical of modern monkeys, and could have set the stage for further specializations in orthograde climbing and suspension during the evolution of modern ape taxa.
9 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 191 Origin of the hominid body plan The fossil record provides more support for the hypothesis that at least some orthograde climbing anatomy characterizing the hominid (great ape and human) clade is homologous. Fossil great apes (e.g., Pierolapithecus, and especially Hispanopithecus and Rudapithecus) of the Middle Late Miocene in Eurasia have orthograde, climbing, and/or suspensory features that in many ways make reasonable ancestral states for modern great apes and humans. However, even if the hominid LCA had many climbing characteristics seen in modern great apes and humans, homoplasy was also common in the evolution of the hominid body plan (Figure 10.1b). For example, the pronograde quadrupedal anatomy of Ankarapithecus and Sivapithecus, early orangutan relatives, shows that 1) either the LCA of great apes lacked the orthograde anatomy of modern great apes, or 2) Ankarapithecus and Sivapithecus represent evolutionary reversals to a more primitive body shape, or 3) Pongo has evolved suspensory characters in parallel with other great apes. ORIGIN OF THE HOMININ CLADE AND BIPEDALISM The end of the Miocene was a time of major changes in global climate. Sea levels lowered at the end of the Miocene until the Strait of Gibraltar closed and much of the Mediterranean Sea evaporated on several occasions, known as the Messinian Salinity Crisis. Africa became overall cooler and drier, with well-documented increases in grasses and decreases in forests in eastern Africa. Against the backdrop of these global and local climatic changes, the LCA of African apes and humans split into a gorilline clade and Pan Homo clade, and the latter split again into panin and hominin clades. The main debates about the origin of the African ape human clade concern biogeography (Begun, this volume Chapter 21), namely whether an ancestor of this clade might have migrated from Eurasia during the Late Miocene or not, and whether this LCA practiced knuckle-walking as part of their adaptation or this behavior evolved independently in Gorilla and Pan (Richmond et al. 2001). Some features of the wrist, such as a fused os centrale and derived distal radius, are biomechanically advantageous for knuckle-walking and are shared among African apes and early hominins, suggesting that the African ape-human LCA had a body plan adapted to knuckle-walking and climbing. However, conclusive answers to hypotheses about the evolution of knuckle-walking require additional fossil evidence. Earliest potential hominins The earliest potential hominins include three genera (Sahelanthropus, Orrorin, Ardipithecus) from the Late Miocene (see Simpson, this volume Chapter 22), some or all of which may represent basal members of the hominin clade. It is important to interpret these taxa with caution. Each are represented by small fossil samples and classified as members of the hominin clade based upon few shared derived traits. Nevertheless, the postcranial anatomies of these taxa bear similarities to fossil hominins that suggest some comparable functional adaptations. Their anatomy implies that terrestrial bipedalism played some role in their locomotor repertoires, yet the degrees to which these taxa relied upon this behavior remain open to debate.
10 192 BRIAN G. RICHMOND AND KEVIN G. HATALA Sahelanthropus tchadensis is known only from cranial fossils found in Chad, dating to about 7 Ma. The only evidence that this taxon was adapted for bipedal locomotion or at least highly orthograde postures is the anterior position and orientation of the foramen magnum, which is correlated with orthograde postures such as those used in a bipedal posture (Zollikofer et al 2005). Certainly, the discovery of postcranial fossil evidence will be critical before we can properly understand the positional behavior and locomotion of S. tchadensis. Orrorin tugenensis is known from cranial and postcranial fossils uncovered in western Kenya, which date to about Ma (Senut et al 2001). The shape of the proximal femur (e.g., long neck with thick cortical bone on the inferior surface, wide shaft, and groove for the obturator externus muscle) was similar to that of Australopithecus, compatible with the loading pattern and muscular actions of bipedal locomotion (Richmond and Jungers 2008). A proximal hand phalanx is as curved as those of modern African apes, suggesting that climbing remained an important component of the locomotor repertoire. As is the case with S. tchadensis, more postcranial fossils are needed before we can fully understand the functional adaptations of O. tugenensis. While fossils of Ardipithecus have been described as belonging to two species (Ar. ramidus and Ar. kadabba), securely-attributed postcranial fossils have only been recovered from one, Ar. ramidus. This species is known from Ethiopia and dates to about 4 4 Ma. Ar. ramidus possessed relatively long, highly curved fingers and toes, adaptations for suspension and climbing (White et al. 2009). The pelvis of Ar. ramidus is short, as in later hominins, and the foot is characterized by a peculiar combination of a widely abducted hallux and what has been interpreted as a relatively immobile ankle. This suite of morphological features suggests that, while bipedal locomotion and/or orthograde postures may have been important components of the behavioral repertoire of Ar. ramidus, its primary mode of locomotion was probably a form of vertical climbing and clambering. Australopithecus Among the early hominins attributed to the genus Australopithecus, a body plan emerged that was in many ways more similar to that of modern humans than to that of other extant apes (Figure 10.3). Several morphological features appeared that are associated with bipedal locomotion, showing that these taxa were adapted to this behavior (see Ward and Hammond, this volume Chapter 23). However, these early hominins also retained some primitive postcranial traits, which are found in extant non-human apes and associated with arboreal locomotion. This complex mix of primitive and derived traits has made it very difficult to draw conclusions about the degrees to which Australopithecus climbed trees (e.g., to find food, avoid predators, and for sleeping at night) as well as traveled bipedally on the ground. It is most likely that different species of Australopithecus differed slightly but significantly in their locomotor repertoires and exact motions used when bipedal, perhaps comparable with differences observed in modern closely-related taxa such as bonobos and chimpanzees. Australopithecus fossils also show the earliest evidence of a change in hand proportions involving shortened fingers and metacarpals relative to thumb length, compared with
11 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 193 the anatomy observed in Ardipithecus and extant apes. Shortened fingers may be the result of selection on manipulative behaviors or could be a pleiotropic effect of selection on reduction in toe length for terrestrial bipedalism, or both. Whatever the cause, shortened fingers make the thumb : finger proportions more equivalent and greatly improve manual dexterity in Australopithecus. Four Australopithecus taxa, Au. anamensis, Au. afarensis, Au. africanus, and Au. sediba, preserve sufficient postcranial anatomy to warrant inferences about the evolution of locomotor behavior and manipulative ability. Au. anamensis represents the earliest taxon whose attribution to the hominin clade has not been questioned. Specimens date from 4 2 to 3 9 Ma, and are known from Kenya. The articular surface of the Au. anamensis proximal tibia is oriented about a flat horizontal plane, rather than being posteriorly-oriented as in modern nonhuman apes. This morphology suggests that the Au. anamensis knee joint was adapted for the type of weight-bearing and function characteristic of bipedal locomotion (Ward and Hammond, this volume Chapter 23). However, Au. anamensis also possessed long forearms and curved manual phalanges, which may imply that they also engaged in some degree of tree climbing. A rich fossil record of Au. afarensis is known from Ethiopia, with fossils dating from about 4 0 to 3 0 Ma (Kimbel and Delezene 2009). The lower limb morphology of Au. afarensis suggests that this species was a proficient biped. They possessed a short pelvis that improves stability in bipedal postures by reducing the distance between the sacrum and hip joint. A short lumbar vertebral column, lumbar lordosis, and a robust sacrum also indicate stability in the lower back, unique to bipeds. However, the iliac blades remain relatively coronally oriented, which may have compromised the ability of gluteal muscles to balance the trunk throughout a full stance phase. This and other features have led several researchers to propose unique forms of kinematics (motion patterns) during bipedal gait. The Au. afarensis lower limb was also characterized by a valgus knee that positions the foot more directly under the body s center of gravity, and a robust calcaneus capable of withstanding the high ground reaction forces associated with bipedal walking and running. However, a suite of primitive traits suggests that suspensory climbing was still an important locomotor behavior for Au. afarensis. These include a cranially-oriented shoulder joint associated in apes with use of the forelimbs above the head, and curved fingers and toes well-suited to the biomechanics of flexed grasping. Furthermore, the lower limbs of Au. afarensis are relatively short compared to their overall body size, making their bipedal locomotion less energetically efficient than that of modern humans. This combination of primitive and derived traits has sparked debate over the degree to which this taxon was exclusively a terrestrial biped and whether the retention of primitive apelike anatomy indicates a combination of bipedal walking and climbing trees (summarized in Kimbel and Delezene 2009). Au. afarensis is the earliest hominin species in which we see definitive evidence of a hand more capable of complex manipulative abilities than the hands of extant great apes. Hand fossils indicate that the Au. afarensis fingers had shortened relative to the thumb. With shortened fingers, Au. afarensis may have been capable of performing a precision grip, an ability that was probably important to early humans for tool use and production, foraging, and/or other manipulative behaviors requiring manual dexterity. Au. africanus, a species known from South Africa between 3 2 and 2 4 Ma, had an overall postcranial morphology that was in most ways similar to that of Au. afarensis,
12 Craniallyoriented shoulder joint 6 Lumbar vertebrae Broad, coronal iliac blade Long femoral neck, small head Valgus knee Mostly adducted hallux Australopithecus Robust humerus Long forearms Primitive wrist Gracile thumb Fingers with moderate length and curvature Moderate lower limb length Curved, slightly elongated toes Laterally-oriented shoulder joint Barrel-shaped rib cage Broad, sagittallyoriented iliac blade Long femoral neck, large head Adducted hallux Homo erectus Tall stature Gracile humerus Short forearms Long lower limbs Robust thumb Longitudinal arch Narrow scapula, broad glenoid Narrow rib cage Narrow, verticallyoriented iliac blades Short femoral neck, large head Short pubis Fully adducted hallux Homo sapiens Gracile skeleton Short clavicle Short radial neck Short forearms Curved saddle joint Robust thumb Large lateral patellar facet Figure 10.3 Characteristic postcranial skeletal features of Australopithecus (left), Homo erectus (center), and H. sapiens (right). The features noted on Australopithecus include the combination of primitive and derived traits characteristic of this genus; those noted on H. erectus are derived relative to Australopithecus; and those indicated for H. sapiens are autapomorphic.
13 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 195 but with its own unique differences. The differences include more apelike limb size proportions, suggesting that climbing might have formed a greater component of their locomotor repertoire compared with that of Au. afarensis (Green et al. 2007). An investigation of hominin inner ear morphologies, which are critical to one s ability to balance, found that the balancing abilities of Au. africanus were probably more similar to extant great apes than to modern humans, implying that they were less proficient at walking and/or running. Au. sediba, known only from the site of Malapa at 2 Ma, possessed a mixture of primitive and derived postcranial traits, different from the pattern seen in other species of Australopithecus. The Au. sediba pelvic blades were oriented in a position more similar to those of the genus Homo, suggesting that they were capable bipeds. The morphology of the distal tibia implies that a valgus knee and arched foot, both derived traits associated with bipedalism, may have been present (Zipfel et al. 2011). However, the hand retains primitive traits linked to the capability for powerful flexion, a pattern seen in species adapted for climbing (Kivell et al. 2011). Additionally, the calcaneus is remarkably gracile and thus poorly-suited for habitually absorbing the impact forces associated with bipedal walking and running. Together, these traits imply that Au. sediba had a unique mode of locomotion different from that of any other hominin, which may have included some combination of both terrestrial bipedalism and tree climbing. The Au. sediba hand may have been capable of dexterous manipulation due to shortened fingers relative to the thumb (Kivell et al. 2011). However, the hand is unique in that the fingers are relatively shorter than those of any hominin species. If this species was adapted for the types of manipulative behaviors practiced by modern humans, then they would have had to accomplish these in a slightly different manner than modern humans. Megadont hominins Evidence of the postcranial anatomy of megadont hominins is far less complete than that of Australopithecus. This grade of hominins is defined based on unusually large cheek teeth (premolars and molars) and robust skull anatomy (Wood and Schroer, this volume Chapter 23), and consists of species in the genus Paranthropus and one species placed in the genus Australopithecus, Au. garhi. At present, there are no postcranial fossils that can be securely attributed to P. aethiopicus and P. boisei. It is almost certain that some of the postcranial fossils from the Koobi Fora Formation, Kenya, and Olduvai Gorge, Tanzania, belong to P. boisei based on the relative abundances of taxonomically-identifiable craniodental fossils of P. boisei and early Homo. However, most postcranial fossils are not associated with craniodental remains, and therefore reliable inferences cannot be made about the postcranial anatomy of these megadont hominins. The same is true for postcranial fossils found at several sites at Bouri, where several hundred meters away in same-aged (2 5 Ma) deposits, craniodental fossils were recovered and made the holotype of Au. garhi. The unattributed postcranial fossils include a partial skeleton, BOU-VP 12/1; if length estimates are correct they represent the earliest evidence of a human-like long femur. The estimation errors for the radius and humerus are too large to make reliable conclusions about the brachial proportions. This skeleton includes a foot proximal phalanx reported to match those of Au. afarensis in length and curvature
14 196 BRIAN G. RICHMOND AND KEVIN G. HATALA (Asfaw et al. 1999), pointing to a unique combination of apelike foot anatomy associated with tree-climbing in conjunction with long lower limbs that improve bipedal walking and running efficiency. The same problem frustrates efforts to understand the postcranial anatomy of P. robustus, which is well represented by craniodental remains at Swartkrans and Drimolen, South Africa. However, craniodental fossils of early Homo are also present at both sites. It has been argued that because a majority of the craniodental fossils represent P. robustus, most of the postcrania should also represent P. robustus (see Wood and Schroer, this volume Chapter 23). However, isolated postcranial fossils cannot be attributed with certainty. Fossils that are dissimilar to those known from the genus Homo at other contemporaneous sites, and therefore most likely represent P. robustus, reveal a mix of primitive and derived traits related to bipedalism and manual dexterity. First metatarsals recovered from Swartkrans and attributed to P. robustus show evidence of a humanlike adducted hallux, and may show attachments for the plantar ligaments that stabilize the arch and help the human foot act as a propulsive lever (Susman and De Ruiter 2004). However, the distal articular surfaces of these first metatarsals suggest that P. robustus would have toed off in a way quite different from that seen in modern humans. Meanwhile, fossil hand bones have features, such as a broad apical tuft and an insertion for the flexor pollicis longus muscle on the thumb s distal phalanx (similar to those in Au. africanus and Au. sediba), that suggest a hand capable of performing precision grips well-suited for tool production and/or use. ORIGIN OF HUMAN FORM The Late Pliocene and Early Pleistocene (3 1 5 Ma) was another period of major climatic change in Africa. Evidence from fossil mammals associated with open environments, paleosol carbon isotopes, and ocean core biomarkers demonstrate a major shift in eastern Africa from more woodland environments to the expansion of more open grasslands (Bobe et al. 2007). The majority of land mammal species were replaced during this time period, marking an accelerated rate of extinction and speciation. Hominins were not exempt from the general mammalian pattern. It is in this context that the megadont hominins (above) and the earliest members of the genus Homo originated. Several hominin taxa, including the genus Australopithecus, disappear in the fossil record and many new hominin taxa appear in this time interval, including the genera Paranthropus and Homo, marking major changes in adaptive strategies and skeletal designs. This time period also witnessed the beginning of what later became a very rich archeological record with the first appearance of manufactured stone tools (Oldowan) combined with cut-marked, butchered mammal bones at least 2 6 Ma, the first dispersal of Homo beyond Africa into Asia by at least 1 8 Ma, and a major increase in brain size and emergence of a substantially more human body shape in early Homo Ma. Transitional species The first species in the genus Homo, H. habilis, was given the name handy man because the holotype (OH 7) included neurocranial fragments along with hand bones
15 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 197 and stone tools nearby (Leakey et al 1964). The authors were impressed by the trapezium s broad, flat saddle joint (carpometacarpal) at the base of the thumb, and the very broad ungual tuberosity on the thumb distal phalanx. In the context of a more complete fossil record, it is now clear that the broad thumb tip was already present in some Australopithecus species and that most of OH 7 s hand anatomy is primitive, including curved phalanges with well-developed flexor ridges. A partial skeleton, OH 62, includes a humerus equivalent in size to, and a femur that appears to be smaller than, the limb bones of A.L ( Lucy ). Although the limb bones in OH 62 are too fragmentary to reliably estimate lengths, the cross-sectional areas and circumferences are more apelike in OH 62 than in A.L Therefore, the hand anatomy and relative limb robusticities of H. habilis are quite primitive and resemble the anatomy of Australopithecus, suggesting a skeleton well-suited to climbing trees as well as bipedalism. When combined with the small body size, brain size marginally larger than in Australopithecus, large mandibles and teeth relative to body size, the primitive postcranial anatomy raises questions about whether or not this species represents an adaptive shift substantial enough to warrant inclusion in the genus Homo (Wood and Collard 1999). The variation in craniodental morphology in non-paranthropus fossils, especially in the fossil assemblages from Lake Turkana basin, makes it likely that more than one species of Homo is present in the Early Pleistocene (see Schrenk, this volume Chapter 25). The second species, typically referred to as H. rudolfensis, remains poorly known and has no securely attributed postcrania. Much later in time, Late Pleistocene deposits on the small Indonesian island of Flores preserve the skeletal remains of a surprisingly small and morphologically primitive hominin species, H. floresiensis (see Jungers, this volume Chapter 30). It is described in the transitional species category because despite being known only from deposits about ka, the craniodental and especially postcranial anatomy more closely resemble H. habilis than later species of Homo. The anatomy of H. floresiensis shows that it was bipedal, with features such as a short pelvis, and robust, adducted hallux. However, H. floresiensis also has a number of postcranial features that are primitively shared with H. habilis, and Australopithecus in cases where the anatomy of H. habilis is unknown. In some respects, H. floresiensis is unique, with a foot (especially forefoot) length longer than in any other hominin, and an estimated body mass and stature equivalent to the smallest known H. habilis and Australopithecus specimens and smaller than those of H. erectus and later hominins. This combination of postcranial anatomy suggests that H. floresiensis was bipedal but in a manner biomechanically distinct from the bipedalism of modern humans. With its short lower limbs and lack of human-like longitudinal arch, H. floresiensis does not appear to be well adapted to long-distance walking and running. The relative limb strengths and long, moderately curved toes suggest that it possessed adaptations for climbing trees. The primitive wrist anatomy indicates that this species lacked the adaptations for enhanced manual dexterity that are characteristic of later Homo. Finally, while cranial anatomy suggests that a primitive form of H. ergaster/erectus could make a reasonable ancestor, the many primitive features of the axial skeleton and upper and lower limbs instead suggests that a more primitive taxon, potentially H. habilis, is a more likely ancestor. This in turn suggests that H. floresiensis underwent a long, separate evolutionary history.
16 198 BRIAN G. RICHMOND AND KEVIN G. HATALA Homo Early H. erectus, also known as H. ergaster (Antón, this volume Chapter 26), marks a major transition in body shape, with many derived characteristics linking it with later Homo, including modern humans. For this reason, it is the earliest uncontested member of the genus Homo. The fossil record of H. erectus is relatively rich and geographically more expansive than any prior hominin species, with fossils and artifacts known from sites in Africa, Asia, and possibly Europe (Antón, this volume Chapter 26). It is the first hominin species found outside Africa, but the very primitive morphology of the Late Pleistocene H. floresiensis (Jungers, this volume Chapter 30) raises the interesting possibility that a taxon more primitive than H. erectus left Africa as well. The first appearance dates of H. erectus in eastern Africa (about 1 9 Ma) and the southern Caucasus (about 1 85 Ma) are close enough that, given the scarcity of fossil sites, evidence is not yet conclusive about where H. erectus first originated. H. erectus differs from earlier hominins in important ways in its postcranial anatomy (Figure 10.3). Much of the evidence of postcranial anatomy comes from KNM-WT 15000, an unusually complete partial skeleton of a roughly 8-year-old male from West Turkana, Kenya. It demonstrates that by 1 5 Ma, H. erectus had lost the primitive, apelike morphology related to tree climbing and instead evolved an upper limb featuring a laterally-facing shoulder joint and more humanlike scapula blade shape, and short forearms with humanlike brachial proportions. Although few hand elements are known, H. erectus shows the first evidence of a robust thumb capable of generating greater forces and resisting higher stresses, arguably as a result of selection for manipulation during tool making and/or use. The trunk and lower limb of H. erectus show evidence of selection for bipedal locomotion, including long lower limbs for speed, efficiency, and distance, and large lower limb joints capable of resisting greater joint forces. H. erectus also sports a barrel-shaped rib cage and changes in pelvic shape relative to pelves of Au. afarensis and Au. africanus, such as an iliac blade that is anteriorly expanded, providing attachment for anterior regions of the lesser gluteal muscles that balance the trunk in the late stance phase of walking. Fossil footprints dated to 1 5 Ma from Ileret, Kenya, show evidence, relative to the 3 7 Ma Australopithecus prints at Laetoli, Tanzania, of a more fully-adducted hallux, shorter toes, and well-developed arch (however, they may belong to P. boisei rather than H. erectus, since both are known from the area.) Based on these and other derived features, researchers have argued that H. erectus habitually used larger home ranges than previous hominins and was adapted to more efficient walking, higher running speeds, and/or perhaps long-distance running. However, some postcranial features, such as a broad pelvis and body shape, raise questions about how efficient H. erectus would be during endurance running. The next species of the genus Homo to appear in the fossil record, H. heidelbergensis, is a large, robust-bodied descendent of H. erectus (Hublin, this volume Chapter 27). Homo heidelbergensis, sometimes referred to as archaic Homo sapiens, is known from fossil evidence in Africa, Europe, and parts of Asia. The Sima de los Huesos cave site in the Sierra de Atapuerca, Spain, has yielded by far the most abundant collection of fossils of this species. The Sima de los Huesos site has produced over 4,000 hominin
17 ORIGIN AND EVOLUTION OF HUMAN POSTCRANIAL ANATOMY 199 fossils representing over 27 individuals including adult males, females, and juveniles. These and other fossils show that H. heidelbergensis was characterized by relatively large body mass with wide pelves that had flaring iliac blades and long pubic rami compared with modern humans, indicating a wide body shape. The pelves are sexually dimorphic in shape, suggesting modifications in females to accommodate larger neonates. The limb bones are robust with large joints capable of resisting high joint forces associated with running and long-distance walking. The hand bones lack the apelike features of the hand of H. habilis. In many of these postcranial features as well as others found in the clavicles, scapulae, and humeri the postcranial anatomy of H. heidelberensis resembles the morphology seen in H. neanderthalensis, which may have succeeded them in Europe, suggesting that this postcranial form is primitive for later Homo. The wide body shape and robust build of H. heidelbergensis suggests that they were not particularly well-designed for endurance running, but were clearly fully- committed bipeds capable of traveling long distances. H. heidelbergensis also provides the earliest evidence of a lower level of size dimorphism, comparable with that seen in Middle Pleistocene and modern human populations (Arsuaga et al. 1997). This suggests that a low level of intrasexual competition, possibly indicating a more modern humanlike pattern of pair-bonding, characterized H. heidelbergensis. H. neanderthalensis, also referred to as Neanderthals, was the first extinct hominin taxon recognized in the fossil record, first discovered in 1828 at Engis, Belgium, and only later recognized as belonging to a distinct species. In addition to distinctive skull and dental morphologies (Harvati, this volume Chapter 28), H. neanderthalensis exhibits a unique mixture of primitive and derived postcranial characteristics (Trinkaus 2006). Many of the features long thought to be distinctive to H. neanderthalensis are likely to be primitive, based on their presence in earlier taxa such as H. heidelbergensis. These include a broad body shape (broad rib cage, wide pelvis with a flaring iliac blade, long pubis and anteriorly positioned sacrum and robust limb bones.) The femora of H. neanderthalensis are curved, lack pronounced pilasters (distinctive crests on the posterior surface), and have short necks and low neck-shaft angles. H. neanderthalensis also exhibits a few autapomorphic postcranial characteristics including robust lower cervical spinous processes, marked curvature of the radius, a pronounced flange on the pollical metacarpal for the opponens pollicis and long pollical distal phalanges. Furthermore, Neanderthals have short distal limb elements including short hands and feet, a low brachial index (radius relative to humerus), and a low crural index (tibia relative to femur), that have interpreted as adaptations to cold climate. Although some postcranial variation exists between the earliest and more recent modern humans, H. sapiens is arguably more distinctive in its postcranial anatomy than other later Homo species (Trinkaus 2006; Figure 10.3). The modern human skeleton is, overall, more gracile than any other hominin taxon. Gracility increased over time in H. sapiens, but even early modern human fossils show evidence of gracility. On average, modern humans have lower body masses, but not necessarily stature, compared with H. heidelbergensis and H. neanderthalensis. Derived, autapomorphic features of H. sapiens include short clavicles, narrow rib cages, narrow scapula with a broad glenoid fossa, short radial neck, distinctive shape of the ulnar trochlea, curved saddle joint at the base of the thumb metacarpal, narrower ungual tuberosities at the ends of the distal phalanges, narrower pelvis with more vertically-oriented iliac blades,