Carol V. Ward

Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis

A long bone of the foot of an early human indicates that its foot was stiff and arched, as in modern humans. The transition to full-time terrestrial bipedality is a hallmark of human evolution. A key correlate of human bipedalism is the development of longitudinal and transverse arches of the foot that provide a rigid propulsive lever and critical shock absorption during striding bipedal gait. Evidence for arches in the earliest well-known Australopithecus species, A. afarensis, has long been debated. A complete fourth metatarsal of A. afarensis was recently discovered at Hadar, Ethiopia. It exhibits torsion of the head relative to the base, a direct correlate of a transverse arch in humans. The orientation of the proximal and distal ends of the bone reflects a longitudinal arch. Further, the deep, flat base and tarsal facets imply that its midfoot had no ape-like midtarsal break. These features show that the A. afarensis foot was functionally like that of modern humans and support the hypothesis that this species was a committed terrestrial biped.

The new hominid speciesAustralopithecus anamensis

Australopithecus anamensis1 is the earliest species of this genus to have been found. Fossils attributed to A. anamensis have been recovered from sediments dating to between 3.8 and 4.2 mya at the sites of Kanapoi and Allia Bay in northern Kenya. A. anamensis is still poorly known in comparison with other early hominid species, but the material discovered so far displays primitive features along with more derived characteristics typical of later Australopithecus species. This mix of features suggests that A. anamensis belongs near the ancestry of this genus. Indeed, it may eventually be determined that this was the earliest Australopithecus species.

Events in Hominoid Evolution

The preceding chapters of this volume have described a number of different approaches and solutions to the interpretation of hominoid evolutionary history. Given the breadth of approaches, it is difficult to compare results among researchers. Despite this diversity, however, there seems to be broad agreement on many issues in the complex evolutionary history of the Hominoidea.

Functional Anatomy and Phyletic Implications of the Hominoid Trunk and Hindlimb

Traditional hominid phylogenies are based on craniodental and mandibular characteristics primarily because these elements comprise the bulk of the hominoid fossil record. Few postcranial elements are known for most fossil hominoids, complicating intertaxic comparisons. Another reason postcrania tend to be neglected in phylogenetic studies is the assumption that they are more responsive to selective and ontogenetic pressures than skulls and teeth, and thereby are more likely to reflect homoplasy and obscure phylogenetic conclusions. Explanations of homoplasy are generally invoked when post-cranial analyses do not support phyletic schemes constructed using cranio-dental characters, e.g., in the cases of Sivapithecus (Pilbeam et al., 1990) and Oreopithecus (Harrison, 1986; Sarmiento, 1987).

Human evolution and the development of spondylolysis.

Study Design. We propose that chronic spondylolytic defects at L5 are influenced by insufficient differential mediolateral distances between inferior articular facets of L4 and the superior facets of S1, which results in these structures impinging on adjacent sides of the par interarticularis during hyperlordosis. Individuals with adequate increase in interfacet distances from L4 through S1 are less likely to develop or maintain defects. Objectives. We test the above hypothesis by comparing the transverse interfacet dimensions throughout the lumbar columns of normal and spondylolytic individuals. Summary of Background Data. Spondylolysis is a common condition, yet its etiology is poorly understood. It is generally considered to be the result of a vaguely defined fatigue fracture through the pars interarticularis. The cause(s) of spondylolysis, however, have not been clearly identified. Methods. Lumbar vertebrae from the Hamann-Todd osteological collection at the Cleveland Museum of Natural History were examined. Thirty individuals with bilateral spondylolysis at L5 were compared with 30 age- and sex-matched controls. Differences in transverse distances between articular facets and in transverse breadths of vertebral bodies were compared using two-tailed t tests. Results. Results show that normal individuals have a significantly greater increase in interfacet dimensions progressing down the spine from L4 to S1 than do those with spondylolysis. These differences are not the result of normal individuals having increasingly large vertebrae, as results are significant even when standardized for vertebral body breadth. Vertebral body size itself does not differ systematically between groups. Conclusions. Spondylolysis is the direct result of contact pressures on both sides of the pars interarticularis resulting from inadequate separation between the inferior articular processes of L4 and the superior articular facets of S1. Individuals lacking sufficient increase in transverse interfacet dimensions in their lumbar columns are at greater risk of developing and maintaining spondylolytic defects.

Early Pleistocene third metacarpal from Kenya and the evolution of modern human-like hand morphology

Significance A newly discovered metacarpal from Kaitio, Kenya, dates to 1.42 Mya and provides evidence for the evolution of the modern human hand more than 600,000 y earlier than previously documented. This bone displays a styloid process, which is part of a distinctively human rearrangement of the wrist associated with enhanced hand function when making and using tools. Prior to this discovery, the first evidence of this anatomy was found only in post–Homo erectus archaic humans. This new find documents its appearance much earlier in time and suggests that an increased reliance on manipulatory behaviors indicated by the archeological record early in the Pleistocene selected for the modern human hand early in the evolution of the genus Homo. Despite discoveries of relatively complete hands from two early hominin species (Ardipithecus ramidus and Australopithecus sediba) and partial hands from another (Australopithecus afarensis), fundamental questions remain about the evolution of human-like hand anatomy and function. These questions are driven by the paucity of hand fossils in the hominin fossil record between 800,000 and 1.8 My old, a time interval well documented for the emergence and subsequent proliferation of Acheulian technology (shaped bifacial stone tools). Modern and Middle to Late Pleistocene humans share a suite of derived features in the thumb, wrist, and radial carpometacarpal joints that is noticeably absent in early hominins. Here we show that one of the most distinctive features of this suite in the Middle Pleistocene to recent human hand, the third metacarpal styloid process, was present ∼1.42 Mya in an East African hominin from Kaitio, West Turkana, Kenya. This fossil thus provides the earliest unambiguous evidence for the evolution of a key shared derived characteristic of modern human and Neandertal hand morphology and suggests that the distinctive complex of radial carpometacarpal joint features in the human hand arose early in the evolution of the genus Homo and probably in Homo erectus sensu lato.

Afropithecus, Proconsul, and the Primitive Hominoid Skeleton

Many distinctive synapomorphies of modern apes (and humans) are found in the postcranial skeleton. These characters reflect a basic adaptation, variably developed and practiced among modern species, to forelimb-dominated arboreal locomotion, including climbing, brachiation, and/or forelimb suspension (e.g., Cartmill and Milton, 1977; Fleagle et al., 1981; Hunt, 1992; Keith, 1923; Stern, 1971; Stern et al., 1977). The morphological pattern shared by modern hominoids has led to the general assumption that locomotor divergence was an initial hallmark of the hominoid lineage, setting them apart from their monkey-like forbears. As more is learned about the earliest hominoids, however, paleontologists realize that not all apes share a similar pattern of postcranial anatomy and locomotor behavior, and that the suite of features seen in extant apes evolved in a mosaic fashion over the course of hominoid evolutionary history (reviews and references in Begun et al., 1997a).

Anterior dental evolution in the Australopithecus anamensis-afarensis lineage.

Australopithecus anamensis is the earliest known species of the Australopithecus–human clade and is the likely ancestor of Australopithecus afarensis. Investigating possible selective pressures underlying these changes is key to understanding the patterns of selection shaping the origins and early evolution of the Australopithecus–human clade. During the course of the Au. anamensis–afarensis lineage, significant changes appear to occur particularly in the anterior dentition, but also in jaw structure and molar form, suggesting selection for altered diet and/or food processing. Specifically, canine tooth crown height does not change, but maxillary canines and P3s become shorter mesiodistally, canine tooth crowns become more symmetrical in profile and P3s less unicuspid. Canine roots diminish in size and dimorphism, especially relative to the size of the postcanine teeth. Molar crowns become higher. Tooth rows become more divergent and symphyseal form changes. Dietary change involving anterior dental use is also suggested by less intense anterior tooth wear in Au. afarensis. These dental changes signal selection for altered dietary behaviour and explain some differences in craniofacial form between these taxa. These data identify Au. anamensis not just as a more primitive version of Au. afarensis, but as a dynamic member of an evolving lineage leading to Au. afarensis, and raise intriguing questions about what other evolutionary changes occurred during the early evolution of the Australopithecus–human clade, and what characterized the origins of the group.

Radiographic assessment of lumbar facet distance spacing and spondylolysis.

Study Design. Spondylolytic defects at L5 are influenced by insufficient differential spacing between the inferior articular facets of L4 and superior facets of S1. These structures then impinge on the intervening L5 pars interarticularis during hyperlordosis, contributing to fracture and resorption of the pars. Objectives. Articular facet spacing was evaluated on clinical radiographs of normal and spondylolytic patients. Summary of Background Data. Spondylolysis ranges from a hairline fracture through the pars to a complete pseudarthrotic defect. Insufficient increase in the distance between articular facets from L4–L5 to L5–S1 has been associated with chronic lytic defects in a skeletal sample. Methods. Anteroposterior radiographs of 39 patients with L5–S1 spondylolysis were compared with radiographs from 42 normal individuals. Differences in transverse distances between lumbar articular facets and pedicles were compared using 2-tailed t tests. Results. Patients with spondylolysis exhibited a smaller increase in interfacet distance from the L3–L4 facet joints to the L5–S1 joints than do normal patients, even relative to vertebral size. Conclusions. Spondylolytic fractures at L5 are influenced by an inadequate increase in interfacet distances between adjacent vertebrae. Individuals lacking sufficient increase in lower lumbar transverse interfacet dimensions are at greater risk of developing and maintaining spondylolytic defects.

Dental Ontogeny in Pliocene and Early Pleistocene Hominins

Until recently, our understanding of the evolution of human growth and development derived from studies of fossil juveniles that employed extant populations for both age determination and comparison. This circular approach has led to considerable debate about the human-like and ape-like affinities of fossil hominins. Teeth are invaluable for understanding maturation as age at death can be directly assessed from dental microstructure, and dental development has been shown to correlate with life history across primates broadly. We employ non-destructive synchrotron imaging to characterize incremental development, molar emergence, and age at death in more than 20 Australopithecus anamensis, Australopithecus africanus, Paranthropus robustus and South African early Homo juveniles. Long-period line periodicities range from at least 6–12 days (possibly 5–13 days), and do not support the hypothesis that australopiths have lower mean values than extant or fossil Homo. Crown formation times of australopith and early Homo postcanine teeth fall below or at the low end of extant human values; Paranthropus robustus dentitions have the shortest formation times. Pliocene and early Pleistocene hominins show remarkable variation, and previous reports of age at death that employ a narrow range of estimated long-period line periodicities, cuspal enamel thicknesses, or initiation ages are likely to be in error. New chronological ages for SK 62 and StW 151 are several months younger than previous histological estimates, while Sts 24 is more than one year older. Extant human standards overestimate age at death in hominins predating Homo sapiens, and should not be applied to other fossil taxa. We urge caution when inferring life history as aspects of dental development in Pliocene and early Pleistocene fossils are distinct from modern humans and African apes, and recent work has challenged the predictive power of primate-wide associations between hominoid first molar emergence and certain life history variables.