A new partial skeleton of Proconsul nyanzae from Mfangano Island, Kenya (KNM-MW 13142) includes five lumbar vertebrae, a partial sacrum, and nearly complete hipbone. Until recently, little was known of the axial and pelvic skeletons of Proconsul, or any early Miocene hominoid. Because torso morphology directly reflects locomotor behavior, these elements provide important new information about posture and locomotion of P. nyanzae. Two basic patterns of locomotor behavior and corresponding torso morphology exist among extant anthropoids. Monkeys have long, flexible spines with powerful epaxial muscles and mediolaterally narrow thoraces. This morphology represents a response to the mechanical requirements of habitual pronograde quadrupedalism. In contrast, hominoids, particularly great apes, have shorter, stiffer spines and broader torsos than do monkeys. The hominoid pattern reflects an emphasis on forelimb abduction-adduction, and on stability and control of the lower back during forelimb-dominated arboreal locomotion. KNM-MW 13142 had craniocaudally elongate vertebral bodies, and probably six lumbar vertebrae, reflecting a long, flexible vertebral column. The narrow, laterally facing iliac blades of KNM-MW 13142 imply the presence of a mediolaterally narrow, dorsoventrally deep thorax. Its wide iliac tuberosities, ventrally located lumbar transverse processes, and distinct accessory processes suggest that Proconsul nyanzae also had strap-like epaxial muscles similar to those characterizing most monkeys. Axial and pelvic morphology of KNM-MW 13142 indicate that P. nyanzae had not undergone an ape-like rearrangement of its torso to adapt to forelimb-dominated arboreality. Instead, P. nyanzae probably retained a more primitive pattern of pronograde quadrupedalism. This morphology indicates that the extant hominoid pattern of torso anatomy arose subsequent to the establishment of the hominid clade.

No abstract

Reconstructing the transition to bipedality is key to understanding early hominin evolution. Because it is the best-known early hominin species, Australopithecus afarensis forms a baseline for interpreting locomotion in all early hominins. While most researchers agree that A. afarensis individuals were habitual bipeds, they disagree over the importance of arboreality for them. There are two main reasons for the disagreement. First, there are divergent perspectives on how to interpret primitive characters. Primitive traits may be retained by stabilizing selection, pleiotropy, or other ontogenetic mechanisms. Alternately, they could be in the process of being reduced, or they simply could be selectively neutral. Second, researchers are asking fundamentally different questions about the fossils. Some are interested in reconstructing the history of selection that shaped A. afarensis, while others are interested in reconstructing A. afarensis behavior. By explicitly outlining whether we are interested in reconstructing selective history or behavior, we can develop testable hypotheses to govern our investigations of the fossils. To infer the selective history that shaped a taxon, we must first consider character polarity. Derived traits that enhance a particular function, are found to be associated with that function in extant homologs, and that epigenetically sensitive data indicate were actually being used for that function, can be interpreted as adaptations. The null hypothesis to explain the retention of primitive traits is that of selective neutrality, or nonaptation. Disproving this requires demonstration of active stabilizing or negative selection (disaptation). Stabilizing selection can be inferred when primitive traits compromise a derived function clearly of adaptive value. Prolonged stasis, continued use of the trait for a particular function, or no change in variability in the trait are evidence that can support a hypothesis of adaptation for primitive traits, but still do not falsify the null hypothesis. Disaptation, or negative selection, should result in a trait being reduced or lost. To infer the behaviors of a fossil species, we must first determine its adaptations, use this to make hypotheses about its behavior, and test these hypotheses using epigenetically sensitive traits that are modified by an individual's activity pattern. When the A. afarensis data are evaluated using this framework, it is clear that these hominins had undergone selection for habitual bipedality, but the null hypothesis of nonaptation to explain the retention of primitive, ape-like characters cannot be falsified at present. The apparent stasis in Australopithecus postcranial form is currently the strongest evidence for stabilizing selection maintaining its primitive features. Evidence from features affected by individual behaviors during ontogeny shows that A. afarensis individuals were habitually traveling bipedally, but evidence presented for arboreal behavior so far is not conclusive. By clearly identifying the ques...

No abstract

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.

A partial skeleton attributed to Proconsul nyanzae (KNM-MW 13142) is described. The fossils were found at a site on Mfangano Island, Kenya, which dates to 17.9 +/- .1 million years ago. KNM-MW 13142 consists of six partial vertebrae (T12-S1), a nearly complete hipbone, most of the right femur and left femoral shaft, a fragmentary tibia and fibula, and a nearly complete talus and calcaneus. This skeleton provides the first pelvic fossil known for any East African Miocene hominoid. The new Proconsul specimen is compared to a large sample of extant anthropoids to determine its functional and phylogenetic affinities. In most aspects of its anatomy, KNM-MW 13142 closely resembles nonhominoid anthropoids. This individual had a long, flexible spine, narrow torso, and habitually pronograde posture, features characteristic of most extant monkeys. Evidence of spinal musculature suggests a generalized condition intermediate between that of cercopithecoids and hylobatids. The hindlimb of KNM-MW 13142 exhibits relatively mobile hip and ankle joints, with structural properties of the femur like those of hominoids. This mix of features implies a pattern of posture and locomotion that is unlike that of any extant primate. Many aspects of the Proconsul nyanzae locomotor skeleton may represent the primitive catarrhine condition.

The discovery of Australopithecus anamensis fossils from strata lying between tephra dated at 4.17 and 4.12 million years ago, and from slightly higher strata not well constrained in age by overlying dated units, provoked the claim that more than one species might be represented: it was suggested that the stratigraphically higher fossils, which include the important tibia, humerus and a large, presumed male, mandible (KNM-KP 29287), might belong to a later, more derived hominid. We have recovered new fossils from Kanapoi and Allia Bay, Kenya, during field work in 1995-1997 that confirm the primitive status of Australopithecus anamensis, the earliest species of Australopithecus. Isotope dating confirms A. anamensis' intermediate age as being between those of Ardipithecus ramidus and Australopithecus afarensis. New specimens of maxilla, mandible and capitate show that this species is demonstrably more primitive than A. afarensis. A lower first deciduous molar (dm 1) is intermediate in morphology between that reported for Ardipithecus ramidus and A. afarensis. Single-crystal 40Ar-39Ar age determinations on the Kanapoi Tuff show that, except for a large mandible, all of the hominid fossils from Kanapoi are from sediments deposited between 4.17+/-0.03 and 4.07+/-0.02 million years ago.

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.