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.
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...
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.