Randall L. Susman

Body Size and Skeletal Allometry in African Apes

The African apes represent a group of closely related primate taxa that differ substantially in adult body size. The close phylogenetic affinity of pygmy chimpanzees (Pan paniscus), common chimpanzees (Pan troglodytes), and gorillas (Gorilla gorilla) is affirmed by both molecular and morphological data. Most biomolecular studies to date, however, have been unable to resolve the chimpanzee-gorilla-human trichotomy into a definitive chimpanzee-gorilla clade that would have humans as a sister group (e.g., Sarich, 1968, and this volume; Zihlman et al., 1978; Bruce and Ayala, 1979; Goodman, 1982). The evolutionary tree for humans and pongids based on cleavage maps of mitochondrial DNA (Ferris et al., 1981; Templeton, 1983) is a notable exception in this regard; these data first group pygmy and common chimpanzees into a phyletic unit that is linked next to gorillas. Humans are then joined to the African ape clade before the orangutan (Pongo pygmaeus). A variety of shared, unique features of the karyotypes of African apes lend strong credence to this phylogeny (Mai, 1983). Morphological analyses of the teeth and locomotor skeletons of pongids and humans [summarized in Ciochon (1983)] also corroborate the Ferris et al. branching sequence.

Functional Morphology of Homo habilis

Olduvai hominid (O.H.) fossils 7, 8, and 35 represent the earliest species of the genus Homo dated at 1.76 million years. The O.H. 7 hand, jaw, and skull and the O.H. 8 foot come from one subadult individual, and the O.H. 35 leg are also those of Homo habilis. The skeleton represents a mosaic of primitive and derived features, indicating an early hominid which walked bipedally and could fabricate stone tools but also retained the generalized hominoid capacity to climb trees.

Evolution of the Human Foot: Evidence from Plio-Pleistocene Hominids

The human foot serves a dual role during locomotion. It functions at times as a mobile structure and at times as a rigid lever. The human foot shows the hallmarks of an arboreal heritage wherein the foot was primarily a grasping organ. Over the course of the human career the human foot has evolved an elaborate plantar aponeurosis, strong plantar ligaments, longitudinal arches, an enlarged musculus flexor accessorius, an adducted (non-opposable) hallux, a remodeled calcaneocuboid joint, a long tarsus, and shortened toes (II to V). Comparisons of the chimpanzee and human foot allow us to reconstruct the pathway of foot evolution. Fossil foot bones of Homo habilis, dated at 1.76 million years, are remarkably like those of modern humans. Foot bones from Hadar, dated at around 3.5 million years, are remarkably chimpanzee-like, with only incipient human traits. The surprising chimpanzee-like qualities of the Hadar fossils strongly support the use of living apes as models of ancestral pongid-hominid morphotypes.

The Locomotor Behavior of Pan paniscus in the Lomako Forest

Studies of free-ranging apes date to the 1930s and 1940s [Nissen (1931) on the common chimpanzee; Bingham (1932) on the gorilla; Carpenter (1940) on the white handed gibbon], and long-term field studies on great apes have been in progress since the 1960s (Fossey, 1972; Nishida, 1968; Rodman, 1973; Goodall, 1968). We have come to understand much about the ecology, social behavior, diet, and life histories of our closest living relatives, the Pongidae. As we have learned more about the great apes, our definition of both ape and human has changed. Whereas it was once thought that among primates only humans (and our fossil forebears) made and used tools, hunted and ate meat, and possessed the capacity for symbolic communication, studies of the great apes have revealed the subtlety of these definitions of humankind. With the advance in our understanding of the fossil record, the subtle transition in brain size and dental reduction from ape to human has been revealed. It is now widely agreed that the initial morphological and behavioral change from ape to human came in the locomotor apparatus and bipedalism. The shift from four- to two-legged progression may well have been the one that initiated the hominid trajectory. In spite of the importance of locomotion in the homidid career, relatively little is known of the locomotion of free-ranging primates, particelarly the great apes.

Bonobos: Generalized Hominid Prototypes or Specialized Insular Dwarfs? [and Comments and Replies]

Neontological, biochemical, and paleontological data indicate that the bonobo, Pan paniscus, is a specialized form that possesses relatively small teeth, is quadrupedally adapted, and is only minimally sexually dimorphic. The various specializations of bonobos could be adaptations to ecological restrictions encountered in the terrestrial island of tropical forest that comprises their home range. Bonobos possess specializations quite different from those present in either Miocene apes or the earliest known hominids and should not be considered as suitable living models of the primitive hominoid or hominid condition.

Electromyographic Studies of the Human Foot: Experimental Approaches to Hominid Evolution

Theories about the functions of the foot muscles have centered on their role in arch support. Previous anatomical and electromyographic studies (reviewed herein) have demonstrated that the arches are normally maintained by bones and ligaments. This study reports an electromyographic investigation of five foot muscles (flexor digito-rum longus, flexor digitorum brevis, flexor accessorius, abductor hallucis, and abductor digiti quinti) conducted on four humans. The three toe flexors act together to resist extension of the toes during the stance phase of locomotion. Despite the large flexor accessorius in humans, neither this muscle nor the flexor digitorum brevis are preferentially recruited over the flexor digitorum lon-gus for any normal posture or locomotion. The abductors affect the mediolateral distribution of pressure by positioning the forefoot. We suggest that the foot muscles play an important role in positioning of the forces on the foot in both posture and locomotion. Future electromyographic experiments on human and ape foot muscles in conjunction with detailed studies of early hominid fossils promise to elucidate the pathways of human locomotor evolution.

Preliminary observations on the feeding behavior ofPan paniscus in the lomako forest of central Zaïre

The feeding behavior and ecology ofPan paniscus was studied over a seven-month period in Equateur, Republic of Zaïre, during 1974–1975. Additional data were gathered during four weeks in 1979.Pan paniscus was found to be primarily frugivorous but bonobo foods also consist of leaves, flowers, pith, invertebrates and small mammals.

Brief Communication : Evidence Bearing on the Status of Homo habilis at Olduvai Gorge

Students of the early hominin career have debated the status of Homo habilis since its discovery in 1960. Today discussion centers on which specimens should be included in the species and what constitutes the holotype. Recent reviews of early Homo suggest that the Olduvai Hominid 8 foot may sample Paranthropus while the OH 7 skull bones, mandible, and hand sample H. habilis. Moreover, some suggest that while H. habilis in Middle Bed I at Olduvai is craniodentally Homo-like, the postcranial skeleton of H. habilis is more like that of Australopithecus. Evidence presented here indicates not only that OH 7 and OH 8 represent H. habilis but also that they come from a single individual. The association of OH 35 with OH 7 and OH 8 is less certain. Morphological, pathological, and taphonomic evidence favors the inclusion of OH 35 in the holotype. However, stratigraphic evidence suggests that OH 35 and OH 8 are not coterminous. With or without OH 35, the holotype of H. habilis ranks as one of the most complete early hominin skeletons and the most complete and functionally informative specimen of early Homo.

Terrestrial adaptations in the hands of Equatorius africanus revisited.

Interpretations of the postcranial anatomy of East African early and middle Miocene large-bodied hominoids (e.g., Proconsul, Afropithecus, Turkanapithecus, Nacholapithecus) have suggested that these diverse primates utilized positional behaviors dominated by arboreal quadrupedalism. Preliminary descriptions of the Equatorius africanus partial skeleton (KNM-TH 28860) and other forelimb specimens, however, have argued that this animal relied more on terrestrial locomotion compared to its contemporaries, possibly similar to extant large papionin monkeys. In this paper, we reevaluate this interpretation by examining intrinsic hand proportions based on the lengths of the third proximal phalanx and fifth metacarpal in Equatorius in reference to a large sample of extant catarrhine primate taxa. We focused on the lengths of these hand bones because the ratio between phalanx and metacarpal lengths has been previously documented to discriminate terrestrial from arboreal mammalian taxa, including primates. The Equatorius hand displays semi-terrestrial hand proportions with a relatively shorter proximal phalanx compared to most arboreal monkeys. Its proximal phalanx, however, is relatively longer than those of habitually terrestrial monkeys (e.g., Theropithecus, Papio). Accordingly, although Equatorius retains some arboreal quadrupedal characteristics, these results corroborate the previous inference that it engaged in more terrestrial locomotion than earlier Miocene apes such as Proconsul. We suggest that the postcranial skeleton of Equatorius evinces the earliest signs of semi-terrestriality in the hominoid fossil record. It is likely that the terrestrial specialization utilized by living hominoids, e.g., knuckle-walking, evolved separately.

Pygmy Chimpanzees in Peril

So far as is known pygmy chimpanzees, or bonobos, occupy only a comparatively small area in the central basin of Zaire. A large multinational company has acquired logging rights in what is believed to be the core of their range, and the authors, who have been studying the animals, believe that this could mean the end of this major population. A reserve is urgently needed, and they suggest a particular area of undisturbed primary forest where the local people would act as guardians and also continue their traditional uses.