Knuckle-walking is a pattern of digitigrade locomotion unique to African apes among Primates. Only chimpanzees and gorillas are specially adapted for supporting weight on the dorsal aspects of middle phalanges of flexed hand digits 11-V. When forced to the ground, most orangutans assume one of a variety of flexed hand postures, but they cannot knuckle-walk. Some orangutans place their hands in palmigrade postures which are impossible to African apes. The knuckle-walking hands and plantigrade feet of African apes are both morphologically and adaptively distinct from those of Pongo, their nearest relative among extant apes. These features are associated with a common adaptive shift to terrestrial locomotion and support placing chimpanzees and gorillas in the same genus Pan. It is further suggested than Pan comprises the subgenera ( a ) Pan, including P. troglodytes and pygmy chimpanzees, and ( b ) Gorilla, including mountain and lowland populations of P. gorilla.African apes probably diverged from ancestral pongids that were specially adapted for distributing their weight in terminal branches of the forest canopy. Early adjustments to terrestrial locomotion may have involved fist-walking which later evolved into knuckle-walking. Orangutans continued to adapt to feeding and locomotion in the forest canopy and their hands and feet became highly specialized for four-digit prehension. Although chimpanzees retained arboreal feeding and nesting habits, they moved from tree to tree by terrestrial routes and became less restricted in habitat.While adapting to a diet of ground plants gorillas increased in size to the point that arboreal nesting is less frequent among them than among chimpanzees and orangutans.Early hominids probably diverged from pongids that had not developed prospective adaptations to knuckle-walking, and therefore did not evolve through a knucklewalking stage. Initial adjustments to terrestrial quadrupedal locomotion and resting stance probably included palmigrade hand posturing. Their thumbs may have been already well developed as an adaptation for grasping during arboreal climbing. A combination of selection pressures for efficient terrestrial locomotor support and for object manipulation further advanced early hominid hands toward modern human configuration.The hands of great apes, like those of other primates, are complex organs which have evolved due to selection for locomotor functions, both in trees and on the ground, and for manipulative functions such as feeding, nesting, arboreal resting postures, grooming, and tactile communication.Previous studies of great ape hands have concentrated mainly on their adaptations to brachiation and on the possibility that great ape hands might represent precursor forms of the human hand. Both of these problems are associated with the Keith-Gregory hypothesis that man evolved from a brachiator. Behavioral and field studies indicate that the hands of great apes, particularly those of chimpanzees and gorillas, are more than mere hooks used for arboreal loc...
The hands of the Hominoidea evidence four adaptive modes which distinguish the lesser apes (Hylobatidae), the orangutan (Pongo), the African apes ( P a n ) , and man ( H o m o ) from one another. The hands of the apes consist of compromises between manipulatory and locomotor functions because selection has operated for precision of grip as well as for special locomotor mechanisms. The human hand is almost totally devoted to manipulation. The hands of gibbons, orangutans and the African apes differ in many features that may be correlated with locomotion. The gibbons and siamang are specially adapted for ricochetal arm-swinging. The great apes possess morphological adaptations for arboreal foraging and climbing distinct from those of the hylobatids. I n addition, the African apes have become secondarily adapted for terrestrial quadrupedal locomotion. Many features that distinguish the hands of chimpanzees and gorillas may be associated with the development of efficient knuckle-walking propulsive and support mechanisms.
While the taxonomic affinity of man X and the African apes is generally undisputed, anthropologists have widely disparate opinions on the extent to which the common ancestors of man, chimpanzee, and gorilla resembled living pongids. Many authors proffer sketches of hominid evolution that include apelike stages (1), but few of these include details of the mechanisms whereby apelike forms evolved into more manlike apes which, in turn, culminated in man. Similarly, authors who suggest alternative theories that emphasize non-apelike precursors (2) have difficulty in delimiting the bio-' mechanical and environmental changes involved in the transition from preman to man.Several theories that have received special notice in discussions on human origins accentuate a "troglodytian" or great-ape stage immediately preceding the emergence of man, the plantigrade biped (1). In this article I assess the possibility of a troglodytian stage in human phylogeny in the light of recent behavioral and morphological studies on the hands of the great apes and man. Brachiationist TheoryIn his initial formulation of an evolutionary pathway that possibly culminated in man, Sir Arthur Keith pro-Circle No. 17 on Readers' Service Card posed three "stages": h3 lodytian, and plantigrac real quadrupedal monke evolved into "brachiatin apes that had a predilc pending themselves in o tures beneath brancher Keith's hypothetical hylc Keith also emphasiz in the second stage of I major difference betweer and "troglodytians" wa size of the latter.Man also is orthogra but instead of hanging a superstratum he stand a substratum.Keith and later brachi point to many features, the forelimb (upper lim that are shared by men example, the "brachiatoi have long arms relative t4 and long forearms in p second to fifth fingers; t table absolute size (but to total hand length in anteroposteriorly flatte strongly angled ribs; pro vertebral column into th a broad sternum, with p sion of the sternal elem4 long collarbones; should cated on the posterior z chest wall; large acromia the shoulder blades; late shoulder joints; considei SCIENCE of the shoulder and elbow joints; a relatively short truncal segment of the vertebral column-in particular, a short lumbar region; no tail; and a smaller ie coccyx and more sacral vertebrae than other anthropoid primates have (4).nsIn addition to the general bodily proportions and skeletal features listed above, the "brachiators" have in comto mon certain characteristics of soft parts that may be related to their semierect )n. and orthograde posturing. These include large muscles that raise the arm ttle and rotate the shoulder blade (5); a te characteristic arrangement of muscle fibers in the diaphragm; characteristic positions of the heart, lungs, and other organs in the body cavity; the close atylobatian, trog-tachment of some abdominal organs to le (3). Arbothe diaphragm and posterior abdominal y-like primates wall; a muscular pelvic diaphragm in ig" gibbon-like the pelvic outlet; and lumbar back Dction for sus-muscles that are sm...
In this paper, I present an updated version of the hylobatian model for the proximate ancestors of the Hominidae. The hylobatians are hypothesized to have been relatively small creatures that were especially adapted for vertical climbing on tree trunks and vines and for bipedalism on horizontal boughs. They were no more disposed toward suspensory behaviours than are modern chimpanzees and bonobos. According to this evolutionary scenario, bipedalism preceded the emergence of the Hominidae. The earliest hominids would be recognized as diurnally terrestrial bipeds that stood with full extension of the knee joints and walked with greater extension of the lower limbs than is common in non-human primates that are induced to walk bipedally on the ground. The wealth of hominid fossils from the Hadar Formation, Ethiopia, and the Laetolil Formation, Tanzania, are generally compatible with the hylobatian model. They show that by ca . 4 Ma B.P. habitually terrestrial, bipedal hominids had evolved from arboreal ancestors. The Hadar hominids had curved fingers and toes, strong great toes and thumbs, and other features that suggest that they were rather recently derived from arboreal hominids and that they probably continued to enter trees, perhaps for night rest and some foraging. The hominid hand bones from Hadar evince no features that are distinctly related to knuckle-walking. They relate neatly to counterparts in the hand of O.H. 7, a specimen that was found with stone tools. However, there is no evidence that the Hadar hominids of 3 Ma ago engaged in tool behaviour.
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