Oreopithecus

Known as the "enigmatic hominoid", Oreopithecus can dramatically rewrite the palaeontological map depending on whether it is a descendant of the European ape Dryopithecus or an African anthropoid.[2] Some have suggested the unique locomotory behavior of Oreopithecus requires a revision of the current consensus on the timing of bipedality in human developmental history, but there is limited agreement on this point among paleontologists.

Simons (1960) considered Oreopithecus closely related to the early Oligocene Apidium, a small arboreal anthropoid that lived nearly 34 million years ago in Egypt.[3] Oreopithecus shows strong links to modern apes in its postcranium and, in this respect, it is the most modern Miocene ape below the neck, with closest similarities to the postcranial elements of Dryopithecus, but its dentition is adapted to a leafy diet and a close link is uncertain. Others claim it to be either the sister taxon to Cercopithecoidea or an even direct human ancestor, but it is usually placed in its own subfamily within Hominidae. It could instead be added to the same subfamily as Dryopithecus, perhaps as a distinct tribe (Oreopithecini).[4] A cladistic analysis of Nyanzapithecus alesi recovers Oreopithecus as a member of the proconsulid subfamily Nyanzapithecinae.[5]

Its habitat appears to have been swampy, and not savanna or forest. The postcranial anatomy of Oreopithecus features adaptations for suspensory arborealism. Functional traits related to suspensory locomotion include its broad thorax, short trunk, high intermembral index, long and slender digits, and extensive mobility in virtually all joints. Its fingers and arms seem to show adaptations for climbing and swinging.

Its foot has been described as chimp-like, but is different from those of extant primates. The habitual line of leverage of the primate foot is parallel to the third metatarsal bone. In Oreopithecus, the lateral metatarsals are permanently abducted so that this line falls between the first and second metatarsals instead. Furthermore, the shape of the tarsus indicate loads on the foot were transmitted to the medial side of the foot instead of the lateral, like in other primates.[6] The metatarsals are short and straight, but have a lateral orientation increase. Its foot proportions are close to the unusual proportions of Gorilla and Homo but are distinct from those found in specialized climbers. The lack of predators and the limitation of space and resources in Oreopithecus′ insular environment favored a locomotor system optimized for low energy expenditure rather than speed and mobility.[6]

Oreopithecus has been claimed to exhibit features that are adaptations to upright walking, such as the presence of a lumbar curve, in distinction to otherwise similar species known from the same period. Since the fossils have been dated to about 8 million years ago, this would represent an unusually early appearance of upright posture.[6] However, a reevaluation of the spine from a skeleton of Oreopithecus has led to the conclusion that it lacked adaptations for habitual bipedality [7][8]

The semicircular canals of the inner ear serves as a sense organ for balance and controls the reflex for gaze stabilization. The inner ear has three canals on each side of the head, and each of the six canals encloses a membranous duct that forms an endolymph-filled circuit. Hair cells in the duct's auditory ampulla pick up endolymph disturbances caused by movement, which register as rotatory head movement. They respond to body sway of frequencies greater than 0.1 Hz and trigger the vestibulocollic (neck) reflex and vestibuloocular (eye) reflex to recover balance and gaze stability. The bony semicircular canals allow estimates of duct arc length and orientation with respect to the sagittal plane.

Across species, the semicircular canals of agile animals have larger arcs than those of slower ones. For example, the rapid leaper Tarsius bancanus has semicircular canals much bigger than the slow-climbing Nycticebus coucang. The semicircular canals of brachiating gibbons are bigger than those of arboreal and terrestrial quadrupedal great apes. As a rule of thumb, arc size of the ducts decreases with body mass and consequently slower angular head motions. Arc size increases with greater agility and thus more rapid head motions. Modern humans have bigger arcs on their anterior and posterior canals, which reflect greater angular motion along the sagittal plane. The lateral canal has a smaller arc size, corresponding to reduced head movement from side to side. [9]

Allometric measurements on the bony labyrinth of BAC-208, a fragmentary cranium that preserves a complete, undeformed petrosal bone suggest that Oreopithecus moved with agility comparable to extant great apes. Its anterior and lateral semicircular canal sizes fall within the range for great apes. [10] Its relatively large posterior arc implies that Oreopithecus was more proficient at stabilizing angular head motion along the sagittal plane.

Oreopithecus had hominin-like hand proportions that allowed a firm, pad-to-pad precision grip. Features present in the hands of neither non-human-extant nor fossil apes include hand length, relative thumb length, a deep and large insertion for the flexor pollicis longus, and the shape of the carpometacarpal joint between the metacarpal bone of the index finger and the capitate bone. [11] At the base of the second metacarpal bone, the facet for the capitate is oriented transversally, as in hominins. The capitate, on the other hand, lacks the waisting associated with apes and climbing, and still present in Australopithecus. Oreopithecus share the specialised orientation at the carpometacarpal joint with A. afarenis and the marked groove for the flexor pollicis longus with A. africanus. It is thus likely that the hand morphology of Oreopithecus is derived for apes and convergent for early hominins. [11]