Miocene Ape Spinal Morphology: The Evolution of Orthogrady

Abstract

The postcranial skeleton of living apes is characterized by a number of derived features that are attributable to the frequent forelimb-dominated orthograde positional behavior such as suspension or vertical climbing. Their axial skeletons exhibit a common specialization, e.g., a decreased number of lumbar vertebrae concomitant with an increase in sacral vertebrae, loss of an external tail, spinal invagination into the thoracic and abdominal cavities, and craniocaudally short and dorsoventrally deep lumbar vertebral centrum. So far, over 30 genera of apes have been discovered from Miocene fossil localities in Africa and Eurasia. However, axial skeletal specimens are available in only a handful of apes. Fossil apes (~19–15 Ma, Kenya) from the beginning and mid-part of the Miocene in Africa (Ekembo and Nacholapithecus) were essentially deliberate arboreal pronograde quadrupeds and retained primitive catarrhine axial skeletal morphology: long and dorsomobile lumbar spine, short sacrum, and absence of spinal invagination (as inferred from ventral position of the lumbar transverse process), though Nacholapithecus shows a hint of an early transition to orthograde positional behavior. However, they did not have an external tail, and their tail loss is almost certainly a shared derived feature with living apes. The penultimate lumbar vertebra of Morotopithecus (20.6 Ma, Uganda) exhibits craniocaudally short and dorsoventrally deep centrum and dorsal position of the transverse process. While these features resemble those in living apes, the evolutionary pattern of the dentognathic morphology and paleobiogeography of Miocene apes suggest that derived lumbar anatomy of Morotopithecus is a product of parallel evolution. European ape fossil record illustrates a progressive evolution toward orthogrady. The stem great ape which spread into Eurasia from Africa between 17 and 16 Ma was pronograde-like contemporary African fossil apes. Pierolapithecus (~12 Ma, Spain) is the earliest known orthograde (but perhaps non-suspensory) ape in Europe although its spine might be less dorsostable compared to extant great apes. Hispanopithecus (9.6 Ma, Spain) had acquired a full suite of orthograde and suspensory characters comparable to extant great apes. Oreopithecus (~8–7 Ma, Italy) also has a fully orthograde spine although its lumbar spine has some distinct features from Hispanopithecus. Some authors have proposed that orthogrady evolved in European and African ape lineages (and Asian as well) independently. However, some others propose that a European orthograde ape dispersed into Africa during the Late Miocene gave rise to the extant African apes and humans. Besides this issue, opinions are divided whether the dorsostable spine in the extant African apes is homologous or homoplastic. Postcranial anatomy of Ardipithecus ramidus suggests to some that the last common ancestor of the extant African apes and humans had an intermediate body plan between pronogrady and orthogrady (“multigrady”) with spinal invagination but without enhanced dorsostability as that in extant great apes. However, an argument continues with regard to this interpretation. Postcranial fossils of African Miocene apes are totally absent after ~12 Ma until the appearance of the earliest putative humans. For further clarification of the evolution of the hominoid spine, new discoveries of postcranial elements of Late Miocene African apes are needed.