Last Common Ancestor of Apes and Humans: Morphology and Environment

Abstract

For much of their history, fossil apes retained many monkey-like features in posture and body structure. They also occupied a range of habitats, of which tropical forest was only a part, and there is evidence of increasing terrestriality in the fossil record as it is known at present (2019). In the early Miocene (18–20 million years ago, Ma), fossil apes were pronograde arboreal slow climbers, associated mainly with forest environments and deciduous woodland and with some indications of terrestrial behaviour, particularly the larger species. Their hands had long and opposable thumbs, and the phalanges were curved. In the early middle Miocene (15–16 Ma), apes were still monkey-like in body plan and posture and were associated almost entirely with non-forest, deciduous woodland habitats, with increasing evidence of terrestrial adaptations. Hand proportions remained the same. Towards the end of the middle Miocene (12 Ma), some fossil ape species had broadened chests, long clavicles, medial torsion of the humerus and re-positioning of the scapula to the back. These adaptations may have been linked with more upright posture, as in the living apes, but unlike them, the hand phalanges were short, robust and less curved, and the thumb remained long. Associated environments were deciduous woodland rather than forest. This body plan was retained in part in some later Miocene apes (10 Ma), some of which also had more elongated limbs and hands (thumb length not known), and hind limbs modified for greater flexibility, analogous with the orang utan. Associated environments were subtropical deciduous woodlands and subtropical evergreen laurophyllous woodland in southern Europe. Other late Miocene European apes had adaptations for living on the ground, and some of these also shared characters of the skull with orang utans. They are associated with more open deciduous woodland habitats. This body plan and environment were retained in the early hominin, Ardipithecus ramidus, but with a more robust postcranial skeleton and incipient bipedalism. Based on shared character states in fossil apes, living apes and early hominins, 27 characters are identified as probable attributes of the last common ancestor (LCA) of apes and humans. The likely environment of the LCA was tropical deciduous woodland with some evidence of more open habitats, and this remained unchanged in the transition from apes to early hominins.