finally appears about 5 mya. Primates fall into one of four groups: Nonopposable thumbs: tarsiers and marmosets Pseudo-opposable thumbs: all strepsirrhines and Cebidae Opposable thumbs: Old World monkeys and all great apes Opposable with comparatively long thumbs: gibbons (or lesser apes) Pandas have evolved pseudo-opposable thumbs by extension of the sesamoid bone, which is not a true digit. Pronation and supination The ability to pronate the manus (hand) and forearm in therian mammals is achieved by a rounded head of the radius, which allows it to swivel across the ulna. Supination requires a dorsal glide of the distal radius and pronation a palmar glide in relation to the distal ulna. Pronation has evolved multiple times, among mammals, chameleons, and varanids. However, the more basal condition is to be unable to pronate. Dinosaurs were not capable of more than semi-pronation of the wrist, though bipedal origins of all quadrupedal dinosaur clades could have allowed for greater disparity in forelimb posture than often considered. Monotremes have forearms that are not as dexterous as therians. Monotremes have a sprawling posture, and multiple elements in their pectoral girdles, which are ancestral traits for mammals. In birds, the forearm muscles supinate, pronate, flex and extend the distal wing. Wings All tetrapod forelimbs are homologous, evolving from the same initial structures in lobe-finned fish. However, another distinct process may be identified, convergent evolution, by which the wings of birds, bats, and extinct pterosaurs evolved the same purpose in drastically different ways. These structures have similar form or function but were not present in the last common ancestor of those groups. Bat wings are composed largely of a thin membrane of skin supported on the five fingers, whereas bird wings are composed largely of feathers supported on much reduced fingers, with finger 2 supporting the alula and finger 4 the primary feathers of the wing; there are only distant homologies between birds and bats, with much closer homologies between any pair of bird species, or any pair of bat species. Flippers Marine mammals have evolved several times. Over the course of their evolution, they develop streamlined hydrodynamic bodies. The forelimb thus develops into a flipper. The forelimbs of cetaceans, pinnipeds, and sirenians presents a classic example of convergent evolution. There is widespread convergence at the gene level. Distinct substitutions in common genes created various aquatic adaptations, most of which constitute parallel evolution because the substitutions in question