The evolution of brachiation in atelines: A phylogenetic comparative study.

摘要

This study examines how brachiation locomotion evolved in ateline primates using recently-developed molecular phylogenies and character reconstruction algorithms, and a newly-collected dataset including the ateline fossils Protopithecus and Caipora. Data are taken from over 500 New World monkey skeletons, representing 19 fossil and extant platyrrhine genera, for 48 ratio and angle characters that are hypothesized to functionally correlate with brachiation, hindlimb suspension, prehensile tail use and leaping. Multivariate analysis finds that 17 characters---representing the shoulder, elbow and wrist; the lumbar and distal caudal vertebral column; and long bone properties---differentiate between ateline brachiators and non-brachiators.; Fossils are added to two molecular phylogenies to create several possible phylogenetic scenarios for platyrrhines. A generalized least squares algorithm reconstructs ateline and atelin ancestral character states on these phylogenies. Histories of the distinguishing 17 characters are mapped out on these phylogenies, producing two scenarios of ateline brachiation evolution that have four commonalities: First, many characters change towards the Ateles condition on the ateline stem lineage before Alouatta splits off from the atelins, suggesting that an ateline energy-maximizing strategy began before the atelines diversified. Second, the ateline last common ancestor is always reconstructed as an agile quadruped, usually with suspensory abilities. It is never exactly like Alouatta and many characters reverse and change towards the Alouatta condition after Alouatta separates from the atelins. Third, most characters undergo homoplastic change in all ateline lineages, especially on the Ateles and Brachyteles terminal branches. Fourth, ateline character evolution probably went through a hindlimb suspension with tail-bracing phase, which is different from the evolution of brachiation in hominoids. The atelines probably diversified via a quick adaptive radiation---with bursts of punctuated change occurring in their postcranial skeletons---due to changing climatic conditions which caused competition among the atelines and between atelines and pitheciines.; The results of this dissertation reaffirm the importance of uncovering the underlying genetics and development behind morphologic characters. Although it appears from this and other studies that homoplasy is rampant during the evolution of forelimb and trunk musculoskeletal characters in atelines and hominoids, it is unknown whether underlying homology may be the cause.