Origin, Homologies and Evolution of Primate Head, Neck, Pectoral and Upper limb Muscles and the Use of Myological Characters to Investigate the Phylogenetic Relationships Within the Primate Clade
Open AccessAs recently stressed by the philosopher of science Leandro Assis, a decline in the number of phylogenetic hypotheses based on morphological evidence has become a pronounced trend in contemporary systematics. It is often argued that most morphological characters are too ambiguous, that the delineation of homology is subjective, that phenotypic features are particularly prone to homoplasy, and thus for all these reasons molecular phylogenies should have primacy over morphological phylogenies. Some molecular biologists even suggest that the only role for morphology is to use it to map synapomorphies onto molecular cladograms in order to interpret phenotypic evolution. But, this proposal exposes one of the main limitations of molecular phylogenetics: the translation of evolutionary phenomena in terms of classifications largely based on traditional morphology. In order to indicate that a group is monophyletic, molecular biologists often appeal to support values and rarely use partial sequences of the genome for diagnoses, because these molecular data usually are not easily translated into the language of the phenotype. In my dissertation I challenge the assumption that morphological data are inherently unsuitable for phylogeny reconstruction and argue that both molecular and morphological phylogenies should play a major role in systematics. First, I review the results of recent molecular studies and show that in the case of the order Primates, molecular data provide a robust phylogenetic tree that can, and should, be used to test the efficacy of other methods for recovering phylogeny. In the main body of the thesis, I focus on the head, neck, pectoral and upper limb muscles of primates with the goal of undertaking a cladistic study of primate relationships using myological data collected systematically and without bias and then validating the results obtained against well-supported molecular phylogenies. In Chapters 2, 3 and 4, I present the data obtained from dissections of the head, neck, pectoral and upper limb muscles of representative members of various vertebrate groups including modern humans, and compare these data with the information available in the literature. The Chapters thus pave the way for a detailed examination of the evolution of the head, neck, pectoral and upper limb muscles of primates, and for the polarization of the phylogenetic myological characters used in the cladistic analyses given in Chapter 5.The most parsimonious tree obtained from the analysis of 166 head, neck, pectoral and upper limb muscle characters in 18 primate genera, and in representatives of the Scandentia, Dermoptera and Rodentia, is 100% congruent with the evolutionary molecular tree of Primates. Therefore, this study shows that morphological characters such as those based on muscles are capable of recovering the phylogenies supported by molecular evidence. That is, morphological phylogenies should not merely have a secondary role in systematics; they should instead be actively promoted in order to complement the information obtained in molecular phylogenies. Morphological cladistic analyses are the only way to reconstruct the phylogenetic relationships among fossil taxa. In this regard, Chapter 5 includes a cladistic analysis of a dataset made up of 92 myological characters that, in the best case scenario, could have a hard tissue signature and thus be potentially observable in the fossil record. The results are encouraging for primates as a whole because the consensus tree obtained in the parsimony analysis of this dataset recovers 15 (75%) of the 20 clades obtained in the parsimony tree of the complete primate dataset, although with respect to the hominoids they are discouraging because this tree groups Hylobates with the Homininae and Gorilla with Homo.Morphological phylogenetic studies are able to directly address questions related to anatomical evolution that cannot be addressed in molecular phylogenies. For instance, the dissertation results indicate that since the Pan/Homo split, modern humans accumulated more muscle character state changes than chimpanzees, and that both these taxa accumulated more changes than gorillas. However, there is no relationship between the absolute number of muscles and the number of accumulated steps, and modern humans have in overall fewer muscles than chimpanzees. The only muscle groups for which modern humans have more muscles than most other primates are the muscles of the face, larynx, and forearm. In the case of the latter modern humans include two peculiar muscles that are related to the movements of the thumb (extensor pollicis brevis and flexor pollicis longus) and modern humans usually have an additional hand muscle inserting onto the thumb, the 'volaris primus of Henle'. Therefore, the results of this thesis show that modern humans have fewer head, neck, pectoral and upper limb muscles than most other living primates, but they are consistent with the proposal that facial and vocal communication and specialized thumb movements have probably played an important role in recent human evolution.
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