The broader evolutionary lessons to be learned from a comparative and phylogenetic analysis of primate muscle morphology.

The present publication reviews the broader evolutionary implications of our long-term study of primate musculature. It summarizes the implications of the study for our understanding of the use of myological characters for phylogenetic reconstruction, for assessing the importance of homoplasy and reversions in evolution, and for our understanding of Dollo's law, the notion of 'direction' in evolution, the common myth of human complexity, the tempo and mode of primate and human evolutionary history, adaptive radiations, the notion that 'common' equals 'primitive' and the influence of morphogenesis on the variability of head, neck, pectoral and upper limb muscles. Among other results our study shows that myological characters are useful for phylogenetic reconstruction. The results also stress the importance of homoplasy and of evolutionary reversions in morphological evolution, and they provide examples of reversions that violate Dollo's law due to the retention of ancestral developmental pathways. They also show that contrary to the idea of a 'general molecular slow-down of hominoids' the rates of muscle evolution at the nodes leading to and within the hominoid clade are higher than those in most other primate clades. However, there is no evidence of a general trend or 'directionality' towards an increasing complexity during the evolutionary history of hominoids and of modern humans in particular, at least regarding the number of muscles or of muscle bundles. The rates of muscle evolution at the major euarchontan and primate nodes are different, but within each major primate clade (Strepsirrhini, Platyrrhini, Cercopithecidae and Hominoidea) the rates at the various nodes, and particularly at the nodes leading to the higher groups (i.e. those including more than one genus) are strikingly similar. Our results also support, in general terms, the assumption that 'common is primitive' and they lend some support for the 'vertebrate-specific model' in the sense that during the divergent events that resulted in these four major primate clades there was more emphasis on postcranial changes than on cranial changes. Our study of primates does not, however, support suggestions that the distal structures of the upper limb are more prone to variation than the proximal ones, or that the topological origins of the upper limb muscles are more prone to evolutionary change than their insertions.