Diversity of the primate musculoskeletal system: modularity, integration, and anatomical variation in humans and their relatives

The concepts of modularity and integration help to explain how morphological transformations during primate evolution enabled modern humans to carry out complex functions such as speaking, chewing, and using tools while also being able to respond to selective pressures. We used anatomical network analysis (AnNA) to assess modularity and integration in the head and upper limb of extant primates. AnNA is the study of complex morphological structures using tools and statistics from network theory. In this context, a module is a group of anatomical elements (e.g., bones and muscles) with more connections (i.e., more integrated) among them than to any other elements outside the group. These modules are identified using community detection algorithms. We analyzed the networks of the head and upper limbs of 20 taxa representing all the major extant primate groups based on dissection of hundreds of specimens, observation of skeletons, and an extensive literature review to ensure that the total sample sizes per taxon would be sufficient to determine the normal muscle phenotype for each taxon. Our results reveal patterns of evolutionary change that are highly relevant for biological and physical anthropology and primatology. For example, the division into left and right orofacial muscle modules seen in humans is not found in bonobos or common chimps, meaning that this division may be related to the more complex, asymmetrical facial expressions displayed by humans. Support or Funding Information NSF grant 1516557 (2015–16) to Rui Diogo and American Association of Anatomists Innovations Program