In a very influential paper, Susman et al. Firstly, the facial expression muscle risorius (Figure 1) has been generally accepted as a unique feature crucial for the evolution of our “gracile” smile and “specially sophisticated” facial communication abilities ( Huber, 1931). To illustrate this fact, in this short paper I will refer here briefly to seven muscles that have long been generally seen as “unique human features” and linked with specific adaptations for our bipedalism, tool use, and vocal or facial communication. Strikingly, despite this scarcity of information, biologists and anthropologists have displayed a remarkable confidence in their stories about the origin and evolution of human soft tissues, including their phylogenetic distribution and “singular functional adaptations.” For instance, the only study that specifically focused on the musculature of bonobos ( Pan paniscus) was that of Miller (1952), which was based on dissections of a single adult and did not provide information for numerous head and limb muscles ( Diogo and Wood, 2011, 2012).
In fact, descriptions of the soft tissues of apes have been relatively scarce and mainly referred to just a few muscles of the head or limbs of a single taxon, in most cases (e.g., Tyson, 1699 Bischoff, 1880 Raven, 1950 Swindler and Wood, 1973 Diogo and Wood, 2011, 2012 Persaud and Loukas, 2014). This is particularly striking because these stories are in reality almost exclusively based on hard tissue data. Just-so stories ( Smith, 2016) are frequent in the literature about human evolution because of our tendency to build simple progressionist narratives about our “special” evolutionary history and place in nature ( Gould, 1993, 2002). Each and every muscle that has been long accepted to be “uniquely human” and to provide “crucial singular functional adaptations” for our bipedalism, tool use and/or vocal/facial communication, is actually present as an intra-specific variant or even as normal phenotype in bonobos and/or other apes. How can we be so certain about the evolution of human facial communication, bipedalism, tool use, or speech without detailed knowledge of the internal anatomy of for instance, one of the two extant species more closely related to us, the bonobos? Here I show how many of these stories now become obsolete, after such a comprehensive knowledge on the anatomy of bonobos and other primates is finally put together. Just-so stories are prominent in human evolution literature because of our tendency to create simple progressionist narratives about our “special” place in nature, despite the fact that these stories are almost exclusively based on hard tissue data. Department of Anatomy, Howard University, Washington, DC, United States.
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