Untangling vertical and horizontal processes in the evolution of handshapes

In Guido’s most recent post in this miniseries on sign language manual alphabet evolution, he discussed the role of character mapping on networks in phylogenetic inference. He pointed out how we used this approach to infer evolutionary pathways of languages and why this step in exploratory data analysis is important, given the complexity of the underlying signal in this data set. In this post, I take up the topic of handshape evolution in more detail, explaining some of the complexities involved in studying sign language evolution and looking specifically at how we can identify vertical and horizontal processes in the evolution of handshapes. We know very little about how signs and handshapes evolve. There have been a few studies—most of them from decades ago—comparing American Sign Language in videos and dictionaries from the early 20th century with then contemporary forms (Battison et al. 1975; Frishberg 1975). One study in particular argued that, as a sign language emerges in a community of signers, crystallizing into a stable linguistic system, signs evolve in a quasi-teleological way from earlier, more gestureor pantomime-like forms to more language-like forms, cutting similar evolutionary pathways leading to more constraints on articulation and to general systematization. But what happens (in this story) once sign languages become linguistic systems? Do they continue evolving, as happens in spoken languages? If yes, how? Investigating these kinds of questions was one of my motivations for tracking down historical examples of manual alphabets for over a dozen sign languages. The pay off (besides the thrill of the treasure hunt) is that, in tracing handshapes through historical examples and comparing them with contemporary sign languages, we can infer or, in some cases, deduce vertical and horizontal evolutionary processes affecting sign languages and handshape forms.