Using phylogenetic analysis methods to compare dozens of sign languages, American and German scientists were able to identify five of their main evolutionary lines. An article about this was published in the journal Royal Society Open Science.
Natural human languages are divided into two main types depending on modality - oral-auditory and sign-visual. The first group has been studied much better and is of greater interest to researchers. But the authors of the new work are confident that sign languages are at least as old as speech languages, so they should be given special attention.
Many of the sign languages include a set of characters associated with a written alphabet. Sign language speakers use this set to create words. Historical examples of such alphabets can be found for many sign languages, dating back to the establishment of educational institutions for the deaf during the European Enlightenment.
In a new study, the researchers set out to find out how European sign languages developed and how they differ from each other. For this, at the first stage, the authors created a database of 40 modern and 36 historical sign alphabets. They then compared these alphabets using a phylogenetic network method that can show the degree of connection between multiple languages at the same time.
This allowed researchers to visualize and understand complex relationships between languages without assuming that the common structures in them are due to a single origin. “For this study, we created the largest cross-linguistic comparative database of known sign languages,” says Johann-Matthis Liszt, one of the study's authors at the Max Planck Institute for World History. "The database has helped us trace the evolution of sign languages over the past few centuries and shed light on the reasons for their diversity today."
By combining the methods of historical linguistics and evolutionary biology, a team of scientists was able to deduce probable relationships between sign languages. The researchers were able to group sign languages into five major evolutionary lines. They showed how languages changed as they spread across Europe and the world.
The results of the study confirmed many of the events associated with the spread of sign languages known from historical records, but also presented some surprises. For example, this study confirmed the impact of French Sign Language on improving education for the deaf and building sign language communities in many regions, including Western Europe and the Americas.