Delving into centuries-old manuscripts depicting dozens of sign language alphabets, scientists have identified five primary lineages that originated independently of each other before diversifying throughout Europe and the Americas.
The discovery, published in the journal Royal Society Open Science, sheds light on the budding of conventional sign languages – using letters from the alphabet – when educational institutions for the deaf arose during the Enlightenment in Europe.
Although the evolution of spoken languages over the past couple of centuries has been extensively studied, knowledge about sign languages – particularly how they relate to each other – has been patchy.
Linguistics researcher Justin Power, from the University of Texas in Austin, US, joined forces with oral linguistic database expert Johann-Mattis List from the Max Planck Institute in Germany and French biologist Guido Grimm to apply methods used by evolutionary biology to study sign language evolution.
“In both biological and linguistic evolution, traits are passed on from generation to generation,” says Power.
“But the types of traits that are passed on and the ways in which they are passed on differ. So, we might expect many differences in the ways that humans and their languages evolve.”
By using phylogenetic network methods, they could visualise the languages’ complex network connections rather than assuming that commonalities are simply due to linear ancestral inheritance.
For instance, similar forms could have been adopted from an unrelated language or coincidentally created independently.
To start with, the researchers built an extensive annotated database comprising more than 2000 handshapes from 40 contemporary and 36 historical manual alphabets.
Their analysis then grouped the languages into five lineages that arose in Austria, Britain, France, Spain and Sweden then spread to other parts of the world from the late 16th to late 19th century.
The results confirm previous historical observations, such as the wide-ranging influence of French sign language on deaf education and signing communities, but also had some surprises.
This included the spread of Austrian sign language to central and northern Europe as well as to Russia. It was also notable that three of the main lineages, from Austria, France and Spain, all seemed to have been influenced by early Spanish manual alphabets, which use the 22 letters of Latin.
“It’s likely that the early Spanish manual alphabets were used in limited ways by clergy or itinerant teachers of the deaf,” says Power, “but later signing communities added new handshapes to represent letters in the alphabets of their written languages.
“When large-scale schools for the deaf were established, the manual alphabets came into use in signing communities by relatively large groups of people. It is at this point where we put the beginnings of most of these five lineages.”
List says it was a revelation to him and Power that the overall picture provided by the data did not surprise Grimm, the biologist on their team.
“While we had assumed that the data would yield a picture extremely difficult to disentangle … Guido showed us that this was not the case and that the patterns we observed can likewise be found in specific cases of biological evolution.”
The blueprint they have developed can now be applied to gain new insights into sign language evolution, says Power, and to generate new hypotheses that researchers can test to further explore their historical development.