For a very long time, the Yi people used a logographic script to write their language. Their tradition says that it was created by someone named Aki in around 700 AD, but the earliest record is from 1485 AD.
Mostly the Yi priests used it for religious, magical, or medical texts. The literacy rate was very low (less than 3% in 1956), so kids weren’t using it to write to Grandma. They didn’t have a strong central government, so didn’t use it as a bureaucratic language. They didn’t appear to use it for accounting or records; when they communicated with the outside world, they used Chinese. As a result, there was zero standardization, and every set of village priests had their own local dialect of the script, resulting in a huge number of characters: 90,000 by one estimate.