William N. answered 04/11/19
Fifty Years Experience in American Sign Language and Interpreting
I have answered this question previously on this message board. Here is the answer repeated here.
ASL developed in North America in the early 19th century with the founding of the American Scool for the Deaf in Hartford, CT. Thomas Hopkins Gallaudet and Laurent Clerc founded this school and began teaching deaf students via sign language. Laurent Clerc was a deaf teacher whom TH Gallaudet had recruited from France to come to America and help him found this school. In France there had been a strong tradition of using French Sign Language to teach deaf students. Laurent brought his French Sign Language to the school and it mixed with indigenous forms of signing that already existed in North America but wasn't as formalized as French Sign Language. The students attending this school brought their individual home sign languages and other existing sign languages to the school and it mixed with the formal French Sign Language that Laurent Clerc was using. This mixture developed into American Sign Language. Even today between ASL and French Sign Language there exists cognates; that is, signs that are the same in both languages. Some linguists estimate around 30% of the signs in the two languages are cognates.
Hope this brief explanation clarifies for you how ASL developed. If you are interested in learning ASL I would be happy to tutor you.
Rregards
Bill N.