There are languages that are considered isolates, meaning they are spoken and written languages that share no connection to any other known language. Basque is an example for that, spoken in the Northern parts of Spain, but does not share any similarities to Spanish, Portuguese, English or French. Common theory is that it just developed in that region and was not influenced by outside factors at all which is linguistically weird since trading with other regions was common. There are also no known languages that descended from Basque, thus it’s not a proto-language either.