Salt, being an important preservative (not to mention necessary for human life), is central - even essential - to human society and so the origin of its use by humans is solidly prehistoric (and so largely outside the scope of this subreddit: /r/AskAnthropology might have some better answers).

Evidence of human salt processing goes back more than 8000 years in Bulgaria and China, and the availability of salt has been an important issue for every civilization.