It is in the history - every time you update a field on an issue, JIRA writes "<fieldname> <from> <to>" into the history. So if you look for the first instance of "Assignee", then the <from> will contain the original assignee.
I suspect there is more to the question than "just look at the history" - could you expand and explain what you're trying to do and how?
JQL is for finding issues, not fields, so that's not really going to work. (You can't say "show me something on an issue" with JQL, the question is always "Show me the issues that match this data) For SQL, read changeitem for the <fieldname><from><to> I mentioned already, and join it to changegroup to link in with the date/time stamp and the issue that the change was made to.
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