User blog comment:SLyfoX999/New TES Game (Idea)/@comment-4057437-20120717000316/@comment-4491632-20120720154107

The Falmer most certainly weren't the only race in Skyrim at the time, let alone Tamriel. The Dwemer had been coexisting with the Falmer for who-knows-how-long before the Nedes arrived, and the Bosmer have been noted as being in Tamriel since the beginning of the first Era. While I'm not entirely sure when the Aldmer came about in Tamriel, considering they're thought of as the ancestors of all elves in Tamriel, it can be assumed that they're there too.

Speaking of beast races, Elsweyr has been recorded as having up to 16 Khajiit kingdoms during the First Era, which means that they likely were around during the Nedic invasion. Argonians have little early recorded history, but it's mentioned that the Aldmer did make contact with them when they began exploring Tamriel, thus, it's very, very likely that they were around there too.