Anderson came close, but had a falling out with the Council after a mission with Saren went awry. I think you can ask Anderson about it after Shepard initially tries (and fails) to explain that Saren has gone rogue. Not sure if you can still ask him about it at this point in the game though.

Captain Anderson was getting "trained" to be the first human spectre.  I don't remember the specifics, but Saren basically screwed him somehow, which made the Council deny Anderson spectre status.

Anderson was the first candidate but then things got messed up and Sheperd is the first Spectre

The first book they released is about Anderson and how things went bad. It's a good read if you wanna learn more about what happened.

" The first book they released is about Anderson and how things went bad. It's a good read if you wanna learn more about what happened. "