I've had the idea that the Precursors are actually Humans and that it's a continuous cycle.
The Precursors look like humans, but they are not humans. You're right about the continuous cycle though. Maybe I should dig up some of Frogboy's old post again? He explained most of this stuff already.
As for the Thalan, they are an insectoid race from a future dimension. They have come to our space-time to prevent the destruction of the universe, claiming that the humans are responsible for it.
Edit: In the TotA tech tree of the Thalan it was mentioned, that they have multiple appendages. I'm quite disappointed that they changed that.