My personal theory? All the races that look the same; Terrans, Thanagarians, Rannians, Kryptonians, Tamaraneans, etc, were all seeded by a mysterious alien race as old or older than the Oans. Where are they now? Where'd they come from? Why did they seed so many planets? What was their goal?
All part of the mystery.
Mostly because I havent worked that bit out.