How Did the Supermassive Black Hole Get Started?

We don't actually know where these super assive ones in the centre of galaxies came from, or how they came to be so large. The it would make you wonder, like, how did all those tall people get so tall? At some point they must have been medium height, rights? Where are all the future tall people? That's basically the question we're asking about these intermediate mass black holes. So it's a mystery. We don't know if these black holes actually started from something small and then passed through this intermediate mass black hole region and then became supermassf black hole. Or if they somehow skipped it.