There's plenty of chemosynthetic life on earth that doesn't need a sun - even if earth style life needed the sun to evolve, if the earth became a rogue planet those organisms might be fine. And life might not have needed the sun to evolve.
Yeah, that doesn't work when you're not receiving any heat whatsoever. There needs to be an energy input. For Earth, that's the sun. Where is the energy coming from otherwise?
