I'd guess abiogenesis on Earth, but there are so many varieties of possible, if unlikely, transfers of life between planets and other domains. Abiogenesis on Mars with a subsequent move to Earth would be interesting.

After that there are many conceivable levels right up to galactic or even universal panspermia. I don't think they are probable, but what do I know?

If I had to guess, I'd say sparsely distributed independent biospheres with occasional cross transfers within a solar system.