The fact of the matter is at some stage you have to stop and say that life formed SOMEWHERE, and you have to say how. Explaining life on this planet as coming from another one, is just regressing the mystery. It's as thick as saying "god did it", because that doesn't explain where 'god' came from. So with life. Saying 'it came from another world' doesn't explain how or why it formed on that other world.
So just get it right! Explain how it could come about on this world, and the job is done!