The idea that life began elsewhere in space and subsequently "seeded" the earth is apparently making a comeback. This idea, called panspermia, is the scientific equivalent of a Hail Mary pass in football, but the interesting thing about such theories of life's origin is the unstated reason for adducing them.
It's believed by advocates of panspermia that life must have originated elsewhere because it's just too difficult to construct a plausible materialistic scenario for the origin of life here on earth, given the terrestrial conditions that were believed to have prevailed 3 billion years ago. In other words, panspermia is the materialist's way out of a philosophicaly desperate situation. If life could not have arisen on earth without some extra-cosmic help then in order to avoid the conclusion that life's appearance is evidence for an intelligent agent involved in its origin, it is proposed that life must have originated somewhere else where conditions might have been more propitious.
If the cosmos shows no signs of life anywhere else besides earth, however, and if the genesis of life on earth remains an intractable problem, a lot of people are going to wonder if there may not be something else involved in its production besides purely physical forces and processes.
For a good read on the difficulties any theory of materialistic abiogenesis, whether terrestrial or extra-terrestrial, needs to overcome pick up Paul Davies' The Fifth Miracle.
