I've always thought it was highly unlikely that life started somewhere else in space and then "seeded" the Earth. It would have had to survive for thousands or millions of years of constant cosmic radiation while making the trip. I think it's much more likely that the simple organic compounds that are found in space fell to Earth and then took part in the 'soup' in which llfe began right here.
That's not to say that life did not start in other places as well, just that the transfer would be highly unlikely.
For example, if life transfer were relatively easy, we should have found traces of earthly life, or its remains, on the Moon. There were hundreds of millions of years over which the transfer could have occurred, if it were possible.