Re: panspermia.

Applying Occam's razor isn't trivial here. The difficulty of the journey to earth makes panspermia less probable, but all the other places where life could then have previously evolved makes it more probable. The issue is - or should be - how these things balance.

If you write down the theory, panspermia has a longer description. However, that's not the correct way to decide between the theories in this kind of case - you have to look a bit deeper into the probabilities involved.