I've noticed a lot people throwing around probability as reason why life would have ET origins, speaking from the perspective of a true layman surely the probability of:
Life starts on Melmac (Which is a billion in 1 event as someone else has already used as an arguement against earth origin)
Melmac has massive catastophic event
Small chunks fly into space and with a few bits of Alf strapped on
Tiny bits of Alf manage to survive the millions of years flying through space
Tiny Alf rock manages to manages to find its way to earth (space is really big. You may think its a long way to the shops, but thats just peanuts to space (DA))
Alf manages to survive the intense heat of entering the earths atmosphere
Each one of those steps has a massive probability of failure, surely probability actually favours life starting here?