the answer in my mind is quite simple:
1. The probability of life evolving on a given planet is miniscule (unknown...but almost certainly miniscule).
2. But the universe is a REALLY big place
3. Therefore the probability of life existing elsewhere in the universe is quite high
4. But the universe is a REALLY big place
5. So the chance of the two lifeforms interacting are miniscule!
__________________
|