In order to not derail other threads, I'm attempting to separate this. ^.^

First, can we even think that there are other species capable of communication and animal type actions outside of earth? I'm assuming yes, because I think that the probability of only one planet hosting life seems odd, however, timing matters. Anyway, would aliens necessarily be comprehensible? Couldn't they be not carbon-based, or have different senses? If they had different senses, they could be aware of entirely different things. We know it's possible to have other senses (think bats), it seems that perceiving waves is all that is necessary. But if they perceived a different set of waves, would we even ever notice them if we were quite near, because they could communicate with each-other, but such waves might not register at all with us.

Maybe I should stop confusing myself...