Perhaps we've never been visited by aliens because they have looked upon Earth and decided there's no sign of intelligent life.

We define ourselves as intelligent. That's odd, because we're doing the definition - We're creating our own definition and saying, 'We are intelligent!'

If we find life out there, and it's not us, we will deem it not intelligent. But what may be equally as likely is that we find life that's vastly more intelligent than we are. If that's the case, we are putty in their hands.

I think that intelligence is such a narrow branch of the tree of life - this branch of primates we call humans. No other animal, by our definition, can be considered intelligent. So intelligence can't be all that important for survival, because there are so many animals that don't have what we call intelligence, and they're surviving just fine.

If Mars formed life, then life on Earth could have been seeded by life on Mars, making every life form on Earth descended from Martians.

I lose sleep at night wondering whether we are intelligent enough to figure out the universe. I don't know.