Life has to exist elsewhere in the universe, your basic logic and probability will show you this. Out of the millions of plantes that exist it is practically impossible that only earth supports/has supported life
Aliens on planet earth, well i'm not so sure. I find it interesting that the USA has a rate of claimed alien abductions that is considerably higher than any other country in the world. As a consequence i'm just not convinced that everyone in the USA is telling the truth. If aliens visited the world, you would exect their landings to be equally spread across the globe. Or you may expect them to be concentrated in areas of high population or density. You would not expect the majority to be in one country.
Plus i like to think of it this way: If you assume all planets in the universe had the same chance of developing life as each other, you could speculate that earth would be an average example. (of course there is no way to prove this, its just speculation) But i this were the case, then you could also speculate that any planet that had life would be roughly as developed as us here (assuming the laws of physics and biology etc are similar on other planets and lead to similar development rates). If most planest are as developed as earth, and we have not yet left the solar system, you could speculate that toher planets may be in a similar situation. This makes the the idea of aliens visiting here seem unlikely.
Of course such a way of thinking about it depends on whether earth is average in terms of development, or is at the arse end of the scale or the top. Something we will never know.
Hope that made sense!
