We would be very ignorant to believe that there are no other planets that can hold life, I just don't think any planet has enough resources to be able to fuel a voyage between solar systems.
Although the US Government has recorded several UFO sightings, some of which have since been wiped from history, and if there were other lifeforms on Earth, who's to say we are even capable of seeing them? Or even them seeing us? It's possible that human beings and an alien race have always co-existed, we just operate in a completely different way and don't effect eachother. Maybe that is the reason for ghost sightings?
I think that if Aliens were to colonise Earth before we reached the technological advancement to be able to colonise other planets and visit other galaxies then we'd most definitely be the inferior race on Earth, and most likely hunted, eaten and enslaved. We'd be treated in the same way that we've treated all inferior creatures on Earth, and who's to say we don't deserve it?
Sorry, i originally wrote the first paragraph but I got a little crazy after reading the rest of the thread.
That's our problem, we can't get along, so we still have to evolve as people to even begin to think about constructing something that can go a million times faster than the speed of light. As humans, could we even sustain that kind of speed without literally puking our guts out?
200 years ago nobody thought it was possible to travel 50 mph without dying.. In 2004 the X-43 reached 7000 mph.