This is soft of a Sci-Fi idea, I was toying with.
Attached to Slashdot: Don't Talk To Aliens, Warns Stephen Hawking
The theory of panspermia is where microbes transmit life to habitable bodies in space in for form or comet's and asteroids. If a planet such as earth were to be impacted by something large, fragments would be launched in to space, these would immediately freeze and preserve microbes almost indefinitely. A few cubic meters of frozen sea water from earth would contain a soup of trillions of single celled microbes and be in a perfect delivery system to survive reentry on another planet. Given the large numbers of fragments ejected in to space from such an impact and the small number of microbes needed to populate a planet, there is a good chance that our DNA has evolved across the whole of our galaxy.
So If you believe that which I think is far easier to believe then faster then light travel. The idea of a Christopher Columbus scenario doesn't make sense. Any intelligent aliens arriving by craft would have a few problems. They are 10's of thousands of years from home. The idea of arriving with an Attack fleet is completely impractical given the times and distances involved. Even the resources just for a small group to make such a journey is more then the whole Human race can generate right now.
The one group that does arrive would be more like Robinson Crusoe trapped on an island with no technology other then what they can scavenge off the ship they arrive in. The some of the more Advanced technologies from back home would be impossible to replicate. Imagine trying to make a semiconductor fab with a small group stranded on an island, it's not going to happen.
They also probably wouldn't be well adapted for our environment. The temperature, mixture of gasses, food etc. I think the first order of business would be like the Avatar movie scenario. To evolve hybrid creatures that can think like them and know their culture but would be bio-compatible with earth. They would have to do this by mixing their DNA with earth creature DNA, which probably would work because of microbial panspermia providing some level of commonality between xenoDNA's. Mostly likely they would want their hybrids to fit in and rise to power based on their technological advantage and then leverage the local population to do the work for them.
Also if it happened with one race of aliens then it's could probably happened with several races, each one landing and hybridizing local populations independently.
The other alternative is they have become a machine race, with their whole beings becoming entirely data. At which point our planet would be little more then a curiosity. As mining would be impractical with such a gravity well and there is little here they would need or want other then scientific curiosity. Maybe study us and how we evolved much the way humans study apes in the jungle. We don't come in to take over the ape civilization, just try to leave then as undisturbed as possible to learn how they think and interact with each other.