Let's assume that there are tones of civilizations in the Universe.
Let's assume they all developed in a different environment.
Let's assume they all developed different technology.
Let's assume they all have different ways of traveling.
Let's assume they all have different ways of thinking.
There are many ways of arriving here.
Maybe one civilization is arriving here with cigar shape ships.
Another civilization could arrive here with a triangle shape ships.
Another civilization could arrive here with some kind of "stare gate" or "portal" .
So the point is, we can't make assumptions because we don't know what kind of technology they posses.
edit on 14-1-2012 by Nikola014 because:
(no reason given)