Also, I find it quite funny how many times these aliens look more human than the species closest to humans on earth. As if evolution universally converged on the same form.
Although the reason for that is quite obvious and rather simple (people are stupid and simply can't imagine something that's much different from themselves could exist and even be intelligent), it actually isn't quite as queer an idea as you might think.
Evolution generally(*) converges to what best fits the environmental conditions, whatever uses the least amount of energy and bears the greatest chance of survival.
Given similar environmental conditions. similarly shaped bodies would likely evolve (and only aliens which come from somewhat similar environments could possibly come to visit, except if they're wearing full-body environment suits -- and then you wouldn't know what they look like!).
For example, on our planet, all vertebrae have five fingers (and more or less exactly the same organs in general). These five fingers may melt into two or into one during fetal development, or one of them may wander away a bit (think dog), the thumb may be opposable or not, but they all start with the same number, and they all are generally alike. Does that make sense? No. But for some reason it's like that. Clearly 8 fingers are better than 5, so why do we not have 8 of them? And now that I think about it, 3 hands are clearly an advantage. If you're a handyman, you know what I'm talking about. So why do we not have a 3rd arm on our back?
Something must at some time have given a strong evolutionary advantage to "The Universal Plan" which is in use on our planet, or it wouldn't have persisted in such an ubiquitous manner. So either you believe that it's like this because "God" made it that way (but, why not any different, why no variation, and why doesn't God use a bit of fantasy?) or it must be the optimum given earth-like environmental conditions (which, too, are kind of an "optimum").
(*) Notable exceptions are conditions which include aggressive races such as homo sapiens, obviously. While nobody knows why homo neanderthalensis died out, it sure wasn't because their race was "unfit" in a Darwinian sense. They were much more advanced than homo sapiens at their time and had by far better adapted bodies. Numerous finds in caves suggest that their brains were quite a bit more advanced than one would think, too. My personal theory is that they died out because they were busy painting walls and thinking about who put the stars on the sky while their smaller but more aggressive cousins were busy throwing javelins at them.
The 1,000 or so species that die out every year also don't die out because they're unfit, but because we are an aggressive, destructive species and we have no respect for their existence.
Edited by samoth, 16 October 2013 - 04:47 PM.