Where are all the aliens? Scotty Hendricks at Big Think lists some common solutions to the Fermi Paradox, among them the Great Filter. I myself like to define that one as follows: A civilization with the power to destroy itself, will destroy itself. (Especially if it’s a human civilization.)

Hendricks goes on:

There are, of course, other possible solutions. Civilized life might just be rarer than we think, or uncommon in the area of the universe we are in. It could be possible that the other life in the cosmos is so alien that we wouldn’t Recognize it if we saw it — as predicted by H.P. Lovecraft in his classic work The Colour Out of Space. Other solutions abound, ranging from the idea that we are alone to the idea that some hyper civilization prevents others from reaching a particular level of technology.

I’m of course partial to the Lovecraftian idea that potential alien life is so weird that we would not be able to understand it or even recognize it as life if we were to encounter it. But on the other hand, since evolution must be regarded as a universal principle, it seems likely that life on other worlds would be quite similar to life here on Earth, as long as it has evolved under similar circumstances. It is only if life can evolve on worlds very different from Earth (or not on worlds at all) that it might also be very different from us in form and purpose.

In any case, no discussion of these things is complete without this: