Here's a thought- Let's assume that life is, in fact, a spontaneous phenomenon that the laws of the universe state will just pop up whenever the circumstances are right- so, basically, leave a habitable planet alone long enough, life will form there out of lightning and mud, or whatever. So, life can and will exist elsewhere in the universe. Now, let's assume that life does, in fact, change gradually to become more suited to it's environment through darwinian evolution. Assuming that these two laws, both of which work entirely through the principles of randomness, hold fast in the model of the universe, it becomes INCREDIBLY unlikely that life on other planets would end up anywhere close to what we know of it. Even if you had a planet exactly like the Earth, organisms would still be entirely different. Think about it- take a billion years, and figure that every time a living thing on the planet reproduced during that time, that's another chance for evolution to pick a different path. There are possibilities beyond human notation of numbers, and since our view of sentience is so narrow (we have one species to analyze, and we don't even understand ourselves yet), it all just seems so astoundingly unlikely that we would even recognize another planet's life if we encountered it as alive, let alone thinking. It could be of a completely different chemical structure, reproduce and move through ways beyond our wildest dreams- it wouldn't have DNA, it would probably have totally different senses than us, and to assume that it would be even approximately the same size is just dumb. Things like fur, blood, irisis, and breasts, some of the most highly evolved features of earth life- it's not likely we'd find them elsewhere.
Now, with that said, let me say that I still like the idea of humanoid aliens. It's a great fictional device, and writing about them is a wonderful creative outlet. Especially when you try to justify their existance. My favorite theory I've seen in various sci-fis is that all life in the universe has a common origin, which is why it's so similar. Getting closer, but not quite. Still, though, you gotta love movie aliens.
Of course, I could be wrong. We could meet incredibly human-like aliens, and therefore have to totally re-think our theories on the formation of life and the universe. If they don't eat us, that is. It's a cookbook!
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