Quote:
Originally Posted by DwnWthVwls
How do you define something? I feel like a lot of things could only happen once, depending on what kind of answer your willing to accept.
-Do you really think there are humans anywhere else? or any of the living thing on this planet? How about diseases?
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I would define humans as
Homo sapiens which is a loosely defined branch from a certain lineage that we can trace back to the earliest life forms on planet earth. You could have something that looked like humans on other planets, but they wouldn't be humans, because they would have no biological ties to our particular lineage.
But natural selection tends to make similar things over and over again in similar environments. Take hair for example. You can find hair on the butt of a bumblebee, but its evolutionary origin is very different from the hair on your head. Still, hair-like structures can be useful in many different ways and so has evolved independently several times.
Similarly, if the traits and collections of traits that make us human would also be useful elsewhere, it seems reasonable to expect similar traits would evolve on different planets.