Life probably has infinite diversity as we move through the cosmos.
But so what?
Except for the exo-biologists, we’re mostly interested in intelligent races with space flight technology. I think there’s a good argument that these folks will look a lot like us.
When people ask about “aliens” I presume they are thinking about intelligent races with whom we could come in contact via travel through space (or even radio).
Different environments lead to differing appearances and capabilities. But if we consider intelligent, technology-developing species that live on solid ground on Earth-like oxygen/water worlds, I’m betting that convergent evolution will lead to the emergence of similar looking species.
Whenever I see bug-eyed monsters in sci fi flicks, I ask myself, on what kind of world could that being have evolved? Unlikely an earthlike planet. Thus they wouldn’t be much interested in Earth. They would seek worlds like their home world.
Let’s stipulate that we’re talking about intelligent races with technology, not just bugs or algae. There’s an argument that they may look somewhat like us.
First, what is “humanoid?” Consider the velociraptors in Jurassic Park. Were they humanoid? Suppose they had evolved for another 65 million years instead of getting wiped out.
Bipedal, upright, head on top, two eyes, mouth/nose for eating, communicating, breathing, tasting, and smelling. Hands for using tools. There are strong evolutionary reasons for all of these.
If we, or they, are crossing the cosmos to find intelligent life, we’re first going to visit earthlike planets, not Jupiters or Neptunes or Mercuries. Water/oxygen planets. Convergent evolution says that similar environmental niches evolve similar beings. Beings with the qualities I listed above.
This also limits the size: how much smaller or larger than us could they be? Brain case, mobility, flexibility all limit size divergence.
I wouldn’t be too surprised if we discovered that upright bipeds with many of our design features turn out to be the most common form for intelligent, technology-developing races on earthlike worlds.
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My aliens are quite diverse, but all the ones that live on dry land (Earthlike planet) have a head on top that contains their brain and many sense organs. You want your eyes as high as possible so you can see the farthest.