Quote:
Originally Posted by Penforhire
On the other hand, since we haven't been able to find much life different than ours then perhaps it is natural to assume even more compatibility. Who's to say which assumption is more correct?
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Every creature on this planet has a common biological heritage, obtained from the chemical makeup of this planet. I wouldn't assume any other planet would duplicate Earth's chemical makeup, so I wouldn't assume any organism that developed on that planet would duplicate our biological makeup.
We already know that all terrestrial organisms use a "right-handed" molecule configuration, though why that is, no one knows. Organisms featuring "left-handed" molecules would not be able to interact with us on a molecular level; foodstuffs would pass undigested through us, antibodies would not be able to recognize alien bacteria, etc.
And there are so many ways in which even the same chemical organisms can be different from ours; imagine, for example, an alien race that had a protein-based chemical composition, but in which those proteins were straight, not tightly twisted like ours? It would require a very different chemical system to process those proteins, and that could determine the difference between a being similar to us, a plant, or a sentient rock.
One of SF's favorite tropes is of finding things that are "like us." It makes for easier storytelling. I think it much more likely that anything we find out there will be overall nothing like us.