While I think there's an excellent chance of life existing elsewhere in the universe (even a slim chance it exists elsewhere in our own solar system in some form), without some common genetic ancestry I'd expect it to look and live quite differently than us. Excellent reasons already given above: environment, adaptation, etc.
One other reason why I'd expect vast differences if you suddently gathered 25 'alien' lifeforms in one room today is the vast timescales involved in universal development. Hopping through Earth's lifecycle history alone, you would meet a wide variety of 'dominant' lifeforms at any given visit: microbes, amoeba, dinosaurs, modern humans... all within a fraction of our planet's total time of existence.
I think generally life may go through similar patterns in evolution (basic locomotion, brain evolution, etc.) but the start times for these runs would vary by millions if not billions of years. Who can predict what humans might evolve into a million years from now, given how we've changed in the last two million? It's not only where and how we might discover alien lifeforms, but when we discover them in their own history of being.
MISSION:TREK's in-depth review of STAR TREK
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