suppose there were another planet similar to earth in composition, temperature, and atmosphere.
(and given the bazillions of stars in the universe, the probability is quite high that, somewhere, there must be)
so, considering another earth-like planet with H2O, carbon, and oxygen... over a couple billion years ... what kind of life would evolve? why wouldn't it make sense that many of the life forms that would result, would be similiar to life forms that either once existed on earth in the past, or currently exist on earth today?
you know, mammal-like creatures like cows or mastadons. or reptile-like creatures like dinosaurs or crocodiles. or at least, your basic fish variants and plant life.
and the things that were different, they wouldn't necessarily be radically different from creatures that could conceivably be found on earth. like a pig-dog. or a lizard-bird. or something.
just a thought.