Thing is, thinking aliens would be radically different from Earth-based life forms is as silly as the little green men theory. There would, no doubt, be notable differences, but, and I point out this is only for the evolutionists amongst us, the species that exist on this planet have mutated and been put through the mangle of life for millions of years. The designs work, that's why the creatures survive. If conditions on another planet are similar to an area on Earth, then there is no reason why the creatures that evolve there wouldn't be pretty familiar. The random mutations would have to cope with the same challenges, and would, most likely, come up with pretty much the same answers.