Quote Originally Posted by halfeye View Post
It is impossible. It's not the life being seeded across the galaxy that's difficult, that's easily possible even without FTL. What's impossible is that life is fractal, you don't get the same results twice, the slightest variation gets different results, and those different results get another lot of different results. Convergent evolution is a thing, birds have wings, bats have wings, insects have wings, and they have a lot in common, they tend to taper toward the tips, they're flatish, and they're fairly smooth. How they're made though is different, and that's what makes for all the variations.
Well here's the thing though, convergent evolution is "a thing" because evolution reacts to external pressures. While the results will not be identical (unless under absolutely identical conditions and in a fully deterministic universe), the same set of pressures will result in broadly the same developments.

Whether or not they'll end up genetically compatible to ours is another thing entirely, but another planet with the same composition, in the same temperature band of the same class of star as Earth, will more or less end up developing the same kinds of carbon-based lifeforms. The details will be different, but with the same stuff to work with, chemistry will create the same building blocks of life. The same kind of organisms will dominate, the same kind of organisms will be the first to leave the water and go on land. The same kind of sunlight and the same kind of water will cause the same kind of plants to grow, causing the same kind of atmospheric makeup. It all goes along the chain from there, to mammalian arboreal species that gradually develop intelligence and society, and eventually arrive at the same kind of mostly hairless, upright bipedal mammal with sexual dimorphism.

The only major wrinkle in this concept is the one or three extinction events that our planet had to go through, with the biggest (or at least most recent) being the asteroid impact that wiped the planet of oversized, energy-inefficient and ambient-temperature-sensitive reptiles, and only allowed smaller and more warm-blooded specimens like mammals to really go forward on land. However, it may just be that it's the kind of "interference" an outside agent seeking to populate the galaxy with similar lifeforms could enact. The effects of a properly aimed asteroid aren't exactly unpredictable, not if you've seen it happen several times already. It may be that without a timely rock to put on some extra pressure, a typical Earth-like planet would forever be stalled in a primal wilds state, dominated by creatures that can't support the energy balance required by an intelligence level that would allow them to form language and tool use.