This is a nit-pick of mine for sci-fi in general. Obviously it stems from the initial concept of "all aliens look basically like humans with maybe weird foreheads or ears". Once people got wise to how to stupid that was, we started to see the explanation that some grand, all-powerful race of humanoids actually seeded life throughout the galaxy/universe/whatever, thus resulting in the abundance of intelligent alien civilizations that all look like humans.
My question is: has this trope ever been done in a way that actually made sense and wasn't just some lazy hand-waving to explain humanoid aliens?
Take Prometheus for example (probably a bad example to start with, but whatever). Supposedly these crazy proto-humans seeded earth with their DNA billions of years ago or whatever. The thing is, those "seeds" would have a HILARIOUSLY low chance of ever turning into anything like a human, even on an earth-like environment. Sure, there might be some convergent evolution and you end up with an intelligent species that has a head, two arms, two legs, and binocular vision (say, maybe something like the Navi from Avatar). But even that's a stretch.
A lot of science fiction uses this trope, and it has always bothered me. I mean, how does it work? Do you just drop a bunch of naked, bewildered humans onto a planet and let them have at it (at which point the fossil record of that planet becomes a major issue)? Do you manipulate the environment of said planet so as to "mold" humanoids? How the hell does it work?
Now, I'm fairly new to sci-fi literature (been on a kick with it recently, starting with the Dune series, then Blindsight by Peter Watts (all of you fuckers need to read this book right away -- if you like Bakker, you will almost certainly like Blindsight), and then finally the first two books of the Hyperion Cantos. I'm about to start the Culture series. But nonetheless I've been into sci-fi as a broad genre my whole life, and I can honestly say I've never seen a satisfyingly believable example of this trope in action.
Surely someone, somewhere, has done this successfully? The idea itself isn't horrible, it's just that the execution is always so vague and lackluster.