As far as I know, there are no plants or animals for whom microgravity causes truly mortal problems, so it's not just humans that are the beneficiaries of this. One possible explanation, if Panspermia theory is correct, is that we all have a common microbial or nanobial ancestor that was adapted to survive for tens of millions of years in microgravity conditions.

Or it could be simpler than that. Keep in mind that gravity isn't the only constant throughout our evolution -- there's also been the weak, strong, and electromagnetic forces as well, all of which are far more powerful than gravity. You can try the statically-charged-comb-deflecting-a-stream-of-water trick to see how easily electrostatic forces can act against gravity; at the cellular scale, in enclosed environments, such forces (and also chemical bonds) become even more pronounced, and can almost always trump gravity entirely. Just ask the capilaries of any Redwood tree. So, if biological systems are so good at ignoring the effects of gravity (due to its comparitive weakness at biological scales), then it seems reasonable that they would likewise be good at ignoring th effects of microgravity.

But I agree with you, something awfully strange is going on. I'm also non-theistic (though I am religious after a fashion), and think that invoking some human-like dude in the sky to explain this away is, in view of the other 50 billion galaxies out there, just a bit provincial. However, to look at the fact that the cooling embers of an ancient supernova have suddenly become terribly self-conscious -- that's us -- and explain it away as some totally meaningless coincidence... well, one must have an awfully strong faith in coincidence, in order to believe that...

Cheers,

Nathan.