The first sentence: Sinaht has been mourning her world for a lifetime, and her mate for fifteen cycles, when she finally finds the way.
The last sentence: Jim laughs and slings his unbroken arm around his First Officer's shoulders, which Spock endures with remarkable ease as they look out at their familiar stars.
The thing that made me want to write it: From watching Nova [an American science show] I learned that Mars once had a rotating molten core, liquid water, a thick atmosphere (maintained by the magnetic field produced by the rotating core's dynamo), and a large satellite, all just like Earth, but when its satellite crashed into it Nars lost the internal dynamo, the atmosphere and the water and thus any chance of sustaining life-as-we-know-it. And then I thought, what if that collision weren't an accident? What if Earth had been saved from the same fate? And when you need time-travelers to save the Earth... (well, one could also call on The Doctor, but he doesn't have photon torpedoes.)
The biggest problem I had while writing it: I've never seen Enterprise, so basing my villains on a race from it was a bit... dodgy, probably.
Why it almost never got posted: Being as that the plot of Star Trek XI (and umpteen other instances of Star Trek) hinged on time travel, it felt redundant to write yet another time travel story. But the plot was so compelling...
The scene that hit the cutting room floor: I was going to put in explicit Kirk/McCoy, even if just a smooch, to make it slightly-slash-flavored gen in analogy with all the het flavoring gen usually gets (both in fanfic and in professional works) but that ended up not fitting in anywhere.
Anything else: I might write this one day, and if I do, the Kirk/McCoy smooch will be in there.