[WallShadow]

Ya see, here's what happened:
Aliens with intent to planet-o-form Earth to their own ecosystem style redirect an earth-grazing asteroid to impact the planet. The lowly monkey-boys infesting the planet detect the incoming rock of ages and attempt to destroy/deflect it by firing every nuke on the planet at it. Of course certain first-world wannabee's quality control being what it is, some don't detonate exactly where they should (that's right, they fall back to Earth and go Boom!). Well, the rock is deflected a little bit, just enough to have it intersect the planet at one of its biggest thin spots--the Yellowstone Caldera. Hit two rocks together, you get a spark. Hit two BIG rocks together, you get a megavolcanic eruption.
This is not quite what the aliens wanted, so they sow lots and lots of biological agents and nanobots to correct the environment. Some of the upper strata of the land-dwelling species are affected, primarily primates. Voila! Apocalypse flustercluck!