The Guillemot is a seabird that lays its eggs on a bare rock ledge on a cliff face. When an egg is accidentally dislodged, its shape causes it to spin in a tight circle, which prevents it from falling off the ledge into the sea. (Springwatch - BBC)
Can we just take a moment to appreciate how fucking awesome this is?
These eggs no doubt started out like all other avian eggs, but they had the problem of rolling off the cliffs. The eggs that were slightly more oblong tended to roll off the cliffs less, and thus the genes contained in those eggs lived to be passed on. Fast forward a few million years, and BAM tight-circle eggs.