Calculating God — Robert J. Sawyer

An alien shuttle craft lands outside a museum. Out pops a six-legged, two-armed alien named Hollus who says, "Take me to your paleontologist." The paleontologist, Tom Jericho, helps Hollus investigate Earth's evolutionary history. It seems that Earth and Hollus' home planet, and the home planet of another alien species traveling with Hollus, all experienced the same five cataclysmic events at about the same time, including events exactly like the meteor that wiped out the dinosaurs.

Both alien races believe this proves the existence of God: i.e.. He's obviously been playing with the evolution of life on each of these planets. From this provocative launch point, Sawyer tells a fast-paced, morally and intellectually challenging science fiction story that lust grows larger and larger in scope.