Dinosaurs died some 65 million years ago in the great worldwide Cretaceous extinction that also snuffed out about half of the species of shallow water marine invertebrates. They had ruled terrestrial environments for l00 million years and would probably rule today if they had survived the debacle. Mammals arose at about the same time and spent their first 100 million years as small creatures inhabiting the nooks and crannies of a dinosaur’s world. If the death of dinosaurs had not provided their great opportunity, mammals would still be small and insignificant creatures. We would not be here, and no consciously intelligent life would grace our earth. Evidence gathered since 1980 indicates that the impact of an extraterrestrial body triggered this extinction. What could be more unpredictable and unexpected than comets or asteroids striking the earth literally out of the blue? Yet without such impact, our Earth would lack consciously intelligent life. Many great extinctions (several larger than the Cretaceous event) have set basic patterns in the history of life, imparting an essential randomness to our evolutionary pageant.
      —Stephen Jay Gould, The Flamingo’s Smile: Reflections in Natural History

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We exist through a strange and unlikely series of events. Life is mysterious, tenuous. We are here temporarily.