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103 gist Richard Lewontin’s comment that a typical mammalian species lasts about 10 million years.*° In the longer term, the extinction of individual species is the norm. More than 99% of all species that ever lived on Earth are now extinct. Those other species lacked the scientific knowledge and technological ability to save themselves from external forces. They also lacked our ability to Anntenee Levan thane aan Mind-Stretcher. Ulmschneider calculated that 99.8% of all intelligent societies that ever existed in our galaxy are extinct. If he is right, explor- ers from living civilizations might find relics on many planets or in orbit around many stars—the ruins of great cities, the monuments of ancient kingdoms, the tombs of the dead. The most active interstellar travelers might be archaeologists studying the remains of departed civilizations. They may always be in demand, for the realm of their researches will be vast in space and deep in time. We may be focusing too narrowly on our species as it now exists. What is the lifetime of a civilization whose members evolve into something dif- ferent? Humankind, declared de Duve, will either evolve or disappear.” This would be true of other intelligent species as well. Isaac Asimoy, in his 1979 book A Choice of Catastrophes, listed poten- tial disasters that could end or cripple the human experiment. Some were Earth-bound, such as the slowing of the Earth’s rotation, tectonic events in the Earth’s crust, changing climate, and the failure of Earth’s magnetic field. Many others came from beyond the Earth: collisions with astronomi- cal objects such as black holes, free-floating planets, clouds of dust and gas, and antimatter; the bombardment of the Earth by comets, asteroids, and meteorites; the effects of collapsing stars; the death of our own Sun; in the very long term, the increase of entropy and the closing of the universe. Asimov added competition from other forms of life, including disease microorganisms, and conflicts among humans. He mentioned but dismissed the possibility of conflict with extraterrestrial intelligence.” Only a year later, the idea that extraterrestrial forces could cause cata- strophic mass extinctions on Earth gained new credibility when scientists described how an asteroid impact could have wiped out most species of dinosaurs 65 million years ago. Researchers later found evidence of another Extraterrestrial Terminators destroy ourselves through conscious acts. Extraterrestrial Terminators —Arthur C. Clarke, 1950**