| Summary: | We are lucky, on Earth. We are lucky because we—as complex and self-aware organisms—are here. We are sustained, given air to breathe, and water, and food, by a very ancient planet: a planet past its midpoint, a planet that is nearer death than birth. Our species is a latecomer. It took some three billion years to bridge the gap from a single-celled organism (originating in this planet’s youth) to a multicellular one, and then a little over half a billion more to arrive at the diversity of species on Earth today, including Homo sapiens . In all this time, the chain of life has remained unbroken. The Earth has been consistently habitable, with an atmosphere, and land, and oceans. Since life began, our planet has never been truly deep-frozen, nor have the oceans boiled away. The Earth is the Goldilocks planet. One recalls, here, the children’s story, where the young heroine of that name walks into the house of the three bears, and in their absence tries out successively their bowls of porridge, their chairs, and their beds. Each time the first and second choices are too hot or cold, large or small, hard or soft—and the third choice is just right . The Earth has been, so far and all in all, just right for life: not just right at any one time, but continuously so for three billion years. There have, though, been some close calls: times of mass extinction. But, life has always clung on to bloom once more. That makes the Earth’s history more remarkable than any children’s story. Other planets have not been so lucky. Mars seems to have been a planet with an appreciable atmosphere, and—at least intermittently—running water over its surface, and may even have begun to incubate life. But the atmosphere was stripped away by the solar wind. Its early lakes and rivers became acid, charged with sulphates. Then, most of the water evaporated and was carried off into space; what little was left became locked away as permafrost and in thin ice-caps. Mars does have weather, including spectacular, planet-wide dust-storms.
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