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74 that many forms of Earth life are governed by deep geometrical rules, observed Grinspoon, it suggests a universal geometry of life.*” Life as We Don’t Know It It is understandable that humans—or the inhabitants of other isolated biospheres—may conclude that their own environments are uniquely suited for life. The evolution of life on Earth has exploited the idiosyn- cracies of carbon and water, making us carbon and water chauvinists. These materials seem special to us, Feinberg and Shapiro argued, after 4 billion years of adapting to and building on their peculiarities. However, living things on other planets may exploit other chemical flows. As Clifford Pickover of IBM’s Watson Research Center put it, life will evolve into whatever embodiment best suits its purposes. Sagan warned years ago that we tend to overlook the possibility that other conceivable laws of nature might also be consistent with life.* Earth may be a very special place—for living things of the kinds we know on Earth. Every planet changes in the course of its history, Ernst Mayr argued, and the sequence of changes has to be just right.** That may not be the whole story. The Gaia theory originated by chemist and biophysicist James Lovelock and further developed by biologist Lynn Margulis suggests that living things can drive changes in the biosphere that benefit the long-term evolu- tion of life. Early photosynthetic organisms arose in an ecosystem that was fundamentally different from the one we know today. They enriched our planet’s atmosphere with oxygen, enabling very different evolutions on land. Life has “Terraformed” the Earth for billions of years.** While individual species go extinct, life survives and evolves. Long-term habitability may depend more on the establishment of a robust and resourceful global living organism than on having a lucky planet and a lucky star, speculated Grinspoon. Once life starts, it is only a matter of time before it develops, in concert with its evolving planet, into a global system of interacting and self-regulating cycles. Life is what a planet becomes.*° The question may not be the probability of life, according to biologist Norman Pace, but, rather, the probability that life, having arisen, survives and comes to dominate a planet. Yet we must be cautious about assuming that Gaia—a description of what happened on Earth—will predict events on other worlds.*’ The exact model we see might be peculiar to our planet. Probabilities: Life Gaia and Her Sisters