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A biologist from a pedestrian planet, peering at some stretch of North America from a height of 500 feet, will conclude that its dominant species is a shiny lozenge-shaped reptilian creature that alternately basks in the sun and sprints at great speed. It is host, he will note, to small endosymbiotic organisms which at intervals emerge, move about slowly, then reenter the host ... They are devoted to the care and feeding of the host. They suck energy-rich organic compounds from the bowels of the planet and feed them to the host, something it is unable to do for itself ... They make over ecosystems to meet the host's needs, replacing vast forests and grasslands with flat surfaces on which the host can bask or sprint more easily, and building hives and dens in which the host can take shelter from the elements. What they get in return is as yet unclear. |