Some serious biologists, and I count myself among them, have begun to wonder that among the enormous and still unknown diversity of microorganisms, one might — just might — find aliens among them. True aliens, stocks that arrived from outer space. They've had billions of years to do it, but especially during the earliest period of biological evolution on this planet. We do know that some bacterial species that have earthly origin are capable of almost unimaginable extremes of temperature and other harsh changes in environment, including hard radiation strong enough and maintained long enough to crack the Pyrex vessels around the growing population of bacteria. There may be a temptation to treat the biosphere holistically and the species that compose it as a great flux of entities hardly worth distinguishing one from the other. But each of these species, even the tiniest Prochlorococci, are masterpieces of evolution. Each has persisted for thousands to millions of years. Each is exquisitely adapted to the environment in which it lives, interlocked with other species to form ecosystems upon which our own lives depend in ways we have not begun even to imagine. We will destroy these ecosystems and the species composing them at the peril of our own existence — and unfortunately we are destroying them with ingenuity and ceaseless energy.