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76 have started independently at two different levels. Davies speculated that we may find a second origin of life on the Earth: pockets of microbes deep in the crust that survived the early asteroidal bombardment 3.9 billion years ago.”! Mind-Stretcher. Calling a phenomenon extreme depends on its context. What would be extreme in an alien environment? If life had originated and evolved under conditions very different from those on Earth, organ- isms that we call extremophiles might be in the mainstream of living things. As Shostak suggested, they might be the most frequent form of life in the universe. What is exotic and rare on our planet could be on The assumption that biology required light as well as liquid water limited the prospects for finding life beyond Earth, observed Davies. Now that we know that life flourishes under Earth’s surface, the possibilities for life elsewhere look more promising. Subsurface life may be widespread among planetary bodies, even among those with totally inhospitable surfaces.” The ability of living things to survive in extreme conditions broadens the habitable zone in our solar system. Over interstellar distances, surface life may be the only kind that we can detect, because of its effect on planetary atmospheres. If we do find evi- dence of surface life, that may imply that many other forms of life also exist, but out of reach for our instruments.* Scientists now talk about life under extreme conditions, tightly linked evolution of living organisms and the planet on which they arise, and cata- strophism. All of those ideas had seemed marginal or even heretical 25 years ago.‘ We had underestimated the resiliency and opportunism of living things—the insistence of life. Bottom Line, Life. A modest gain for believers. Though we still lack confirmed evidence of extraterrestrial biology, the variety of environ- ments capable of supporting life appears to be much greater than we thought in the past; the constraints on what living things need to survive have loosened. Probabilities: Life common elsewhere.