One amino acid does not a protein make—let alone a being.
—Preston Cloud, Oasis in Space
Once life evolves, it tends to cover its tracks.
The discovery of extremophilic microbes has radically changed our conception of where life might be able to exist in the Universe—it causes us to reassess the concept of habitable zones. Scientists now realize that habitats suitable for microbial life are far more widely distributed in our solar system, and surely in the Universe as well, than was considered possible even in the most optimistic views of the 1980s and before. On the other hand, these same studies are showing that complex life—such as higher animals and plants—may have fewer suitable habitats than was previously thought. But just because life could exist in a place doesn't mean it is actually there. Life can be widely distributed in the Universe only if it can come into being easily. In this chapter we will examine current knowledge and hypotheses about how life may have first formed on Earth and in what type of environment this may have taken place.
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