There are many debates about the origin of life on Earth, but whether there is life elsewhere in the universe is a still more contentious topic, a minefield of unanswered questions. We considered one of these in chapter 14: how likely or unlikely was the "origin of life"? Another basic question is whether all life in the universe is based on nucleic acids and proteins, like ours, or whether it can have different molecular or physical hardware. If so, what are these alternatives? Even if we could answer these questions, we would then have to ask how many planets in the universe could support life, and estimate how many actually do so. Has all extraterrestrial life (if it exists) evolved as ours has? Perhaps most of all, people want to know whether there are intelligent species on other worlds, and if so whether we could communicate with them.
There is scant evidence to help answer these questions, but people nevertheless speculate and even hold firm opinions about them. This has generated an entire discipline, "exobiology" or "bioastronomy", complete with dedicated international conferences and publications. Exobiology is probably the only "scientific" subject ever to have thrived in the virtual absence of data. There is no denying its fascination.
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