Charles Stross: "How habitable is the Earth?"
The point is this: we are finely tuned survival machines that have evolved to survive in a niche on one particular planet in one particular epoch. Even our own planet is unimaginably hostile to our kind of life for most of its history. And while survival outside that niche is possible with the assistance of a horrendously complex toolkit we call "civilization", we've yet to try it somewhere where we can't count on the basics (free oxygen and triple point water).
