[pfdietz]

The past survival of livable conditions on Earth is no guarantee they will persist for very long in the future. There's observer selection bias to consider. For example, the O2 level in the atmosphere could drop below that needed to sustain higher life. As I understand it, there's been no feedback mechanism identified that stabilizes O2 at current levels, so the persistence of adequate oxygen over the last 500 million years could just be an unlikely accident.

In the absence of burial of photosynthesized materials atmospheric O2 will disappear in a few million years as reduced materials are exposed by geological processes and then oxidized. So, the time constant for O2 fluctuations is rather short.

If and when we spread into the galaxy, we may find many planets where livable conditions were snuffed out before intelligence could arise. In our own solar system, both Mars and Venus may have been more habitable than Earth earlier in the history of the solar system, but now are forever ruined.