If you are time-traveling, not very:

So here's the upshot: of the 4.6 Gy of Earth's known history, there's only been enough oxygen in the atmosphere for us to survive for about 0.5 Gy. For roughly 90% of the Earth's history we couldn't even breathe the air. And about 10-25% of the time, there have been ice ages so savagely fierce that the glaciers reached the tropics: odds are good that any meat probe landing on solid ground during these periods would rapidly die of exposure. So historically, Earth has only been inhabitable about 8% of the time assuming you are lucky enough to find some solid ground. Once you factor in the random surface distribution, we're down to about 2% survivability.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.