Photo by Krissy Venosdale.

Four and a half billion years is a hard number to digest. That’s the age of the Earth, and a lot has happened in that time. The geologic record contains dramatic climate swings, the formation of entire continents, the proliferation of new species -- as well as mass extinctions. But no matter what has happened in the past, life goes on. Well, in the case of mass extinctions, at least some life does ...

To help people get their heads around our role in all this, geologists use the analogy of a clock: If you compress all of the Earth's history into a single day, humans do not show up on the scene until a minute before midnight.