Earth is about 4.6 billion years old. The geologic time scale, read from rock layers and fossils, organizes that history into eons, eras, periods and epochs, like chapters in a very long book. Most of the story has no complex life in it at all. This week the crew unrolls a paper timeline where each sheet stands for 100 million years, places life's milestones on it from a textbook table, and finds that humans fit in a sliver thinner than a pencil line.