YOHO NATIONAL PARK (British Columbia) • If the history of earth is condensed to fit into a single, 24-hour day, life emerges sometime before dawn. Photosynthesis evolves around mid-morning, and the atmosphere becomes oxygen-rich right before lunch.

But most of the day is utterly boring; all organisms are microscopic and occupied with little more than belching gases and oozing slime. It isn't till 9pm, about half a billion years before the present, that we see the first complex, multicellular beings. Scientists call this juncture the "Cambrian explosion" - the moment when billions of years of bacteria gave way to the rapidly evolving beings we know as animals.