One of the most challenging concepts in thinking about evolution is the idea of "deep time," the profoundly different timescale involved in geological and evolutionary processes than those we deal with in our daily lives. It is rare for those of us in North America to see buildings that are more than 100 years old. If the geological history of the world were represented by the height of the Empire State Building, the time since mastodons walked across North America 14,000 years ago would be represented by the thickness of a single dime.