This lesson provides a real-sense time scale experience to better visualize the vast extent of geological time. Using familiar linear analogs, students come to realize that the five main classes of vertebrates (fishes, amphibians, reptiles, mammals and birds) each emerged several tens of millions of years apart, over the past 500 million years. Students also discover when the earliest fossils of each major vertebrate class first appeared, over several hundreds of millions of years, and gain a sense of their gradual changes over time.