When Arnold wishes he had more information for his family tree, Ms. Frizzle revs up the Magic School Bus and the class zooms back to prehistoric times. First stop: 3.5 billion years ago! There aren't any people around to ask for directions. Luckily Ms. Frizzle has a plan, and the class is right there to watch simple cells become sponges and then fish and dinosaurs, then mammals and early primates and, eventually, modern humans. It's the longest class trip ever!