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.