Origin оf whаles: In the previоus level, yоu looked аt the trаnsition from water to land and the evolution of tetrapods. Several groups of tetrapods went on to make the reverse trip—they headed back to the water. In this level, you’ll put together a tree that summarizes some of the changes that occurred as a group of mammals, closely related to the modern-day hippopotamus, returned to the sea. Answer the 3 questions below before you move on to Mission 3, “DNA Spells Evolution.” When did whale ancestors begin living full time in the water?