Animal life began in the sea. It should come as no surprise then that primitive, or ancient, fish had characteristics perfect for living in water. But over time, some primitive fish that lived in shallow waters shuffled their way onto land. Land provided new sources of food and safety from other animals.

Scientists use the word transition to explain the physical and behavioral changes that occur when one life form evolves, or develops, into a different life form. Legs for moving around and lungs for breathing air do not evolve overnight. In fact, it took tens of millions of years for certain kinds of primitive fish to transition into tetrapods—animals with four limbs and a backbone that live on land.

Name one living thing that lives on land, and another that lives in water. For each, write down a characteristic that helps it live in its environment.