Photo courtesy David Pu'u/Corbis

Books, TV shows and movies sometimes mention people with gills. In "Harry Potter and the Goblet of Fire," Harry eats "gillyweed" during the Tri-Wizard Tournament. Soon he has sharp pains in his neck. Harry reaches up to his throat and feels "two large slits just below his ears, flapping in the cold air…" He has gills!

In the movie "WaterWorld," some of the people have mutations that give them gills. Will we ever see people with gills? Could a mutation like this let people swim in the water all day long like the fish do?

One way to answer this question is to look at the world around you. There are lots of mammals that swim like fish. Whales swim. So do dolphins. Manatees, too. All of these mammals live their entire lives in the water. But they all have lungs just like people do. You never see a mammal with gills in nature. Why is that?

The main reason is because a mammal would need to have GIGANTIC gills. Gills work for fish because fish are cold blooded. Cold blooded animals need a lot less oxygen than warm blooded animals do. Imagine that you have a 100-pound human and a 100-pound fish sitting on a couch. A human being needs maybe 15 times more oxygen than the fish does. If the human then gets off the couch and starts swimming, the human needs even more oxygen. So does the fish, but fish are super-efficient in the water.

Fish also use their mouths and gill flaps to constantly pump water over their gills. Lots of fish (like certain sharks) have to swim constantly so enough water flows through their gills.

Go look at a goldfish sometime and notice how much of its head is taken up by its gills and gill flaps. Now imagine a human being with gills and gill flaps, but the gills are 15 times larger. You would need a whole new torso just to hold the gills! And that would be pretty gross.

That's why people don't have gills. Compared to air, there just is not enough oxygen dissolved in water to keep a warm-blooded animal going.