Around four hundred million years ago, our ancestors turned toward land—first with their eyes, and then their fins, which soon became legs. Was it a good decision? Their descendants’ views seem to be divided.

A Gary Larson cartoon is lodged in my memory: two froggy/fishy creatures gaze longingly from the water onto the beach. One of them is holding a baseball bat in its fin/hand, and there’s a carelessly struck ball on the sand. The caption says: “Great moments in evolution.” When I was later reading Carl Zimmer’s book At the Water’s Edge (about how life emerged onto land and then returned to the sea) and saw that the first chapter is entitled “After a Lost Balloon,” of course I immediately thought that the author had pinched Larson’s joke about the ball. But the truth turned out to be much more interesting.