The father of evolution was a nervous parent. Few things worried Charles Darwin more than the challenge of explaining how nature's most complex structures, such as the eye, came to be. "The eye to this day gives me a cold shudder," he wrote to a friend in 1860.

Today biologists are beginning to understand the origins of life's complexity—the exquisite optical mechanism of the eye, the masterly engineering of the arm, the architecture of a flower or a feather, the choreography that allows trillions of cells to cooperate in a single organism.

The fundamental answer is clear: In one way or another, all these wonders evolved. "The basic idea of evolution is so elegant, so beautiful, so simple," says Howard Berg, a Harvard researcher who has spent much of the past 40 years studying one of the humbler examples of nature's complexity, the spinning tail of common bacteria. "The idea is simply that you fiddle around and you change something and then you ask, Does it improve my survival or not? And if it doesn't, then those individuals die and that idea goes away. And if it does, then those individuals succeed, and you keep fiddling around, improving. It's an enormously powerful technique."