But nearly 150 years after Darwin first brought this elegant idea to the world's attention when he published The Origin of Species, the evolution of complex structures can still be hard to accept. Most of us can envision natural selection tweaking a simple trait—making an animal furrier, for example, or its neck longer. Yet it's harder to picture evolution producing a new complex organ, complete with all its precisely interlocking parts. Creationists claim that life is so complex that it could not have evolved. They often cite the virtuoso engineering of the bacterial tail, which resembles a tiny electric motor spinning a shaft, to argue that such complexity must be the direct product of "intelligent design" by a superior being.

The vast majority of biologists do not share this belief. Studying how complex structures came to be is one of the most exciting frontiers in evolutionary biology, with clues coming at remarkable speed.

Some have emerged from spectacular fossils that reveal the precursors of complex organs such as limbs or feathers. Others come from laboratories, where scientists are studying the genes that turn featureless embryos into mature organisms. By comparing the genes that build bodies in different species, they've found evidence that structures as seemingly different as the eyes of a fly and a human being actually have a shared heritage.

Scientists still have a long way to go in understanding the evolution of complexity, which isn't surprising since many of life's devices evolved hundreds of millions of years ago. Nevertheless, new discoveries are revealing the steps by which complex structures developed from simple beginnings. Through it all, scientists keep rediscovering a few key rules. One is that a complex structure can evolve through a series of simpler intermediates. Another is that nature is thrifty, modifying old genes for new uses and even reusing the same genes in new ways, to build something more elaborate.

Sean Carroll, a biologist at the University of Wisconsin–Madison, likens the body-building genes to construction workers. "If you walked past a construction site at 6 p.m. every day, you'd say, Wow, it's a miracle—the building is building itself. But if you sat there all day and saw the workers and the tools, you'd understand how it was put together. We can now see the workers and the machinery. And the same machinery and workers can build any structure."