Blueprints for Bodies

A developing fly larva looks as featureless as a grain of rice. But it already bears a map of the complex creature it will become. Across the larva, different combinations of genes are active, marking it off into invisible compartments. These genes turn on other genes that give each compartment its shape and function: Some sprout legs, others wings, others antennae. An invisible anatomy becomes visible.

Flies aren't the only animals that build their bodies this way. Scientists have found that the genes responsible for laying out the fly's body plan have nearly identical counterparts in many other animals, ranging from crabs to earthworms to lampreys to us. The discovery came as a surprise, since these animals have such different-looking bodies. But now scientists generally agree that the common ancestor of all these animals—a wormlike creature that lived an estimated 570 million years ago—already had a basic set of body-plan genes. Its descendants then used those genes to build new kinds of bodies.

To appreciate how this tool kit can generate complexity, consider the velvet worm. The velvet worm creeps along the floors of tropical forests on nearly identical pad-shaped legs. It is, frankly, a boring little creature. Yet it is also the closest living relative to the single most diverse group of animals, the arthropods. Among arthropods, you can find a dizzying range of complex bodies, from butterflies to tarantulas, horseshoe crabs, ticks, and lobsters.

Scientists studying body-plan genes think arthropods started out much like velvet worms, using the same basic set of body-building genes to lay out their anatomy. Over time, copies of those genes began to be borrowed for new jobs. The invisible map of the arthropod body plan became more complex, with more compartments and new body parts sprouting from them.

Some compartments, for example, developed organs for breathing; later, in insects, those breathing organs evolved into wings. Early insect fossils preserve wings sprouting from many segments. Over time, insects shut off the wing-building genes in all but a few segments—or used some of the same genes to build new structures. Flies, for example, have just one pair of wings; a second pair has turned into club-shaped structures called halteres, which help flies stay balanced in flight.