"The segments have all become different, the appendages have all become different, but the machinery for making appendages is the same," says Sean Carroll. "Evolution is a tinkerer, an improviser."

How We Got a Head

The human head is, inch for inch, the most complex part of our body. Not only does it contain our brain, but it also packs in most of our sense organs: eyes, ears, a nose, and a tongue. The intricate bones of the skull add to the head's complexity, from the cranium that keeps the brain safe to the jaws that allow us to eat. Thousands of variations on the theme exist—think of hammerhead sharks, of anteaters, of toucans.

All those heads become even more remarkable when you look at two simple sea creatures that are the closest living relatives of the vertebrates (animals with backbones). These humble organisms have no heads at all. But they have the makings of one in their genes.

The larvacean, a tiny gelatinous tadpole, lives in a floating house it builds with its own mucus. Its nervous system, such as it is, is organized around a simple nerve cord running along its back. Even stranger is its cousin, the sea squirt. It starts out as a swimming larva, with a rodlike stiffener in its tail. When it matures, it drives its front end into the ocean floor, eats most of its nervous system, and turns its body into a basket for filtering food particles.

At first glance, these creatures seem unlikely to hold any clues to the origin of the vertebrate head. But a close look at the front tip of larvaceans and larval sea squirts reveals a small brainlike organ where a vertebrate would have a head. "There are 360 neural cells there. Compared with the vertebrate brain, that's nothing," says William Jeffery, a biologist at the University of Maryland. Yet scientists have seen a strikingly familiar pattern in how that tiny cluster of cells develops. Some of the same genes that build our own brains are at work there, and in roughly the same areas—front, middle, and rear.

Jeffery and his colleagues have also found that sea squirts have what appear to be primitive cousins of neural crest cells—the kind of cells that build much of the head in the developing embryos of vertebrates. Like our own neural crest cells, the sea squirt's emerge along the back of the developing embryo and migrate through the body. But instead of making a skull, neurons, and other parts of the head, they turn into pigment cells, adding brilliant colors to sea squirt bodies.