One of creationists’ favorite arguments is that so intricate a device as the eye—with a light-regulating iris, a focusing lens, a layered retina of photosensitive cells, and so on—could not have arisen from Darwinian evolution. How could random mutations have spontaneously created and assembled parts that would have had no independent purpose? “What good is half an eye?” the creationists sneer, claiming the organ as prima facie proof of the existence of God.

Indeed, even Charles Darwin acknowledged in On the Origin of Species that the eye seemed to pose an objection to his theory. Yet by looking at the fossil record, at the stages of embryonic development and at the diverse types of eyes in existing animals, biologists since Darwin have outlined incremental evolutionary steps that may have led to the eye as we know it.

The basic structure of our eyes is similar in all vertebrates, even lampreys, whose ancestors branched away from ours about 500 million years ago. By that time, therefore, all the basic features of the eye must have existed, says Trevor Lamb of the Australian National University. But vertebrates’ next closest kin, the slippery hagfish—animals with a cartilaginous cranium but no other bones—has only rudimentary eyes. They are conical structures under the skin, with no cornea, no lens and no muscles, whose function is probably just to measure the dim ambient light in the deep, muddy seabeds where hagfish live.

Our eyes are thus likely to have evolved after our lineages diverged from those of hagfish, perhaps 550 million years ago, according to Lamb. Earlier animals might have had patches of light-sensitive cells on their brain to tell light from dark and night from day. If those patches had re-formed into pouchlike structures as in hagfish, however, the animals could have distinguished the direction from which light was coming. Further small improvements would have enabled the visualization of rough images, as do the pinhole-camera eyes of the nautilus, a mollusk. Lenses could eventually have evolved from thickened layers of transparent skin. The key is that at every stage, the “incomplete” eye offered survival advantages over its predecessors.

All these changes may have appeared within just 100,000 generations, biologists have calculated, which in geologic terms is the blink of an eye. Such speedy evolution may have been necessary, because many invertebrates were developing their own kinds of eyes. “There was a real arms race,” Lamb says. “As soon as somebody had eyes and started eating you, it became important to escape them.”