Charles Darwin considered the evolution of the human eye one of the toughest problems his theory had to explain. In “On the Origin of Species,” he wrote that the idea that natural selection could produce such an intricate organ “seems, I freely confess, absurd in the highest possible degree.”

But Darwin dispelled that seeming absurdity by laying out a series of steps by which the evolution could take place. Making this sequence all the more plausible was the fact that some of the transitional forms Darwin described actually existed in living invertebrates.

Now, a team of American and European researchers report that they have discovered an eye that could represent the first step in this evolution. They have found, in effect, a swimming eyeball....