Charles Darwin regarded explaining that natural selection could produce such an intricate organ as the human eye as one of the biggest challenges to his theory. "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." "It is possible...that in the course of evolution, our own eyes started out as swimming eyeballs. Only later did the job of catching light get relegated to only some cells, which could send signals to their neighbors. And only much later did these specialist cells relay signals to brains."