In a marine worms eyes,the theory of evolution

Charles Darwin considered the evolution of the human eye one of the toughest problems his theory had to explain. 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.

This is in no way the ancestor of the human eye,but its the first time we have had a model of it, said Yale Passamaneck,a postdoctoral researcher at the University of Hawaii. He and his colleagues report the discovery in the online journal EvoDevo.They made the discovery while studying a species of brachiopods,or lamp shells,which live in shells but are marine worms. Lamp shells have existed for over half a billion years,but their biology has remained a mysteryincluding the question of whether they can see. Four-day-old lamp shell larvae,have puzzling dark spots on either side of the front end of their bodies. Recently,Carsten Lüter,a biologist at the Berlin Museum of Natural History,and his colleagues dissected the eyespots of some lamp shell larvae. They discovered that each spot was actually a pair of neurons,one for capturing light and one containing pigment. The neurons connected to a brainlike clump of neurons inside the larva.Their anatomy suggested the spots were simple eyes. They contacted Passamaneck and his team,experts on the genes for animal photoreceptors and discovered that photoreceptor genes were active in the dark spots. Passamaneck checked to see if the photoreceptor genes were active at other stages. But the opposite happened. He discovered that the genes were active much earlier,36 hours after fertilisation,when the lamp shell embryo was a cup-shaped mass of a few hundred cells.

To see if the embryos were doing something with the light,Passamaneck and his colleagues put a light on one side of a dish of embryos. The embryo is covered with tiny beating hairs,which it uses to swim in a spiral pattern. He found that after 20 minutes,twice as many embryos would end up on the illuminated side of the dish as on the dark side.Passamaneck and his colleagues hypothesise that the cells can detect the direction of light because itsblocked in some directions by the embryos yolk. It can then use this information to change the rhythm of its hair. Its possible,Passamaneck said,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 some cells,which could send signals to their neighbours. And much later did these specialist cells relay signals to brains.CARL ZIMMER