Evolution of the eye, computer artwork. It is thought that the eye originated as a patch of light-sensitive cells (left) on the organism's surface. When a mutation in some organisms led to the cells being positioned in a small depression, it became possible to detect the direction of the light source. As the depression became deeper and the aperture narrower (centre), this information became more precise and it was possible to make out shapes. Next a protective layer formed over the opening, allowing cells inside the eye to begin to specialise. Some organisms went on to form lenses at the front of the eye (right), which allowed the eye to focus.