During evolutionary divergence since more than 3.5 billion years ago, there have been at least 64 times that “sight” (or awareness of “light/dark”) has evolved––each independently from one another. Bacterial phototaxis (movement toward light) was first recognized more than a century ago, but the method by which such small cells can sense the direction of illumination has remained puzzling. The unicellular cyanobacterium, Synechocystis sp. PCC 6803, moves with Type IV pili and measures light intensity and color with a range of photoreceptors.
Authors [see ref] show that individual Synechocystis cells do not respond to a spatiotemporal gradient in light intensity, but rather they directly and accurately sense the position of a light source. They demonstrated that directional light-sensing is possible, because Synechocystis cells act as spherical microlenses, allowing the cell to see a light source and move towards it. A high-resolution image of the light source is focused on the edge of the cell opposite to the source––triggering movement away from the focused spot. Spherical cyanobacteria are probably the world’s smallest and oldest example of the evolution of a camera eye.
eLife http://doi.org/bcgd (2016)