According to this it started with a light-sensitive spot on the skin probably in order to see light a day which would help you to survive.

http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth? Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.