Scientists believe that, in the very early evolution of animals, the precursor of modern eyes had a single photoreceptor cell. How could a single photoreceptor cell have evolved into a complex eye like the eye in a human or octopus?
To begin to answer this question, scientists have studied the anatomy and function of different types of eyes in a variety of contemporary animals. None of these contemporary animals was an evolutionary ancestor of humans or octopuses. However, the different types of simpler eyes in some contemporary animals can suggest a possible sequence of intermediate steps that may have occurred during the evolution of the human eye or octopus eye.
This evidence also indicates how each intermediate step in the evolution of a complex eye could have been useful and contributed to increased fitness (the ability to survive and reproduce).
Light-Sensitive Sensory Organs in Several Types of Contemporary Animals
(e.g. in a limpet that scrapes algae off of rocks and glides or creeps along, similar to a snail)
(e.g. in a snail that feeds on sponges and moves by gliding or creeping along)
(e.g. in an octopus, an active predator that relies on vision to hunt prey; it moves by swimming or using its arms for crawling.)
Each type of light-sensitive organ can be useful for the animals that have it. For example, eye spots which detect light vs. dark are useful for limpets that are active at night and not active during the day. When the limpet's eye spots detect light, the limpet clamps its conical shell down on the rock to avoid predation and drying out during the day.