The eye has been called a miracle of evolution, but scientists, beginning with Darwin, can explain the likely evolution of the eye via natural selection. Considering the visual systems of other creatures, from zooplankton, through insects, fish, and birds can help us to understand the many ways our remarkable human visual system is similar or different. Correctly detecting and interpreting movement is critical to survival, and thus, current neurologic theory posits that complex body movements, vision, and cognitive power have evolved together hand-in-hand.

Our nighttime (rods/scoptopic) and daytime (cones/photoptic) visual systems are very different, and have interesting evolutionary histories. Rods are up to 10,000 times more sensitive to light than cones, and are grouped into large ‘receptive fields’ that pre-process visual information before it is transmitted to the brain. Due to the pervasiveness of electric lighting, many modern humans have not developed a comfortable ability to operate under fully scotopic conditions.

Color vision is quite variable within the human genome, and thus color perception can vary considerably between people. Cones are even more energetic than the rods, and require a constant supply of blood providing oxygen and nutrients. To economize, they are also concentrated in a small area, called the fovea, that provides most of our highly detailed and color vision. Our eyes are constantly scanning, in movements called saccades, so our brains can fill in visual details and create a coherent image. We keep learning about new complexities of the eye, such as how the rods and cones interact, how daily and seasonal changes may affect visual perception, and how these may also impact childhood visual development.