ABSTRACT
How can evolution explain an organ as complex as the eye? Did it suddenly appear whole, and if not, what good is a half-evolved eye?
As it turns out, quite a bit. In just a few short steps, outlined in the chapter, each step improving on the last, a patch of photosensitive cells can become a complex lensed eye, complete with cornea and lens. Paired, optically reversing eyes evolved multiple times in different animal lineages, all rooted in the same Pax gene family.
In our line they likely emerged in early vertebrates, about 520 million years ago. Photopigments, the chemicals sensitive to light, appeared even earlier. To a first approximation, their combined sensitivities match the solar radiation spectrum on Earth's surface. Initially incorporated into four different cone receptors, first one and then a second were lost in early mammalian ancestors. That's because in night-dwellers, they conveyed no advantage, and incurred a metabolic disadvantage. It wasn't until 21-45 million years ago that Old World monkey and ape ancestors re-evolved a third cone type, producing so-called "three color vision".
Other themes visited include night vision's evolution and adaptations; the differing acuity of the cone and rod systems; and the visual brain.
