Table 1 Summary of the steps in the evolution of vertebrate eyes as proposed by Lamb et al. (2007)

>580

Bilaterally symmetrical animals evolve

Some components may have functions other than in light detection, and may work together in a simple light response system in the absence of any visual organ

Yes. Similar to the light sensing capability of soil-dwelling nematodes that lack any type of eyes^a

Various G-protein-coupled signaling cascades evolve and initially may function only in sensory systems other than vision (e.g., chemoreception)

Early opsins (G-protein-coupled receptor proteins) evolve

Early rhabdomeric and ciliary photoreceptors evolve

580–550

Ciliary photoreceptors and opsins continue to be modified

Organ serves as a simple light detector

Yes. Similar level of complexity as found in some modern non-vertebrate chordates

550–530

Ciliary photoreceptor gains more complex signal transmission capabilities

No image-forming capabilities, but the organ can detect shadows or serve a circadian function

Yes. Similar level of complexity as found in modern hagfishes and larval lampreys

Eye-field region of brain bulges to form lateral “eye vesicles” outside of the newly evolved skull

Lateral vesicles invaginate, bringing the proto-retina next to the proto-retinal pigment cell layer

A transluscent layer of cells (a primordial lens placode) evolves and prevents pigmentation of skin over the light-sensing organ

530–500

Photoreceptors develop cone-like features

Eye has image-forming capabilities and can operate over a relatively broad spectrum of light and range of light intensities

Yes. Similar to the eyes of modern adult lampreys

Duplication of genome creates multiple copies of phototransduction genes

Cell types of photoreceptors diverge in form and have distinct opsins

Retinal information processing capability increases as neural changes take place

Lens placode invaginates and forms into a simple lens

Iris develops and basic pupil constriction is possible

Extraocular muscles evolve

500–430

Myelin evolves and increases efficacy of neural transmission

Eye has strong image-forming capabilities, including for adjusting the amount of incoming light and accommodating the lens to focus at different distances

Yes. Similar to the eyes of many modern fishes

Rhodopsin evolves from cone opsin and rod bipolar cells evolve (possibly from rod photoreceptors)

Highly contractile iris evolves

Refractive capabilities of the lens improved

Intraocular eye muscles evolve, allowing accommodation of the lens

Retina contains both rods and cones and has more efficient processing capability

< 430

In tetrapods, the lens becomes elliptical to compensate for added refractive power of the cornea in air

Eyes as found in modern amphibians, reptiles, mammals, and birds

Yes. You are reading this page

Eyes become specialized in different groups according to different conditions (e.g., nocturnal vs. diurnal, predators vs. prey, etc.)