Evolution of the Eye in Vertebrates

The eyes of humans and other vertebrates originated from a single light-sensitive organ, which in early chordates was located on the top of the head. Research has shown that the ancestors of vertebrates completely lost their original vision when they adopted a burrowing lifestyle. Later, when vision became necessary again, evolution split this organ into two parts and moved them to the sides of the head.

Types of Light-Sensitive Cells

Animals have two main types of light-sensitive cells: ciliary and rhabdomeric. In ciliary cells, pigments are located on modified cilia, while in rhabdomeric cells, pigments are found on numerous folds of the cell membrane. In protostome animals, such as insects and mollusks, eyes develop from the outer covering (epidermis) and consist only of rhabdomeric cells, whereas ciliary cells are found deep within the brain.

Features of Vertebrate Eyes

In vertebrates, eyes develop not from the skin, but from the wall of the embryonic brain. The retina is a genetic "chimera": light is detected by ciliary rods and cones, while the signal is transmitted to the brain through rhabdomeric neurons. For a long time, it was unclear why two different evolutionary lines are combined in a single organ.

Research on the Origin of Eyes

To study this question, data from molecular phylogenetics, paleontology, and anatomy were combined. Scientists analyzed RNA sequencing results from individual cells in mice and ancient fish (lampreys). They compared the transcriptomic profiles of modern retinal cells and cells of the pineal gland—an endocrine organ that in some modern reptiles and amphibians functions as a single "third eye."

Evolutionary Stages

About 560 million years ago, the ancestors of vertebrates adopted a bottom-dwelling lifestyle, burrowing into the mud to filter food, which led to the loss of their lateral eyes. Only the unpaired parietal organ remained, helping to determine depth and distinguish day from night. This organ contained both types of light-sensitive cells, but they functioned independently.

When later chordates returned to active swimming and hunting, evolution did not recreate eyes from the epidermis. Instead, the median brain visual organ split and shifted to the sides of the head, forming the basis for the retina, while its remnant in the center of the brain became the pineal gland. Thus, vertebrate eyes developed from brain tissue.

Formation of the Retina

In the ancient parietal organ, ciliary and rhabdomeric cells worked independently. To form a fully functional retina, evolution had to unite them into a single electrical circuit. This role was taken on by ancient motor cells, which previously stirred spinal fluid with their cilia. Their descendants—bipolar neurons—now receive signals from rods and cones and transmit them to the optic nerve.

The Role of the Pineal Gland

The pineal gland, a relative of the eye, does not perceive light directly but responds to visual signals. It produces the hormone melatonin and regulates the body's circadian rhythms, controlling sleep and wake cycles.