Scientists have proposed a new model for the evolution of the human eye

Paired eyes of vertebrates, including humans, may have evolved from a complex median photosensitive organ of an ancient one-eyed ancestor that lived about 600 million years ago. This was reported on July 26 in the journal Science Daily.

The authors reconstructed the possible origin of the vertebrate retina by comparing the location, physiology, and functions of photosensitive cells in different groups of animals. The work is a scientific review and an evolutionary hypothesis, rather than a description of the found remains of a specific one-eyed creature.

According to the proposed model, the distant ancestor of vertebrates was a small worm-like marine organism that fed by filtering plankton. Previously, its evolutionary line probably possessed paired lateral eyes or groups of photosensitive cells.

After the transition to a sedentary or burrowing lifestyle, the need for advanced lateral vision decreased. Paired organs could have been lost, while a group of photosensitive cells located in the center of the head remained and formed a primitive median eye. Such an organ probably did not create a detailed image, but it made it possible to distinguish between light and dark, determine the alternation of day and night, and navigate relative to the surface of the water.

Later, the descendants of this organism switched back to active swimming. The need for vision increased, and parts of the complex median photosensitive organ could be used to form new paired eyes. According to the authors, at the same time, the lateral retinas and the structures of the future pineal complex developed from it.

This reconstruction may explain the unusual structure of vertebrate eyes. Their retinas develop as an extension of the nervous tissue of the brain, whereas the eyes of many invertebrates, including insects and cephalopods, were formed along a different evolutionary path.

According to the researchers, the trace of the ancient median organ has been preserved in the pineal complex of vertebrates. In humans, its part is the pineal gland, a gland that produces melatonin and is involved in regulating circadian rhythms.

At the same time, the human pineal gland is not a full-fledged eye and does not form images. Information about lighting is transmitted to structures regulating circadian rhythms mainly through the retina and neural pathways. In a number of other vertebrates, the median or parietal eye has been preserved in a more pronounced form.

The sequence proposed by the authors — the loss of the lateral eyes, the preservation of the median photosensitive organ and its subsequent use for the formation of paired vision — remains an evolutionary reconstruction. Additional comparative, genetic, and paleontological data will be required for its further verification.

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