An international team of scientists has concluded that the eyes of vertebrates, including humans, might have evolved from a single central eye of an ancient creature that lived around 600 million years ago. The findings were published in the journal Current Biology.
The authors of the study attempted to shed light on one of evolution’s mysteries: why the visual organs of vertebrates differ from those found in most other fauna.
Animals typically possess two types of light-sensitive cells: rhabdomeric and ciliary photoreceptors. In insects, mollusks, and crustaceans, rhabdomeric cells form the primary eyes, while ciliary cells are involved in regulating biological rhythms and responding to light levels.
The situation is reversed in vertebrates. The retinas of humans utilize ciliary cells for light detection, but they transmit signals to neurons that exhibit characteristics of rhabdomeric cells. This specific combination is not observed anywhere else in the animal kingdom.
To understand the origin of this structure, the scientists analyzed the localization and functions of light-sensitive cells across 36 major animal groups. They concluded that the common ancestor of vertebrates resembled a small, worm-like creature with two lateral eyes and a single central eye located on the top of its head.
Later, the ancestors of vertebrates adopted a less active lifestyle and began burrowing into the seabed. In such conditions, complex lateral eyes became redundant and gradually disappeared, while the central eye persisted, as explained by the neurobiologists.
Subsequently, these creatures returned to an active swimming existence, which required functional vision once more. The researchers propose that evolution repurposed the retained central eye. It became more complex and then divided into two parts that migrated to the sides of the head.
This, the authors suggest, explains the unique structure of the human retina. However, the ancient central eye did not completely vanish. The scientists believe it has been preserved in the form of the pineal gland, located deep within the human brain. This gland currently plays a role in sleep regulation through melatonin production.
In several modern animals, a similar “third eye” remains visible. For instance, the tuatara, an ancient reptile from New Zealand, has a light-sensitive organ on top of its head with its own retina and lens.
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