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A few species of vertebrates still retain the median eye on top of the head. In this frog, the median eye appears as a small light-blue spot between the regular eyes.
Photo Credit: TheAlphaWolf
(CC BY-NC 4.0)
Scientific Frontline: "At a Glance" Summary: Evolution of the Vertebrate Eye
- Main Discovery: All vertebrates evolved from a distant worm-like ancestor possessing a single median eye, which eventually gave rise to modern paired eyes and the brain's pineal gland.
- Methodology: Researchers conducted an extensive comparative analysis of light-sensitive cells across diverse animal groups, evaluating their specific physiological functions and anatomical placement within the body.
- Key Data: The identified ancestral organism lived approximately 600 million years ago, and its primitive median eye survives in modern vertebrates as the pineal gland, an organ that regulates sleep cycles via melatonin production.
- Significance: The findings explain why vertebrate retinas originate from brain tissue rather than the skin on the sides of the head, distinctly separating vertebrate optical evolution from that of invertebrates like insects and squid.
- Future Application: Tracing the evolutionary path of these optical structures provides a foundational framework for analyzing the neural circuits responsible for retinal image processing in modern neurobiology and ophthalmology.
- Branch of Science: Evolutionary Biology and Sensory Biology

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