Evolution of light sensors: ancient cone-like cells are actually rods

New evidence suggests that the short 'cones' of jawless sea lampreys may be physiologically equivalent to rods.

• Vertebrate rod photoreceptors are traditionally believed to have evolved from cone photoreceptors after jawed and jawless fishes diverged.
• This traditional view is challenged by findings regarding the short 'cones' in sea lampreys.
• The physiological equivalence of these short 'cones' to rods raises questions about the evolutionary timeline of photoreceptors.

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