A new study from the University of Sussex out today reveals that fish see the underwater world in a completely different way to humans and the discovery could change how we understand the evolution of human vision.

Using advanced brain imaging researchers found that zebrafish eyes are tuned not to the colours of the rainbow, but to “whiteness”, helping them detect nearby objects in murky water. This ancient visual strategy, scientists say, may have laid the foundations for how human colour vision later evolved.

This discovery sheds light on how the first visual systems evolved hundreds of millions of years ago and suggests that colour vision originally developed as a way to “see distance”, not colour itself.

“For fish, colour isn’t just about seeing the rainbow — first and foremost, it’s probably about knowing what’s close,” said Professor of Neuroscience at the University of Sussex, Tom Baden, who led the research. “Underwater, light becomes increasingly green or blue with distance. By tuning their eyes to ‘whiteness’, zebrafish can tell what’s nearby and worth paying attention to.

“The same thing happens in air, only over much longer distances,” Baden added. “It’s the reason distant mountains look pale and bluish — a trick artists have used for centuries.”

How they made the discovery

The team used two-photon brain imaging to monitor zebrafish neurons as the fish viewed visual patterns in different colours of light. They also used genetic tools to switch off individual types of light-sensitive cells - called cones - to test how each contributed to vision and behaviour, and discovered that:

• The brain responds strongly to broad-spectrum (“white”) light, but much more weakly to any form of “coloured” light.
• Red and ultraviolet cones are essential for normal “white” vision, driving the signals that detect movement and shapes.
• Green and blue cones act in opposition, suppressing visual signals when the stimulus is not white.
• Removing red and UV cones caused near-blindness, while removing green and blue cones increased visual responses in the brain.

Why are these findings significant?

The findings support a new evolutionary theory, proposed in two recent papers from the same team,  that colour circuits in early vertebrates did not first evolve “to see colour”, but to help animals navigate underwater, where light quickly loses both brightness and spectral range with distance.

Understanding this also helps place our own vision in context. Human red, green, and blue cones - the basis of human colour vision - do not directly match the fish red, green, and blue types. Instead, human red and green cones both trace their ancestry to fish red cones, and human blue corresponds to fish ultraviolet.

This means the human visual system stems entirely from the core, light-detecting channels of that ancient design. The suppressive “auxiliary” circuits that fish still use to filter the underwater background were lost in mammals, simplifying the system but preserving its basic wiring logic.

“Our results show how the roots of human colour vision lie in a very different ecological world,” Baden said. “We’ve kept the part of the system built for detecting what’s nearby - the active, ‘white-seeking’ part - while the filtering components that once handled the watery background disappeared as our ancestors moved onto land.”

Yet mammals are the exception, not the rule. Most other tetrapods (amphibians, reptiles including birds) did not reduce but instead expanded upon the ancestral cone system, adding new photoreceptor types, and colour-processing circuits that appear to be far more complex. However, how those expanded systems work, and whether they retain traces of the ancient ‘white-seeking’ logic seen in fish, remains largely unknown.

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Background
This work builds on two conceptual papers from 2024

• Ancestral photoreceptor diversity as the basis of visual behaviour (Nature Ecology & Evolution)
• From water to land: Evolution of photoreceptor circuits for vision in air (PLOS Biology)

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