A short while ago, I talked all about proteins. Continuing with that theme, I explored a rabbit hole around the opsin protein family in animals.

  • The Ur-Opsin Hack: GPCRs are versatile protein receptors that normally change shape in response to a bound chemical to send a cellular signal. Just before the Cambrian explosion (600–700 million years ago), an early GPCR mutated so it could couple with vitamin A. Because vitamin A naturally changes shape when hit by a photon, this mutation gave early animals the ability to detect light. We call this evolutionary ancestor the Ur-opsin gene.
  • The 2R Hypothesis: A major driver of the rapid biodiversity during the Cambrian explosion was that early vertebrates duplicated their entire genome twice. This gave them four copies of many essential genes, providing massive evolutionary freedom to mutate these extra copies and develop wildly different traits without breaking the original biological functions.
  • Tetrachromatic Origins: Thanks to those whole-genome duplications, the Ur-opsin gene split into four distinct versions, making early vertebrates tetrachromatic. They possessed SWS1 to see UV light, SWS2 for blue, RH2 for green, and LWS for red. Today, most vertebrates—including fish, reptiles, and birds—still utilize this tetrachromatic vision.
  • Convergent Cephalopods: Cephalopods (like octopuses and squids) also inherited the Ur-opsin gene. However, because their evolutionary line never underwent those massive genome duplications, they remained monochromatic. Interestingly, their eyes look structurally identical to ours—complete with a cornea, lens, retina, and fluid—but this is purely a masterpiece of convergent evolution.
  • The Mammalian Downgrade: Early mammals spent most of their evolutionary history surviving as nocturnal creatures. Since discerning distinct colors isn't highly useful in the dark, they eventually lost the SWS2 and RH2 genes. This is why most mammals today are only dichromatic.
  • The Yellow Filter: Around 80 million years ago, proto-primates emerged into the daylight. To protect their eyes from damaging UV radiation, they evolved a yellow filter over their lenses. Blocking out UV light created a new environmental pressure that forced their remaining SWS1 receptors to shift and start absorbing blue light instead.
  • Monet's UV Vision: If you physically remove that yellow lens, the human brain actually interprets UV light as a whitish-blue or pale violet. A famous example is the painter Claude Monet: in 1923, after developing severe cataracts that muted his color perception, he had the lens of his eye removed. Suddenly able to perceive UV wavelengths, he began painting with incredibly vivid blues and violets.
  • Return to Trichromacy: About 35 million years ago, Old World monkeys experienced a local duplication of the LWS (red) gene. This new duplicate, called MWS, was evolutionarily pressured to shift its sensitivity to pick up green light. This specific mutation is the reason Old World primates—including humans—regained trichromatic vision.
  • Conformational Lock: The amino acid sequence similarity between SWS1 and LWS is roughly 40–45% across all vertebrates. This highlights a core rule of proteins: once a protein’s specialized function is defined and optimized, the gene is essentially locked into its conformation. You need a mechanism like gene duplication to lift the evolutionary pressure, allowing the spare copy to freely mutate and explore new functional potential.
  • The Mantis Shrimp Exception: Mantis shrimps possess up to 33 different opsin genes, allowing their eyes to detect dozens of specific colors. However, unlike vertebrates, these crustaceans are small and lack the complex brains needed to overlap and interpolate neural signals from different cones. While vertebrates can perceive millions of blended colors thanks to the processing power of horizontal, amacrine, and ganglion cells in the retina, mantis shrimps rely on a massive sheer number of receptors because they process each color as a hardcoded, distinct channel.