THE first vertebrates to walk the land could sense colours better than we can. The fish from which amphibians, birds and mammals evolved probably had four colour receptors in their eyes, whereas humans and many other primates now have only three, says Mickey Rowe, a vision specialist at the University of California at Santa Barbara.

A wide variety of species, from starlings to chameleons and goldfish, have four colour receptors. "The fourth pigment is usually in the ultraviolet," says Rowe, so flowers that look, say, white to us might appear different to some birds.

DNA sequencing shows that the genes for each of the photoreceptors are similar in birds and fish that have all four. That indicates the common ancestor they shared with mammals also had four colour receptors. However, the earliest mammals were nocturnal animals that had little use for colour vision and lost two receptors. In some primates, one of the remaining two then evolved into green and red receptors.

Although ancestral amphibians sampled the spectrum better than we can, says Rowe, we may perceive the spectrum better. Colour vision depends on how the brain processes the signals from the eye, not just what the eye detects, and humans devote large areas of their brains to vision. No one knows what other mammals perceive, but humans can distinguish up to two million hues.

Source: Palaeontologia Electronica (vol 3, issue 1, article 3)