The first prehistoric fish that clambered onto land glimpsed its new world in full color and could even see wavelengths of light that our eyes can't, a new study suggests.
Scientists have found that the retinas of Australian lungfish (Neoceratodus forsteri) contain visual pigment genes that are more similar to those of tetrapods—four-legged land animals with backbones—than those of other fish. They also found evidence the fish can see in ultraviolet.
Australian lungfish are thought to be the closest surviving relatives of the first land animals. The "living fossils" have remained virtually unchanged since first appearing in the fossil record 135 million years ago. They still live in Australian rivers.
Helena Bailes of the University of Queensland in Australia and her colleagues analyzed the Australian lungfish DNA that codes for opsin, a visual pigment found in cone cells required for seeing in color. Comparing it to other creatures, they found that it more closely resembled the opsin of amphibians and reptiles than other fish.
"The visual system of N. forsteri may represent an evolutionary design most closely reflecting that present just prior to the emergence of land vertebrates in the Devonian Period," Bailes said.
The team also spotted four types of cone cells in the lungfish eyes, suggesting the fish can see in colors we humans can't. "From looking at the DNA sequence, they certainly have the potential to see in the [ultraviolet] range and further into the red range than humans," Bailes said.
Cones are light receptors in the eye that are sensitive to color, while "rods" are better at seeing in dim light. Humans have three types of cone cells in their eyes: red, green, and blue. "That's why TVs are made of red, green and blue pixels," Bailes told LiveScience.
Color vision is good for increasing contrast between objects and could have been used by the first land-lubbing creatures to find prey or elude predators, Bailes said.
A past study by her team found that the eyes of lampreys, a jawless living fossil whose origins stretch further back than even the lungfish, also had cone visual pigments, suggesting these jawless, fully aquatic fish also could see in color.
"It's thought that color vision evolved in these lamprey fish, through the lungfish to land vertebrates," Bailes said.
The new findings are detailed online in the BMC Evolutionary Biology journal.
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