Some 365 million years ago, during the early Devonian period, the Sarcopterygian (or lobe-finned) fish emerged from the sea and gave rise to the first terrestrial tetrapods.
During the course of their evolution, the tetrapods became adapted to life on land. One big challenge faced by the earliest tetrapods was how to interpret the rich tapestry of visual information to which their aquatic ancestors were all but oblivious.
One would think that, having evolved from fish, the visual systems of the early tetrapods would have been poorly adapted to life on land. But new research, just published in the open access journal BMC Evolutionary Biology, suggests that the first terrestrial vertebrates, had highly sensitive eyes that were adapted to colour vision.
In the new study, Helena Bailes of the University of Queensland, and her colleagues, examined the retina of the Australian lungfish Neoceratodus forsteri (above), a “living fossil” which first appeared 135 million years ago. This fish is believed to be the closest extant relative to the first terrestrial vertebrates, and, as such, its visual system is likely to be similar to that of the fish from which land vertebrates evolved
The Australian lungfish has small eyes and catches prey by electroreception (by sensing the electrical activity of the preys’ nervous system). Therefore, researchers have, until now, believed that it has poor eyesight, and little attention was paid to its retina, or to the light-sensitive opsin proteins expressed in the retina.
Recently, however, the Australian lungfish retina was found to contain four distinct types of photoreceptor, the cells which transmit visual information to the brain. These cells were also found to have certain morphological features that are characteristic of the photoreceptors of terrestrial organisms.
Apart from the partial DNA sequence of one of its opsin genes, nothing else was known about the visual pigments of the Australian lungfish. This prompted Bailes and her colleagues to investigate further. They therefore set out to characterize the full complement of opsin genes expressed in the N. forsteriretina.
5 opsin genes were isolated from the retina the Australian lungfish, and the entire coding sequences of these genes were determined. Significantly, the DNA sequences suggested that this species of lungfish probably has the ability to see ultraviolet light, as well as longer wavelengths of light which correspond to red hues that are invisible to humans.
Comparison of the DNA sequences with those of opsin genes from other organisms revealed that the N. forsteri opsins share greater similarities to the opsin genes of tetrapods than to other fish species. This surprising finding may be explained by the fact that the lungfish inhabits shallow waters, and therefore has spent most of its evolutionary history in a brightly-lit environment.
Bailes, H. J. (2007). Visual pigments in a living fossil, the Australian lungfish Neoceratodus forsteri. BMC Evol. Biol. 7: 200. doi: 10.1186/1471-2148-7-200. [Full text]