The human iris. (Image courtesy of Petr Novák, Wikipedia.)
Davide Pisani, professor of evolutionary biology at The National University of Ireland, described the evolutionary basis of vertebrate vision in a biology department seminar on molecular paleobiology last Monday.
He began the talk by describing his approach to phylogenetic research into the biosphere, which encompasses the “catholic view” that paleontologists should not rule out any information regarding the origins of life.
The first timescale Pisani investigated for evidence of early vision was the Lower Cambrian, about 745 to 520 million years ago. He highlighted how the limited fossil record allows only for guesswork regarding potential ocular remains.
The only fossils that demonstrate pieces of potential eye pigment from this time period are those of organisms from the genera Haikouichthys and Olenellid. Pisani emphasized that unfortunately the fossil record is not only physically sparse in this ancient time period, but also mute in regards to the origin and early evolution of vision.
Pisani then spoke about genomic information, an alternative method for determining evolutionary linkages. Conserved genetic information between current visually capable species and ancient species could potentially elucidate the desired relationships.
After Pisani detailed this more successful type of evolutionary research, he then described the type of vision he investigated. Rather than looking at simple non-spatial vision (light sensitivity), Pisani was interested in the visual modalities and origin of phototopic vision (bright-light vision) and scotopic vision (dim-light vision).
Of these types of vision, there were two specific visual pathways: those of vertebrates and those of arthropods. The most important point in common between these two groups is the use of a visual pigment which belongs to the same protein family.
The visual pigment in question is an opsin added to a chromophore. Both the arthropod and vertebrate visual pigments are members of the opsin family, which consists of seven retinal binding proteins.
Animal opsins belong to one protein family, which evolved through a series of gene duplications. Although these two proteins are different in vertebrates and arthropods, they are evolutionarily related through a protein family as previously stated.
Of the multiple types of vision described (spatial, non-spatial, phototopic, and scotopic), opsins are used for phototopic (bright-light), polychromatic (full-color) vision.
At the root of the tree from which arthropods and vertebrates diverged in terms of visual opsins, there are animals still alive today that have opsins not used for vision, such as the lancet and sea squirt. These animals use opsin-1. This fact not only links all of these animals genetically in reference to visual proteins, but also demonstrates that the genetic divergence event most likely involved a transition from non-visually capable opsins to visually-capable opsins.
What all of this genetic interconnectivity implies is that visual abilities reach far back into the Lower Cambrian to a distinct protein family that allowed for advanced phototopic vision. All of this research is an enormous improvement over the extremely sparse fossil record of vision in the distant pre-Cambrian past.