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Eye evolution in the animal kingdom

For Darwin’s contemporary opponents the eye was so perfect and so complex that it could not possibly have developed as a result of natural selection. However, new evo-devo research findings show that the development of the eye is one of the cleverest and most convincing examples of evolution in animals.

"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree."

Charles Darwin, The Origin of Species, 1st Ed., p. 186.

This quotation is still used by creationists and supporters of “intelligent design” around the world as a criticism of evolutionary theory and Darwinism along the lines of “Even Darwin had to acknowledge that….” In his book “The Origin of Species” Darwin freely admitted that he did not know how the highly complex eye of vertebrates could have been formed by natural selection; he nevertheless believed that his theory was right and would one day be used to explain how the mammalian eye developed to “a moderately high degree of perfection”. In the light of current knowledge, the evolution of the eye is even one of the neatest examples to prove that animals evolved over many millions of years.

The mammalian eye, an anatomic miracle and model of evolution © University Eye Hospital Heidelberg
Back in the 19th century, biologists were already extremely enthusiastic about the strikingly different kinds of eyes in animals, including the compound eyes of insects and the vertebrates’ simple lens eyes. The different eye types can be arranged in morphological order from primitive to highly complex forms, thereby giving researchers an idea as to how the mammalian eye might have developed over time. However, certain components of the eye and cell anatomy suggest that not all eye types have evolved from a single common descent, and that there exist eye types in a broad range of different animals that cannot be grouped into a sensible phylogenetic tree. The famous evolutionary researcher Ernst Mayr concluded that photosensitive, eye-like organs have independently developed in animals on at least 40 occasions. The best-known example is the lens in the camera-type eyes that are found in vertebrates and octopuses. Although the eyes of both groups are of similar complexity and effectiveness, the fact that in vertebrate eyes the nerve fibres pass in front of the retina whilst in octopus eyes they do not shows that they must have developed independently.

A gene that induces eye formation

The famous experiments carried out by Walter Gehring and his team at the University of Basel from 1994 onwards have completely changed our perception of eye evolution. Gehring’s group showed that when a certain gene (called “eyeless”) is activated in parts (legs, wings) of Drosophila fruit flies where it would not normally be active, the flies grew extra eyes on the body parts in question.

Drosophila head under the scanning electron microscope. A normal eye and an additional eye on the antenna induced by Pax6. © Biocentre Basel

Counterparts of the gene are also found in mice  and humans. The gene codes for a transcription factor (DNA-binding protein regulating the transcription of other genes) and is referred to nowadays as Pax6. It also regulates the development of camera-type eyes in mammals. The Pax6 genes of the different species are interchangeable. When a mouse Pax6 gene is transferred into Drosophila, the gene produces Drosophila eye tissue. The Pax6 is found in a broad range of animal taxa and is in all cases extremely important for the development of eyes. Thomas Holstein from the Institute of Zoology at the University of Heidelberg writes: "The Pax6 transcription factor is very probably one of the most important examples of proteins that has been conserved in bilaterians and one of the most spectacular results of the comparative functional analysis of Pax6 genes..... The results suggest that urbilaterians possessed a Pax6 gene and that this particular gene was already involved in eye development." From these findings it can be concluded that the common ancestor of all multicellular animals - with the exception of sponges, worms and jellyfish - which very probably lived as inconspicuous worms in Precambrian oceans, already possessed eyes, albeit very simple ones.

Worms as a model for the oldest types of eye

The elegant conclusions drawn by evo-devo researchers do not end here. Detlev Arendt from the European Molecular Biology Laboratory (EMBL) in Heidelberg found the simplest eyes that exist on earth, consisting of two cells, a photoreceptor and a pigment cell, in larvae of the ragworm Platynereis dumerilii (see BioPro article of 21st Dec. 2009). These eyes were exactly what Darwin had postulated in “The Origin of Species”: “The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment cells and covered by translucent skin, but without any lens or other refractive body.”

Platynereis. Front part with eye spots. © Udo Ringeisen, EMBL Heidelberg

The receptor cell contains opsin proteins that enable the perception of light; opsins also have the same function in other animals. Platynereis larvae develop very simple larval eyes that consist of a single photoreceptor cell and a single pigment cell. As the adult eye matures, it acquires a spherical lens and more photoreceptors.

Arendt and his team discovered a band of cells endowed with cilia in the brain of Platynereis. These cells reminded the researchers of vertebrate photoreceptors and they also contained the same type of opsin (c-opsins) that was found in the photoreceptors of camera-type vertebrate eyes. However, in Platynereis the cilia and c-opsin are not involved in vision, but instead they control the animal's light-dependent biological clock. However, the photoreceptors of the cup-shaped eyes of the adult worm belong to the rhabdomeric type of photoreceptors and contain r-opsins instead of c-opsin. This is also the case in Drosophila compound eyes and the camera-type eyes of squid. The researchers from Heidelberg believe that the rhabdomeric photoreceptors differentiated into the retinal ganglion cells that are involved in the transfer of nerve signals. It is highly likely that our common bilaterian ancestor contained both types of photoreceptors and opsins. Insects and cephalopods have rhabdomeric photoreceptors. The evolution of vertebrate eyes involved both types of receptors, but it was the ciliary photoreceptors that evolved into eyes.

The findings from evo-devo research confirm the assumptions of previous evolutionary researchers and comparative anatomists, i.e. that the different highly differentiated eye types such as the compound eyes of insects, which contain rhabdomeric photoreceptors, the camera-type eyes of squid, which also contain rhabdomeric photoreceptors, and the camera-type eyes of vertebrates which contain ciliary photoreceptors developed independently from each other. However, they did not develop de novo, originating instead from the common simple structures of photoreceptors and pigment cells with the same cellular components and the same genetic tools such as the Pax6 gene and the opsin family genes.

Literature:
Detlev Arendt and Thomas Holstein: Evo-Devo-Forschung. In: Evolutionsbiologie (V.Storch, U. Welsch, E. Wink, Eds.), 2. Ed., Springer-Verlag Berlin/Heidelberg, 2007.
Sean B. Carroll: The Making of the Fittest. DNA and the ultimate forensic Record of Evolution. Norton & Co., New York/London, 2006.
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