Which animals have the most beautiful patterns
For Darwin's opponents, the perfection of the eye was seen as an example of irreducible complexity that could not have come about through natural selection. On the contrary, new findings from Evo-Devo research have turned eye development into one of the most beautiful and convincing examples of evolution in the animal kingdom.
“The assumption that the eye, with all its inimitable devices: adapting the lens to different distances, allowing changing amounts of light and improving spherical and chromatic deviations, was created through natural selection appears, as I openly admit, to be extremely absurd . "
Charles Darwin, The Origin of Species (German 1860)
This quote was and is used by the creationists of all countries up to the supporters of the "intelligent design" of our day to campaign against the theory of evolution and Darwinism, according to the pattern: "Even Darwin had to admit that ..." . What Darwin, however, admitted with his incorruptible honesty was that he did not know how the highly complicated camera eye of the vertebrates could have come about through natural selection; he had no doubt that his theory was nevertheless correct and that one day it would also explain this miracle of creation. In the light of what we know today, the origin of the eye is even one of the most beautiful confirmations of the evolution of animals.
An eye-catching gene
The famous experiments by Walter Gehring and his colleagues at the University of Basel since 1994 have completely changed our conception of eye evolution. They showed that a gene that creates complex eyes on the head in Drosophila also induces the formation of eye tissue in unusual places such as the legs or wings when it is activated there.
The gene is also found in mice and humans; It codes for a transcription factor (a DNA-binding protein that switches the transcription of other genes on or off), which is now known as Pax6 and which also controls the development of the eyes in mammals - in this case camera eyes. The Pax6 genes of the different species are interchangeable. When the gene isolated from mice is transferred into the body of the fly, it ensures the formation of fly eye tissue there. The Pax6 gene has been found in species from a wide variety of animal phyla, and it is important for eye development everywhere. Thomas Holstein from the Institute for Zoology at the University of Heidelberg writes: “The transcription factor Pax6 is probably the most famous example of a protein conserved within the Bilaterians and for spectacular conclusions obtained from the comparative functional analysis ... The results allow the conclusion that the Urbilater already possessed a Pax6 gene, and they make it very likely that the function of the primordial Pax6 transcription factor also had something to do with eye development there and rib jellyfish - who lived as a presumably inconspicuous worm in the Precambrian seas, already eye-obsessed, if probably very simple.
The worm as a model of the oldest eyes
The elegant conclusions of Evo-Devo research do not end there. Detlev Arendt from the European Molecular Biology Laboratory (EMBL) in Heidelberg found the simplest eyes in the world, consisting of two cells, a light receptor and a pigment cell, in larvae of the sea ringworm Platynereis dumerilii (see BIOPRO article from December 21, 2009). They correspond exactly to what Darwin had already postulated in his epochal work: "The simplest organ that can be called an eye consists of an optic nerve surrounded by pigment cells and covered by translucent skin, but without a lens or other refractive body."
Opsin proteins, which also perform this function in the eyes of all other animals, serve to perceive light in the receptor cell. As the adult worm develops, larger cup-shaped eyes arise from an accumulation of large numbers of light receptor and pigment cells near the larval eyes. The Pax6 gene is also involved in this.
In the worm's brain, Arendt and his team found a band of cilia-bearing cells that are reminiscent of the light receptors of vertebrates, and in these cells the same type of opsin (so-called c-opsins) that occurs in the visual receptors of the vertebrate camera eye. In Platynereis, however, the ciliated cells and the c-opsin are not involved in the visual process, but control the light-dependent biological clock. The visual receptors of the worm's cup eyes, on the other hand, are - like those of the complex eyes of Drosophila and the camera eyes of cuttlefish - of the so-called rhabdomer type, which contain the opsins of another type, the r-opsins. According to the Heidelberg researchers, the rhabdomer cells in vertebrates have differentiated into ganglion cells of the retina, which are involved in the transmission of nerve impulses. It seems that the Urbilater that Holstein speaks of had both types of light receptors and opsins. In the case of insects and cephalopods, the rhabdomer type was used as the visual cell. Both types of receptors were involved in the evolution of the vertebrate eye, but the ciliary type became the visual cells.
The findings of the Evo-Devo research confirm the assumptions of earlier evolutionists and comparative anatomists that the various highly differentiated eye types such as the complex eyes of insects of the rhabdomer type, the camera eyes of the squid of the rhabdomer type and the camera eyes of the vertebrates of the ciliary type have developed independently of one another. But they did not emerge de novo, but rather stem from common simple structures of light receptors and pigment cells, and for their evolution they were able to fall back on the same cell building blocks and the same genetic tools as the Pax6 gene and the genes of the opsin family.
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