How did the eyes develop

The eye is a fascinating organ, it allows us to take a picture of our outside world and at the same time allows a look inside, into our inner soul. Evolution has produced at least three fundamentally different types of eyes. First, there is the camera eye with a lens that projects the incident light onto a retina; it has achieved its best education in vertebrates and octopuses. Second, the complex eye of insects and other arthropods, which is composed of numerous individual eyes, each consisting of a lens and a group of photoreceptor cells. And thirdly, the mirror eye, which in the scallop shell, for example, has both a lens and a reflecting parabolic mirror that project the light onto a retina. Most animals' eyes are located on the head and transmit their signals to the brain, which processes the information and forwards it to the effector organs, for example the muscles. In the mussels, however, which do not have a head, the eyes are attached in a row on the edge of the mantle, which allows them to see when the shells are slightly open. With certain annelids that live upside down in a tube, the eyes are sensibly attached to the rear end.

Extremely perfect organ
Darwin's theory of evolution has shaken the worldview of the 19th century, and yet doubts about it have persisted in circles of fundamentalists. In particular, the evolution of the eyes was difficult for Darwin and his contemporaries to understand, as it seems absurd that such a perfect organ as, for example, the eye of an eagle could have been created purely by chance variation and selection. In his book "On the Origin of Species" Darwin therefore wrote a special chapter on the difficulties of his theory of evolution and treated organs of extreme perfection - such as the eye - separately. In it he suggested that the highly developed eyes emerged from a very simple prototype, which consisted only of a visual cell (photoreceptor cell) and a dye cell (pigment cell), but which already enabled the wearer to see direction. The cup-shaped pigment cell shields the light from one side so that the animal can determine the direction of incidence of the light. However, Darwin immediately mentioned that the prototype could not be explained by selection, because this could only start when the organ in question is already working to some extent. If this is the case, selection can contribute to a gradual improvement of the organ: Additional photoreceptor and pigment cells can be recruited or, for example, lens-forming cells, until the eyes are largely perfected. Since the prototype cannot be explained by selection, its creation must have been a very rare and coincidental event. This fact was largely overlooked by the neo-Darwinists, who based Darwin’s theory with genetic principles. Neo-Darwinists like Ernst Mayr therefore assumed that the eyes in the 40 to 60 different animal phyla developed independently of one another. They represented the classic dogma that the lenticular eye of vertebrates and the complex eye of insects had no common ancestors - an opinion that can be found in almost all textbooks.

Controlling the control gene
However, our more recent molecular genetic studies have shown that the different types of eyes found in the animal kingdom are all controlled by the same master control gene - a kind of main switch - called Pax6 and therefore have a common origin. We managed to induce eyes on antennae, legs and wings by targeted activation of the Pax6 gene of the fruit fly Drosophila and the corresponding (homologous) gene of the mouse (see picture). In addition, the prototype postulated by Darwin has actually been found in certain flatworms and certain larvae of annelid worms. Darwin’s hypothesis was correct, and in the light of more recent research, the theory of evolution is no longer a mere theory, but a scientific fact that can no longer be denied.