Why hasn't evolution corrected eye defects?

Evolution: commonality between humans and blind cave fish mutants

When fish develop over millions of years in the eternal darkness of caves, then they can do without eyes. A blind African specimen has thrown off additional know-how ballast that is no longer needed, researchers led by Nicholas Foulkes from the Karlsruhe Institute of Technology now report after genome studies in the magazine Current Biology: The blind animals can no longer produce certain enzymes that can be produced by UV radiation Fix any DNA damage quickly and efficiently. It is noticeable that hardly any other organism on earth can afford something similar, from protozoa to plants and fungi to marsupials - with one strange exception: humans and their closer mammalian relatives. Did their line of ancestors go through a permanent dark phase in which they were able to consistently avoid light and UV radiation?

Foulkes and his colleagues had the genetic makeup of Phreatichthys andruzzii investigated, a blind fish from Somalia whose strains have lived in lightless cave waters for millions of years. It became clear that various mutations have disabled some DNA repair mechanisms that are otherwise important in fish and other vertebrates. This affects repair enzymes that repair DNA damage caused by UV radiation or reactive oxygen radicals. The cave fish, for example, no longer build photolyases, which specialize in separating dimerized thymine residues on a DNA strand using UV radiation.

Photolyases or other photoactivated DNA repair enzymes are found in plants, fungi, protozoa and many animal phyla and are also very useful in vertebrates: UV damage is one of the most common threats to properly functioning cells. They may even be so crucial that only animals like the cave fish P. andruzzii With his typical way of life in eternal darkness, he can afford to lose the enzymes without serious disadvantages compared to the competition in the evolutionary race.

This invites the scientists to speculate about why photolyases are only missing in all real mammals, and thus also in humans. Have the ancestors of all placenta animals gone through a long phase of evolution in which they developed almost exclusively in the dark? In fact, this scenario has been seriously discussed for some time: According to this, the mammalian ancestors occupied the ecological niche of night activity, especially at the time of the dominant dinosaurs. To this day, some features - for example in the anatomy of the sense of sight - indicate that the mammals were once nocturnal animals. Perhaps this went so far over a few millions of years that our ancestors were temporarily unable to get any sunlight at all - in an evolution bottle neck of permanent night activity and thus a phase in which the UV protection through photolyases could then fail, as in the case of cave fish, without any drastic consequences . Placental mammals can also repair UV-induced DNA damage - using other mechanisms - but not as quickly and efficiently.