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What do blind cavefish, dinosaurs, and sunburnt humans have in common? A lot more than you can realize, according to a thought-provoking new study.
New research published today in Somalian blind cavefishPhreatichthys andruzziiare missing an ancient DNA repair kit that protects many animals from the harmful effects of sunlight. This observation makes total sense, given that this cave-dwelling species has lived, and subsequently evolved, for millions of years in complete darkness. The lost system, called photoreactivation DNA repair function, uses energy from visible light to repair DNA caused by ultraviolet (UV) light, and it is found in many organisms, including bacteria, fungi, plants, and many animals. This repair kit is one of the reasons why plants can be found in the sun, when people get cancer when we sunbathe.
Indeed, placental animals, a group that includes humans, do not have this special DNA system. An intriguing aspect of this new research is that it's potential evidence for the "nocturnal bottleneck" theory-the idea that the ancient ancestors of modern mammals lived underground or were exclusively nocturnal in order to avoid being eaten by dinosaurs. Like the blind cavefish, these early mammals evolved in total darkness for a prolonged period, resulting in the gradual loss of the light-activated DNA repair function. It's from this group of "bottlenecked" mammals that all placental mammals emerged, which may explain why modern placental mammals still lack this genetic trait. And why are humans being so dependent upon sunscreen.
Or at least that's the prevailing theory. As some experts told Gizmodo, this new study is really interesting, but it will be necessary to strengthen the apparent connection between the lifestyles of our ancient ancestors and our missing DNA repair kit.
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Use it or lose it. It is a simple phrase, but it eloquently explains an evolutionary process known as "relaxed selection." It happens when the survival, or fitness, of a species is no longer dependent on a certain physical characteristic or behavioral trait. An unnecessary or superfluous trait will be dealt with in the genetic code responsible for that trait degrades over time, or if mutations turn it into something else. This does not happen overnight, and it often leads to vestigial traits. In humans, we see this in our wisdom teeth, tonsils, appendix, and coccyx, otherwise known as the tailbone. Eventually, these traits will continue to disappear as soon as possible.
For the Somalian blind cavefish, relaxed selection resulted in the gradual loss of its photoreactivation DNA repair function. This feature is likely to occur during the earliest stages of life on Earth as a protection against the harmful effects of sunlight. Nick Foulkes, the lead author of the new study and researcher at the Karlsruhe Institute of Technology in Germany, said
Foulkes studies the effects of evolution on animals living in extreme environments and how it changes their DNA. This is the Somalian blind cavefish-a species that has not been exposed to visible light for more than 3 million years. Genetic analysis showed that the DNA of Somalian blind underwent mutations underwent mutations that wrecked its light-activated DNA repair function. What's more, the fish also lacks a D-box enhancer-a regulatory enhancer that coordinates and boosts DNA repair in response to sunlight by activating other genes. Interestingly, other cavefish species appear to have their DNA repair kits.
"The Somalian cavefish has been living in complete darkness for years, but it has been studied for years," said Foulkes told Gizmodo. "So these other species may be isolated in the dark long enough to lose photoreactivation. But it is a matter of course, but it is there that other wines have been stored in this cave.
As noted, the only other group of animals known to have lost this function are placental mammals.
"This raises the fascinating possibility that our ancestors may have shared a similar lifestyle with cave animals."
"These plants are found in all living things, from plants, fungi, and bacteria to fish, amphibians, reptiles, birds, and even mammals like marsupials-kangaroos, koalas, and opossums being good examples," said Foulkes. "Only placental mammals like this lack this. Precisely why has a long time, been a mystery. Now, we have found this ancient genetic repair kit, comes as a big surprise. These fish will not have seen the sun or suffer the effects of UV radiation for millions of years. So photoreactivation, which depends on light, may serve no useful purpose. This invention is also useful in this field.
Indeed, this could be an example of convergent evolution, in which same or similar features (or in this case, the disappearance of traits) appear in non-related species. The commonality is prolonged exposure to darkness. But where are the dark cellfish trapped in dark caves for millions of years, early mammals lived in the dark, or due to nocturnal living or subterranean lifestyles, to avoid being predated upon by dinosaurs. Mammals first evolved around 200 million years ago, spending around 130 million years among non-avian dinosaurs.
"The finding is fascinating as it is the first report of an organism that has lost its UV-mediated DNA repair, and the fact that they seemingly lost it because of their dark environment fits nicely with the hypotheses that humans have lost the "Nicolas Rohner, a research scientist at the Stowers Institute and an expert on cavefish who was not involved in this study, told Gizmodo. "The findings are surprising because related study in another cavefish species has found the opposite. In this study the authors found cavefish to have [functional] DNA repair. This means that different cavefish species use different strategies to survive under seemingly similar conditions. "
Aniket Gore, a scientist who works at the Laboratory of Molecular Genetics at the National Institute of Child Health and Human Development, said the new study is a "very well done, careful analysis."
"Gore, who does not participate in the new research, told Gizmodo. "The study highlights that unique model organisms, such as the cavefish, reveal answers to fundamental issues related to human health and development."
As for the claim that dinosaurs are the reason humans need sunscreen, Gore said that's a bit of a stretch. Other methods of repair include DNA repair, NER (nucleotide excision repair) and BER (base excision repair).
"One possibility is that NER and BER can fix changes in the DNA sequence independent of light and then they can work even in the dark or overcast conditions making them highly effective repair systems to fix mutations."
In other words, perhaps mammals have not been solely dependent on the photoreactivation repair kit, given the effectiveness of other DNA repair mechanisms.
Gizmodo also reached out David M. Grossnickle, an evolutionary biologist from the University of Chicago and an expert on early mammal evolution, to get his take on the new study.
"The apparent convergent evolution in placentals and Somalian blind cavefish is really interesting," Grossnickle told Gizmodo. "The study provides indirect support for the hypothesis that placental mammals lost the photoreactivation DNA repair function because they were nocturnal and not exposed to sunlight."
Grossnickle said this is a welcome addition to our growing knowledge of early mammal ecologies and behaviors, but it's important to acknowledge that two other groups of living mammals-pouched marsupials, such as kangaroos, and egg-laying monotremes, like the duck-billed platypus -Have retained the photoreactivation DNA repair function. In addition to these three living groups (placentals, marsupials, and monotremes), Grossnickle has been reported to be very early mammal groups that were very different in the Mesozoic era, but are now extinct.
"Placentals did not show up on the scene until the end of the Mesozoic," explained Grossnickle. "So the study does have implications for placental mammal evolution, but it does not indicate that all mammals experienced a nocturnal bottleneck in the Mesozoic. I urge bail in applying what we learn about placental origins to our story of all mammals. "
The connection between blind and skinburn may be a leap, but it is not completely outrageous. The idea that we are plausible, given that mammals first emerged back then. What is needed now is more evidence, both paleontological and genetic, to strengthen this extraordinary claim.
Until then, please pass the sunscreen.
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