This is absolutely fascinating. It seems there's a very common (among living creatures) visible light dependent DNA repair mechanism, the discoverer got the Nobel for it. This fish, along with modern mammals, has lost it, suggesting that mammals at some point evolved in the dark.
But I don't see in my searches where it mentions that mammals don't have this mechanism anymore? You'd think the Wikipedia would mention it.
"The photolyase mechanism is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism." https://en.wikipedia.org/wiki/Photolyase
Not just creatures: fungi, plants, and bacteria all have it. Also
Cave fish that haven't been isolated as long (more then 3 million years) still have it. So it's really really weird that placental mammals (which includes all non egg laying or marsupial mammals, including us) don't have it. It lends additional support to the Nocturnal Bottleneck theory https://en.wikipedia.org/wiki/Nocturnal_bottleneck
Mammals don't live in the dark. Some of them do but there are non-nocturnal mammals too. The idea in this article is that this is an evidence that mammals might have evolved in the dark (such as in caves).
The idea in this article is that mammals and a blind cyprinid have evolved in the dark and this explains the lost of this mechanism (evidence that mammals might have evolved in the dark). It was not necessary to keep a defense against UV that plants, fungi and other animals still conserve. The part that troubles me is the "but the other creatures still have it".
UV rays do not enter well in the deep water layers so marine fishes living under 60m deep shouldn't have any use for this mechanism. Why mammals had lost it and those fishes still keep it?
I would suggest to read the former statement as "but most other creatures are still untested for the mechanism".
Or maybe we are missing other possible explanations here. I'll add a different hypothesis: "because (or this is evidence that?) placentate mammals have evolved in the dark of a womb and other animals have external embryos (like kangaroos) or have delicate eggs".
My hypothesis would be that the real purpose of this mechanism is protecting the embryo development when even a small punctual UV damage could trigger a cascade of much more serious consequences. This would explain why marsupials and other fishes still need it but diurnal placentates not.
Crocodyles, Platypuses or marine turtles that dig nests in the soil still keep this mechanism?.