Evolution most often result in the modification of an existing structure, rather than the appearance of something totally new. For example, when birds evolved wings, they modified their forelimbs instead of evolving wings out of nothing. It is the same at the molecular level. An existing gene may be modified for new functions, instead of a new gene evolving out of nothing. Gene duplication, meaning evolving an extra copy of an existing gene, is important in evolution, because there is a working copy of a gene while an extra copy can mutate and acquite new functions.
Duplication of body parts is not as easy. All amphibians, reptiles birds and mammals still have a maximum of only 4 limbs, the same number of the earliest tetrapods. In contrast, loss of limbs has happened many times. The giraffe also has the same number of neck vertebrae as most other mammals. So it evolved a long neck not by adding new bones, but by enlarging the bones. Nevertheless, duplication of body segments have happened before. Millipeds and centipeds have many body segments, and their ancestors almost certainly had fewer body segments. Snakes have a lot more ribs than most other vertebrates, certainly a lot more than their lizard ancestor. Therefore it is indeed possible for animals to duplicate existing structures.
Humans cannot breathe underwater because our skin is waterproof, since it is covered by a layer of dead cells. Amphibians can breathe through their skin because they do not have a layer of dead cells on their skins. Parts of our throat used to be gill arches. According to Dollo's Principle, it is not possible for an animal to re-evolve a lost structure. That means after examing a lot of different animals, he has not found any example in which a species has re-evolved something that its ancestor had but that has been lost through evolution. That means snakes cannot evolve the legs they lost, and therefore humans, snakes, birds or any reptile will not be able to re-evolve the gills of their fish ancestor. To evolve gills, reptiles mammals and birds would need to modify another body part and change it into gills. Whether that can happen or not is up to chance and luck.
· 1 month ago