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Appearance

The ''vipers'' are semi aquatic humanoids species with webbed fingers on both hands and feet, they also have 2 fins on each leg and their legs are covered in scales from the feet up to their knees.

Also, they are moderately fit or at least like the average gym rats naturally and can grow to be massive when trained properly as adults.

Breathing

Having both lungs and a pair of gills located on their chest right underneath the neck grants them the ability to breath underwater and on land.

Question

The vipers live in the same world with mermaids, and both species are descendants of humans. Considering both vipers and mermaids evolved alongside one another, why did the mermaids evolve a fish tail while the vipers evolved fins on the legs, or should the real question be what reason did vipers have to live on both land and water and not be fully aquatic like mermaids?

Images

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Evolutionary biology is largely about the pressures of an organism's environment, so the two forms of your question are one and the same. As such, there are many reasons why they may be formed that way:

Location, location, location

Merfolk have tails, and can only swim. Your vipers have legs that are better-adapted (but worse than tails) for swimming, and better-adapted than tails for walking. This implies that their natural habitat consists of both land and water - coastal shallows, inland lakes, etc. Without the power of a tail for speed, swimming deeper/further out in the ocean will put them at a disadvantage compared to a reef.

The need to be out of (or in) the water

There are many reasons why a water-dwelling creature might need to be out of the water. Perhaps their normal habitat has dry and wet seasons, like the lungfish, which requires them to breathe air to survive. A moderate metabolism and the inability to reduce it or hibernate would produce organisms that retain mobility and can seek resources during the dry months.

Another reason to escape from water to land (or land to water) is natural predators occurring in either (or both). A viper may be able to outrun its water-dwelling predators or outswim its land-bound ones.

Dietary and other health restrictions inherited from human ancestors may make it untenable to live entirely underwater. Vitamin D from the sun, along with other vitamins and minerals found in land plants and animals, may not be available in a deep-water environment.

Freshwater vs saltwater dwellers

I've already addressed location, but this is kind of tangential. The ocean is, of course, salt water. It takes a specific biochemistry to handle the salinity of the ocean, which your vipers may not possess (but the merfolk do). Fresh water is pretty much only found in close proximity to land, in lakes and rivers and basins. Even if they can survive in the brackish waters where fresh meets salt, the ability to survive on land is beneficial in areas where the climate includes dry or freezing periods. As such, they may have evolved to survive in the more transient nature of coastal and tidal regions, as opposed to the relatively constant deep-ocean saltwater environments.

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The fundamental problem in your schema are scales, which are difficult to justify even as a regression.

Justification of coexistence of vipers and mermaids could "simply" be vipers are "incomplete mermaids"; i mean: men in aquatic environment (many rather close small islands in rather deep sea) evolved in vipers and when population growth forced them to remain waterborne almost continuously first developed a membrane between the legs an then fused them together getting a dolphin-like tail (but powered by legs and not tail muscles).

In any case fins cetacean-style are much easier to justify than fish-like (spiky) fins.

Also real gills are difficult to justify; apnea-enhancements (again, cetacean-style) would be more likely.

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  • $\begingroup$ there are mammals with scales, pangolins $\endgroup$ – John Aug 4 '17 at 14:26
  • $\begingroup$ @John: You are right. I forgot about them. They are a very different kind of scales, but difference is not relevant here. $\endgroup$ – ZioByte Aug 4 '17 at 14:47
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Justification is easy, they evolved on different continents, or in different environments like Chris's fresh vs salt. perhaps the "vipers" like more semi-aquatic than aquatic, like something on this list. Swamp monkeys or marine otters might be good inspiration.

The anatomy is the big issue, gills have two problems. First getting gills on a primate is not going to happen, the best you will get is something like whales that can hold their breath for a long time. there is no gas permeable tissue to work with on the outside of the body of mammals. You need magical intervention or genetic engineering for gills. The second problem is they are way too small, to breath through them they would need to be as large as the chest cavity, air contains more oxygen than seawater. Our lungs are huge, the gills need to be even bigger than that.

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