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My fish people live in underwater nations. They look like a mix between the Creature from the Black Lagoon (left)

enter image description here

and the sea creatures from Luca:

enter image description here

with the exception of some minorities that resemble fish more than people.

In general, my fish people are humanoid, have fins to breath like fish, have sharp, human-like teeth, have green or blue scales, and also have human lungs to breath above the water.

However, there is one problem I face. In my world, there is evolution for most of the fictional fauna. I am trying to figure out how could the fish people evolve to be humanoid, instead of being like whales and dolphins. For the particular details:

  • The fish people have opposable thumbs

  • They communicate and can understand audible language

  • They are able to build and have underwater kingdoms

  • While they wear some form of clothing, their soldiers definitely wear lots of armor

  • They can walk on land, and are upright and bipedal

  • They are able to both breath underwater and breath above-water

  • Some are born with tails for swimming, others are not, and are more land-oriented

My question: What physical and environmental conditions could cause my fish people to evolve into humanoids, instead of evolving to be like whales or dolhpins?

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    $\begingroup$ Aren’t fish people already humanoid? $\endgroup$
    – sphennings
    Jan 4, 2023 at 2:13
  • $\begingroup$ @sphennings my mistake on the wording. What I mean is what provided conditions would cause them to evolve into fish people. $\endgroup$
    – Crafter
    Jan 4, 2023 at 3:27
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    $\begingroup$ Probably a great many thing. How does this question have a single valid answer as required on this site. $\endgroup$
    – sphennings
    Jan 4, 2023 at 5:49
  • $\begingroup$ And please add information of importance to your question. Not everyone is familiar with Black Lagoon or Luca. $\endgroup$
    – Joachim
    Jan 4, 2023 at 10:40
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    $\begingroup$ Arms and even digits were probably evolved underneath the water, and facilitated migration onto land. This is somewhat true even in our world. It might look something humanoid-ish, just because their evolutionary ancestors were vertebrates, and their fins were rigid and boned. However, the upright posture and lack of tail are very difficult to reconcile... I don't see any plausible way to accomplish that. $\endgroup$
    – John O
    Jan 4, 2023 at 17:06

2 Answers 2

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They are descended from amphibians.

Earth has no bipedal amphibians, but I feel like we can imagine a world that is generally far flatter than Earth (perhaps there is a lot less tectonic activity), leading to a whole lot of swampy land. Like imagine if North America was almost entirely around sea level and the whole thing was one big swamp or marsh. The thinking being that this creates a lot more pressure for things like lungfish or frogs to become common and widespread and is the evolutionary path that eventually leads to "fish people".

Getting into and out of the water is an important evolutionary trait. Spine, lungs, gills, webbed appendages. But swamps have a lot of water loving trees, so the ability to climb trees, perhaps to harvest fruit (and avoid predators) was also important, so you end up with an evolution towards something like an amphibious monkey.

Perhaps there even are amphibious monkeys around. That was the more land-loving branch off the evolutionary chain, found further inland. The fish people took the other direction and became more water loving, developed more around the coastal areas, but still retain their evolutionary abilities to get out of the water, climb trees and do things with their opposable thumbs, but perhaps the coastal areas are also unusually rich in sources of protein (the shallower oceans yielding a lot more food, promoting a lot more oceanic life in general), so the coastal amphibians had fewer food concerns and got more free time in which to develop intelligence.

None of this happened on Earth, mainly because we have giant forests, giant plains, etc, while your world has a whole lot of giant swamps. We got monkeys which branched into humans, you got "water monkeys" which branched into "fish people".

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They fill the same niche as otters or beavers

Whales and Dolphins are not the only terrestrial mammals to return to a watery life-style. Other than being mammals instead of true amphibians, otters check all the boxes for a good precursor species to your intelligent lifeforms. The are tetrapodal, social, tool users, and have with hand-like appendages and a broad swimming tail. The wide spread existence of otters across 5 cotenants suggests that the niche is there for the taking. Beavers are also a good possibility since they are known for thier complex building skills, though generally not quite as social as otters.

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However, if you had a world without otters, it is possible that some kind of frog or salamander would eventually evolve to fill the niche instead. While no species of amphibian on Earth is particularly known for its brain power, there is no reason that intelligence could not evolve in one.

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