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I want to know what evolutionary pressures would lead to merfolk. Here are characteristics required:

  • must be around human height, strength, and intelligence

  • must swim at least 4 mph

  • it having gills is preferred, however lungs are fine too

  • must be capable of complex tool use

  • would be better if they had an improved sense of sight

  • must be able to detect water vibrations

Note: By merfolk I don's specifically mean the half fish variety just aquatic humanoids with the listed characteristics. Also the world is in a medieval level of technology.

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  • $\begingroup$ Why would they obviously have gills? Have you considered and dismissed lungs or do you specifically want them to not need to surface for air? Needing an improved sense of sight implies they wouldn't be deep water dwellers, would you be happy with an answer that resulted in whiskers (like those of seals) for vibration sense? $\endgroup$ – FrankRebin Jan 11 at 1:39
  • $\begingroup$ @FrankRebin I wanted them to not need to surface for air, so that's why I said that. However if someone wants to answer with lungs that would be fine, since that feature was kind of in a concept phase (I shouldn't have put must have). Answers that resulted in whiskers would be perfectly fine for vibration sense. $\endgroup$ – Hello There Jan 11 at 2:29
  • $\begingroup$ I'd suggest being open to some of the ancestors of dolphins, or even just slightly varied dolphins. They have the bones for hands, and you could fairly easily give a dolphin short fin-like arms to allow manipulation. They are capable of impressive feats with just their mouth. Add hands, and they are slightly slower dolphins but capable of greatly increased tool use. No gills, though. For gills, you can't beat the axolotl as a starting point. $\endgroup$ – DWKraus Jan 11 at 4:34
  • $\begingroup$ Gills, and "Large Mammal" (implied by them being part human) is simply not compatible. Gills work fine for low-energy creatures, but they just cannot work as well as lungs to provide oxygen while still maintaining an elevated body heat. $\endgroup$ – PcMan Jan 11 at 6:24
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As for what evolutionary pressures would result in something like merfolk- A planet with very little habitable land would not be able to support the evolution of terrestrial humanoids. Species evolve where there's sufficient resources for them to exist, of course! If you don't want to go that route, maybe intense competition with other sapient species drove one particular lineage back into the water?

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Axolotl style!

Axolotl style.

https://news.yale.edu/2020/01/28/tiny-salamanders-huge-genome-may-harbor-secrets-regeneration

Behold our distant cousin, the wily axolotl. It has external gills and very spiffy ones too.

Except for gills, I personally could satisfy all of your merperson requirments.

  • I am around human height, strength, and intelligence

  • I could swim at least 4 mph, if I ever worked out.

  • I am capable of complex tool use

  • I would be better if I had an improved sense of sight but eventually I will get glasses.

  • I can detect water vibrations. Big ones, but maybe with practice little ones.

Yes the gills are what is left. And the way I will accomplish that is to evert my lungs! They will look much like what the axolotl has except I have only 2 lungs. Initially I thought they might come out my mouth or possibly nose but that would not be a very good look. Instead they can come out of apertures around my neck - like the wily axolotl! I tuck them back in when it is time to stop swimming.

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