If a mermaid was a fully mammalian creature that split on the evolutionary chain about the same time as humans did, would it be likely that mermaids could sleep? Would they rest one half of their brain at a time like a dolphin? Could they pass out?

Bonus question: Does it make more sense for the legs to evolve into the tail, or for the legs to shrink and for the spine to elongate to support the tail?

  • $\begingroup$ To clarify, you want the mermaids to be hominids that diverged from the ancestors of humans some time after the split between the human and chimpanzee lineages? $\endgroup$
    – zeta
    Apr 25, 2015 at 18:48
  • $\begingroup$ Neither humans nor chimpanzees have tails (only tailbones), so it seems probable that their most recent common ancestor also did not have a tail. Given that, it seems like it would be difficult for a "mermaid" descendent to re-evolve a tail. $\endgroup$
    – zeta
    Apr 25, 2015 at 18:53
  • $\begingroup$ In addition to dolphins, most marine mammals have offset sleep cycles, so that at least one half of their brain is awake at all times. Seals spend some of their time fully awake and swimming with all 4 limbs, and some of their time with only one side of their brain awake (the limbs on the opposite side doing the swimming). $\endgroup$
    – trichoplax
    Apr 25, 2015 at 20:45
  • 1
    $\begingroup$ It sounds like a good follow-up question might be, "how can a mermaid evolve from the same ancestors as humans and apes?" $\endgroup$
    – Mikey
    Apr 25, 2015 at 22:58

3 Answers 3


I'm going to say, yes it does make sense.

Given that the the only differences between mers and people are the tail, gills, and other assorted environmental adaptations, I don't see why not. What I am saying is that as long as they have a roughly similar brain, sleep is no problem. From what I can gather, most of the really unique things sleep does occur in the brain, not the body. The body just goes into autopilot while the mind is .....out. As long as there are adaptations in place to ensure that they can get oxygen from the environment and a reasonable assumption of safety, sleep can occur.


Bonus answer: You say that the mermaids evolve from a fully mammalian creature, but you don't state if it is the same ancestor as humans evolved from. So there are two possible answers:

Same Ancestor

As Sumelic stated they would probably have no tails. In that case the fishtail would probably evolve from the legs. Because of the way evolution works, creatures adapt their bodies by the way they use them. If you don't have a tail, you would use your legs to swim, so through the course of evolution the legs would evolve into the fishtail. It would look mostly like the tail of a seal. Keep in mind that the arms would also evolve over time, so you should find a reason why they didn't turn into flippers. They need their hands for some reason.

Different Ancestor

If they don't have the same ancestor and this ancestor has a tail. Then the tail would probably evolve much in the same way as a dolphin or whale.

Overall I think the first option is the most likely one, since mermaids have arms and a face much like a human has.

  • $\begingroup$ In other words, you end up with Selkies rather than traditional mermaids if you go with option 1. (en.wikipedia.org/wiki/Selkie) $\endgroup$
    – eharper256
    Apr 27, 2015 at 8:39
  • $\begingroup$ @eharper256 When I understand that Wikipedia article correctly, Selkies are creatures which can transform between an aquatic and a bipedal shape. Merfolk are usually depicted as purely aquatic. $\endgroup$
    – Philipp
    Apr 27, 2015 at 13:45
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    $\begingroup$ "Because of the way evolution works: Creatures adapt their bodies by the way we use them. If you don't have a tail, you would use your legs to swim, so through the course of evolution the legs would evolve into the fishtail. It would look mostly like the tail of a seal." -- That is not at all how evolution works. $\endgroup$ Feb 24, 2016 at 5:52

Yes, they must sleep. However, the way they sleep will vary. Mammals in the ocean, like whales, sleep with half of their brain at a time. It will not be the same as land animals. Check here for more info.

  • $\begingroup$ The process of sleeping half a brain at a time is known as "logging". $\endgroup$
    – 4oursword
    Apr 25, 2015 at 15:30
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    $\begingroup$ @4oursword Really? I always thought that it was called "Browsing on Buzzfeed"... $\endgroup$
    – Mermaker
    Apr 27, 2015 at 13:08

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