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While reading this question about how non-primate mammals could develop a bipedal stance, I became curious about the broader reasons why animals could evolve to walk on two legs.

I found some insights at this question, which inquires about felines in particular. The accepted answer states,

Moving to trees and/or a rocky area would help. Hands and an upright posture are good for reaching and carrying things. They are also good at manipulating things and tool use. An upright posture is also good for spotting predators.

This is a great explanation of why bipedalism might arise on land, but I don't think it holds up underwater.

  • There are no fruit-bearing trees underwater, and I can't think of similar plants that creatures would need to be "upright" to reach or climb.
  • Even if an alien world has marine trees (big kelp maybe?) I imagine swimming vertically is a more efficient way to reach food. For instance, even though crabs have legs, they can swim in the open ocean. If you can swim vertically, why would evolving to reach vertically be worthwhile?
  • While being vertical might help spot predators in a grassy plain, much of the ocean is relatively flat and featureless. I imagine most prey can see predators coming regardless of height.

Given the drastically different environmental factors posed by life underwater, what environmental factors, if any, would cause bipedalism underwater? What would a humanoid-fish-or-crustacean-producing biome look like?

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    $\begingroup$ iam not biologist so iam not sure can this work or not, but what if they are the type that like to jump upward to catch their prey, using the surface to kick off like rocket, maybe that can help to make them turn bipedal and straight upward like this crocodile jump or shark jump which also reason they straight up is to see the upper surface $\endgroup$ – Li Jun Feb 13 at 9:25
  • $\begingroup$ Are they purely bipedal, or do they also have a set of fins & tail for swimming? $\endgroup$ – Carl Witthoft Feb 13 at 16:21
  • $\begingroup$ @CarlWitthoft I'm alright with either. $\endgroup$ – Zxyrra Feb 13 at 16:33
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    $\begingroup$ FWIW, I could see mermiforms easily enough. Developing arms is perfectly sensible. Why you'd develop two legs underwater is harder to imagine. (I take it "because they're amphibious" doesn't work?) $\endgroup$ – Matthew Feb 13 at 21:21
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Given that a humanoid form on Earth comes from tree dwelling creatures that start roaming on a Savannah, arguably one biome or environment that may encourage a humanoid form would be Kelp Forests.

The problem in a marine environment is that there are very few places to hide, and hydrodynamic form is generally more beneficial than an articulated arm. But, what if you had a creature that specialises in hiding in kelp forests, grabbing on to the kelp and using it as leverage to move out of the way of a fast oncoming predator? They may also end up becoming ambush predators in time, using some kelp as a launching point.

The point being, arms with hands that are designed to grip items like kelp will later be used to grab other things that give them an advantage in the environment. They may one day be used to throw stones or sharp shells or the like at other creatures for defence or hunting. In any case, water is one of those environments where hands are generally not useful, let alone arms, so you need to change things up a little to force out a humanoid form.

But when you get right down to it, that still entails having to grab something in the first instance, whether that is a tree branch or a line of wavy kelp. The fact that it could act as a hiding spot from the creatures in the open sea is probably all the more reason why it may happen, but in order for it to do so you'd probably need much larger kelp forests that are around in the one place for a lot longer than they currently have been.

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I'm sorry to say, but nothing would, unfortunately. There are very good reasons there aren't any fish with legs. Any creature that isn't (partially) land dwelling would not have need for legs over something like fins or tails. Any directional movement underwater is significantly easier with fins and tails, and bipedalism would simply have no evolutionary advantage whatsoever.

Arms are a different thing, they could be useful when higher intelligence is formed, for the manipulation of tools and the like. Using tools with fins is near impossible, and just using mouths, jaws, or beaks would not be ideal.

This means in your world the possibility of merfolk or merfolk-like creatures would be much higher than an ocean dwelling bipedal humanoid. Although from an evolutionary standpoint, it would probably more likely to evolve a sentient squid-like or tentacle based creature over merfolk.

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    $\begingroup$ there are actually several fish with legs. leg evolved before animals came out on land, the big benefit of limbs in water is they are much stealthier and work better in shallow or fast moving water. of course both of these are incomparable with bipedalism but that is not the same thing as saying legs have no use. $\endgroup$ – John Feb 17 at 1:32
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Many underwater creatures have legs. They're the kinds of creatures that are bottom-dwellers, like crabs, lobsters, etc. So the only problem you have to solve is: what would cause a bottom-dweller to have only two legs instead of 6/8/10/12?

The advantage of bipedalism is the ability to move faster over long distances. Not faster over a short interval--there are many creatures that are faster sprinters than humans--but very few creature can compete with humans at running a marathon. Compared to 4 legs, it's very efficient at converting energy to movement on land.

The problem with being underwater is that the weight of water on earth is such that it makes moving with feet much slower than moving by swimming.

Therefore: if the liquid being moved in had significantly less weight than water, I think that's the one situation where the benefits of bipedalism could make a bipedal creature underwater feasible.

How do you get that to happen?

  1. Low-gravity planet. This is the simplest solution.
  2. A different liquid than water is the one being moved in, one with much less density. This is the complex/far-fetched solution so I'll spend more time explaining, even though I think the first solution more viable.

For something lighter than water to be the liquid, this would probably require the planet to have significantly higher pressure and/or temperature. For reference, Jupiter is ~50% liquid hydrogen because of its insane pressure and weight. However, Jupiter's environment doesn't work for our purposes in many ways; also hydrogen is very reactive so wouldn't work as the neutral medium for life to exist in.

So instead we pick helium, which is basically inert. It has a molecular weight of 4 g/mol instead of 18 g/mol for water. 1 I'm not completely sure if helium would work as the medium for life to exist in, you'd have to ask a biologist that, but if it did then maybe the following would work:

According to this chart at a pressure of 4 mPa (39 atm) and temp of 100K (-280F) you would have a substance that is mostly liquid helium, somewhat gaseous helium. Can life exist in those conditions? Maybe not, but I wanted to at least give you the groundwork for this idea if you want to research it further.

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