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Assuming the same rules for physics apply to this world as they do for our own, how would a humanoid race that lives at the bottom of the sea be able to walk around like humans do on land? What kind of evolutionary trait would provide this phenomenon?

It should also be noted that they cannot swim unless on the surface of a body of water, and a select few can manage travel by land, although not for an extended period of time.

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    $\begingroup$ The humanoid would need legs to walk on terrain and a a more flexible spine and stronger back muscles to swim faster $\endgroup$ – Charon Aug 28 '16 at 19:06
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    $\begingroup$ Humans don'y move flawlessly on land ^.^ that's why we fall :P $\endgroup$ – Durakken Aug 28 '16 at 19:06
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    $\begingroup$ kids and old people fall* Unless you are in a low quality horror movie you aren't going to see many people falling down. $\endgroup$ – Charon Aug 28 '16 at 19:07
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    $\begingroup$ Evolution will make sure they don't walk on the seabed, but dive. Unless they have some mean deiety like you, that forbids it. $\endgroup$ – Karl Aug 28 '16 at 19:17
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    $\begingroup$ @Karl There are some animals that live on the ocean floor but are either unable to swim or can do so only infrequently. But since swimming, like flying, is so useful, and unlike flying is easy from a biological perspective, there needs to be a good reason why not to swim. Most bottom-dwellers are at the bottom of the food chain where they feed on detritus and hide from enemies in the sand or rocks, so there's no reason for them to swim, and so they increase their overall density (developing shells, etc.) to be tougher instead. It's hard to think a humanoid would live like this though. $\endgroup$ – IndigoFenix Aug 29 '16 at 5:55
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There are two big barriers to walking around on the sea bed like we do on land. Ignoring the obvious one about breathing, which we assume is solved, the others are:

  1. bouyancy
  2. drag

Bouyancy means that any humans who tried walking on the sea bottom would end up bouncing along like a moonwalk1, or just floating up. They also wouldn't be able to get any traction to propel themselves forward, which means they would be very slow. You see this any time someone actually in the water tries to walk.

A human body in water also has much more drag than one in air, which means that even if you had the same forces propelling you forward, and you didn't tend to float, a humanoid would move much slower than on land. Combined with the above they would be extremely slow.That would make a humanoid ancestor easy prey for anything that was more streamlined and had 3D movement in the water. A shark would make easy pickings of one.

Fortunately the answer to both of these is the same - much more weight (or technically density). A bottom-walking humanoid would have to be much, much heavier than a human, They would also have to be proportionately stronger to overcome the drag.

Given that they are going to have to be slow, and wanting to avoid being prey for the local sharkoids, this leads the humanoid ancestors down the road of armour. If you have to be slow and heavy, you may as well be well protected. This is the direction all bottom-walking life has gone on earth.

So coming back to the present day intelligent speices, since the humanoids are already heavy and well-armoured, and given that projectile weapons work very badly underwater (drag again) your humanoids are perfectly set up for some awesome sword versus shark battles - the underwater equivalent of knight versus dragon. At least you could if only your humanoids could find a way to kindle fire, and thus do metallurgy, underwater. But that's a subject for another question.

1. Armstrong, not Jackson.

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So first of all, we are talking about a humanoid race, not humans in this situation. This gives us slightly more leeway.

Since this race would be living and breathing underwater they would not have the problem of floating since they would be inhaling the same substance in which they lived.

This race would also be able to walk very easily on land if they had the ability to leave the water, because we as humans experience pressure and drag on our bodies as well, just on a much smaller scale. Since these humanoid creatures are born under the water and grow up walking in a substance much heavier and denser than air, they would have almost super human capabilities if they had the capability to leave their home. This is because air puts much less pressure and drag on a body than that of water.

As I said more than once however, your next problem is their ability to leave the water.

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Humanoids living and walking along the bottom of an earth-like planet would have to put up with extreme pressure, wouldn't they? So, first of all they'd probably be shaped differently - shorter, squatter, beefy-er, etc. Their bone structure would likely be more dense.

Given that they could breathe air, they might find walking on the surface of the earth a little like flying - definitely liberating.

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