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My question is how/why does my creature evolve into having arms when it has none?

Context: I am currently creating a evolutionary tree for the first animals of my planet. My planet is the size of earth with a similar climate to earth in the Carboniferous era.

It’s mostly covered in swamps, jungles, savanna, and what I like to call mushroom fields(fields of moss and Prototaxites) It’s rather warm.

One of the first creatures is a two legged, fast, long necked, type of reptile. Its a herbivore and it’s main competition is its cousin which is like it but more heavyset.

It’s main source of food is a small parasitic fern that is found high on trees.

I think it might need arms to climb the trees or defend itself against predators but I need help in defining its future as hopefully the first sentient race of the planet.

enter image description here

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  • $\begingroup$ Thanks for the drawing, it really helps understanding what is your creature in a few seconds :). Do you know its size or is it still a mystery to you? $\endgroup$ Nov 23, 2021 at 17:28
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    $\begingroup$ About 7 or 8 feet tall. $\endgroup$ Nov 23, 2021 at 18:45
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    $\begingroup$ note at some point in this creatures ancestor it had to have 4 limbed(or more) ancestors , you can't evolve an upright limb posture without it, so why not keep those limbs. the thing as drawn had to have a four limbed ancestor. $\endgroup$
    – John
    Nov 24, 2021 at 1:27
  • $\begingroup$ One of its ancestors had 2 large red flaps of tissue running down its neck. It had muscles in it and was used to wave around to get the attention of a mate. If I just carried this feature on do you think it could evolve limbs? $\endgroup$ Nov 24, 2021 at 2:00

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No vertebrate had evolved a new limb for the entirety of 400 million years of their (limbs) existence. They have only changed their function or were lost. A limb is too complex of an organ, requires a lot of pre-existing stuff (bones, muscles, etc) to evolve this late in the evolutionary timeline. So your creature really has only three options here:

  1. Just redesign it to have some frontal limbs that can evolve into arms with time.
  2. Its tail can shift to become prehensile, though it might be questionable if such a thing can be dexterous enough to allow tool manufacture (You only have what amounts to basically just one long finger).
  3. Their face tentacles can become prehensile and more sturdy, allowing for grabbing and manipulating objects. Honestly, if you don't want to redesign your proto-species, it's the best path you can take, IMO.
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  • $\begingroup$ No new limbs were needed. We already had 4. While it is no guarantee that it would happen, if you have a (partially) limbless organism that desperately requires re-limbing or it will become extinct, it might just come up with something that can plausibly resemble an arm. It's dinosauroid after all, and if his story takes place something like in modern times, that's 70 million years in which something might happen. $\endgroup$
    – John O
    Nov 23, 2021 at 15:16
  • $\begingroup$ "No new limbs were needed. We already had 4." Tell that to the elephants. Evolution does not work like that. It isn't sentient, it has no anticipation and it has no goals. If you "desperately" need limbs without having ones - you don't start evolving them pronto, you join the extinction club. $\endgroup$ Nov 23, 2021 at 19:33
  • $\begingroup$ Agreed, limbs are just to basal to developmental pathways to evolve new ones, the best you can manage is turning something else into something vaguely limb like, as the answer already stated . $\endgroup$
    – John
    Nov 23, 2021 at 21:56
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    $\begingroup$ Sentience is an illusion. Evolution does work like that. Some join the extinction club, others get their limbs. Nothing magical happened when they first developed 400 million years ago. There was pressure to adapt, adaptations occurred, limbs evolved. Nothing is ever guaranteed, but nothing is ever really out of reach either. It might be that you guys are just a little too discriminant in what constitutes "limb". $\endgroup$
    – John O
    Nov 23, 2021 at 22:27
  • $\begingroup$ @JohnO that's not how development or evolution works, and the existing developmental pathways precluded limbs which are segmentally distinct sections of the body. limbs evolved before complex alteration of developmental segmentation. It cant' evolve afterwards. If by limbs you mean things with bones, muscles, and multiple joints, it can't happen in terrestrial vertebrates. $\endgroup$
    – John
    Nov 24, 2021 at 1:22
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What this creature needs is a path where the future hands are always useful.

Probably start with the studs. You don't have any on its front, you should put some there.

Once they are there, the creature can start using them. First off, it could use them to bump into things. Knock down fruit from trees and the like. Once it's in the habit, having groves and things would let it catch things in stud and push up or down or sideways, which could be useful.

After that, some musculature that would let the stud be moved in place independent of the whole body would help. Even a small ability to shift could be useful.

Once the muscles start to develop, the true arm evolves. The stud is likely to be shed at some point to allow more dexterity.

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  • $\begingroup$ how are they more useful than just using existing limbs or structures. limbs evolved as control surfaces, where a nub is actually useful. $\endgroup$
    – John
    Nov 23, 2021 at 21:58
  • $\begingroup$ The question posited no existing limbs. Fish manage to evolve fins. $\endgroup$
    – Mary
    Nov 23, 2021 at 23:48
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    $\begingroup$ fins evolved extremely early in fish, when most hox genes were very new and developmental pathways had little baggage. And again nubs are useful as control surfaces underwater, out of water they serve no function that existing structures cannot do far better. terrestrial vertebrates are so far removed from their ancestors that developmental pathways don't leave any room for anything like limbs. heavy alteration to the segmentation of the body plan would be needed to develop limbs, this is literally the hopeful monster's problem. $\endgroup$
    – John
    Nov 24, 2021 at 1:19
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You work with what you have got.

advanced creature with hands

Thanks to my associate Mushi for illustrating the concept. Your creature has perfectly good appendages. It evolves hands from its feet. Where do you think our hands came from?

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The tentacles that release enzymes will probably grow larger and more muscular, to allow it to pin down prey. Over time, they would evolve into arms on it's face.

Things don't normally just evolve. They need precursors. The only easy to evolve into arms are those tentacles, so they will have to do.

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