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My previous question asked if a stegosaur could become bipedal. I guess I should have asked if and how it could become humanoid-like. Asking if this is possible made me think about all the other vague humanoid shapes that have evolved over the course of Earth's evolution. Penguins, ground sloths, therizinosaurs, and of course humans. What were the driving forces behind all of these forms? What kind of world changes could pressure life forms to be this way?

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    $\begingroup$ Human bipedalism is most likely a function of humans being persistence hunters. Calorically, two long legs are more efficient than four for long duration runs. $\endgroup$
    – Dragongeek
    Aug 2 '20 at 0:40
  • $\begingroup$ @Dragongeek - Maybe turn that into an answer? $\endgroup$
    – elemtilas
    Aug 2 '20 at 4:03
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    $\begingroup$ Actually it is lack of pressure, or better said, lack of specialization. Primates are generalists; as a consequence, primates in general, including humans, are not all that much modified from the basic mammalian body plan -- four limbs of more or less equal length, five fingers on each limb, primitive skeletal features such as clavicles, shoulder blades, unfused radius/ulna and tibia/fibula, mobile wrists etc. Compare how much a primate resembles a lizard, against how much a kangaroo, or a cow, or a horse, or a hippopotamus, or a bat etc. have evolved away from that basic body plan. $\endgroup$
    – AlexP
    Aug 3 '20 at 18:59
  • $\begingroup$ interesting point $\endgroup$
    – Joe Smith
    Aug 4 '20 at 4:09
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If you use natural selection as your primary thesis for change, then you have to articulate how bipedal movement gives the species a clear survival advantage over alternatives, which includes describing how four legs is a clear disadvantage.

A species won't just rear up on its hind legs because it's convenient for them. Ostensibly, there would have to be millions of specific dominant genetic mutations over time that ultimately result in shifting the center of gravity, spinal adjustments, changes in size and mass, reconfiguring the pelvis to accommodate the extra burden and a wider range of lateral movement, adjustments in the feet to provide more balance and stability, and if you don't want the forelimbs to be vestigial, adjustments to the shoulder, elbow, and wrist are also necessary.

All of these things are required to take something like a stegosaurus and turn it into a biped, and all of them have to happen independently of each other, only because the odds of simultaneous non-destructive mutations that both create a clear survival advantage and function in synergy with each other are ludicrously remote.

I'm not trying to argue against the idea as much as dissuade you from trying to come up with explanations for everything because nitpickers are ALWAYS going to find holes in something as complex as this. If you want to write about bipedal stegosaurs, just do it and only provide hints at the process that got them there.

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  • $\begingroup$ Fair point. Haters going to hate. $\endgroup$
    – Joe Smith
    Aug 29 '20 at 23:13
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The humanoid shape is merely a function of specialization of the bipedal shape.

Once you already have the bipedal shape, the function of evolution is to specialize components that are already there to become better, rather than create new function, as least from a general standpoint. That is, if a quadruped requires the ability to manipulate objects, it's not going to develop new limbs to do so, rather, it'll adapt a set of limbs it currently has and the advantage lies in developing sets of limbs for specific tasks. One pair of limbs, the one close to the head, become specialized for manipulation (whatever is the case - claws to attack, for instance, or wings to first glide and then fly) and the other set becomes specialized for running. The ultimate end (or, at least, the current ultimate end) to this dichotomy of specialization can be said to be found in the upright human form - one pair of limbs exclusively on the ground for motion and the other pair exclusively for object manipulation.

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Well, one reason bipedalism is useful, is it requires less energy per step. Studies on energy efficiency in bipedalism A stage in between being on four legs and being on two, would be being able to run on two legs for short sprints, like bearded dragons can. Bearded Dragon Running This adaptation could be needed for getting away from predators quickly, or to surprise and catch prey for the larger animals. A video I think would be very helpful Ancient crocs An ancient crocodilian species that evolved the ability to run on two legs in what could be a precursor to your bipedal reptilians. It evolved this ability due to competition from other predators in the area, so that could be a main reason for your creatures evolution.

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