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I'm writing a fictional story where non-primate mammals (and marsupials) evolved into different species of humanoids which roamed the earth.

In my world humanoids have evolved from animals such as bears , sloths and koalas including extinct animals such as Chalicotheres.

I'm wondering what evolutionary pressures could lead to the rise of such species.

enter image description here

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    $\begingroup$ Just to be clear, are we talking about humanoid in terms of body shape, or are we talking about the full package; humanoid body AND intelligence? There are some cases where either the humanoid shape could form without the intelligence or vice versa, although the latter is a little more exotic. Narrowing down your requirements will lead to better answers. $\endgroup$ – Tim B II Feb 12 at 22:45
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    $\begingroup$ Just the body shape although I'm not excluding any answers that includes the evolution of intelligence. But mainly I'm focused on the body shape. $\endgroup$ – Pierre Feb 12 at 22:51
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    $\begingroup$ @TimBII Is the latter more exotic? Intelligence is beneficial in tons of environments, and we see other arguably "intelligent" species in plenty of niches (ex. octopuses, dolphins, pigs?). What we don't see is anything converging on human form anywhere else. $\endgroup$ – Zxyrra Feb 13 at 4:46
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    $\begingroup$ @Zxyrra yeah wow; that is the sort of question that deserves an answer in its own right on this forum but for now, let me just say that descending from the trees and walking upright gave us arms almost custom built for tool use, which heightens intelligence, and then the development of language for coordination in both attack and defence allows for strategy to be applied... While your examples are good for emergent intelligence, I'm of the view that to extend to an intelligence capable of abstractions and synthesis like ours would be very difficult for any of those animal examples to achieve. $\endgroup$ – Tim B II Feb 13 at 5:08
  • $\begingroup$ @TimBII I asked a question based on this discussion here. $\endgroup$ – Zxyrra Feb 13 at 5:28
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Evolution is a combination of random mutation and natural selection.

In our case, we exist because one of our ancestors had a mutation that caused the lumbar curve, i.e. where the lower spine curves the wrong way. That allowed standing and walking upright, which made it easy to see over tall grasses (a relatively recent development itself on an evolutionary scale) and use hands for carrying tools or weapons instead of walking, which in turn drove increased intelligence to better make/use said tools and weapons, all of which were huge selection benefits. Many other, lesser mutations refined that new form over time into what we are now, but that was the key.

Our primate ancestors were a bit smarter than other mammals, but nothing else really set them apart—or does so today, if you exclude us. If the same mutation happened in other mammals, you could easily get other bipeds, and they would likely evolve higher intelligence as a result just like we did. Convergent evolution to where they all appeared "humanoid" probably depends on how strictly you define the term.

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Feasible

Primates evolved from a non-primate precursor, according to scientists, around 65 million years ago. So, there's nothing preventing the body structure of humanoids from arising given the right set of influences into a humanoid shape. The real question is why humans are human shaped. And the answer is intelligence.

Humans were designed with intelligence in mind. Being bipedal with thin but flexible fingers isn't an advantage unless you can do something better with the arms than if they were feet, like use and make simple tools or manipulate the environment to your advantage. Bears, for instance, derive an advantage from having four powerful limbs for movement, so if you want to have them become a humanoid, you need to give them a reason to use arms - and that's intelligence.

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  • $\begingroup$ to be fair you don't nee human like intelligence, just great ape like intelligence is enough. you just need prevalent tool use. the tricky part is getting grasping forelimbs in the first place, which is not all that common in mammals. $\endgroup$ – John Feb 13 at 2:33
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So there’s according to Wikipedia 12 theories on why humans became bipedal, I remember there being more.

Anyways here’s a nice explanation for reasons to give your species human-like bipedalism: https://en.m.wikipedia.org/wiki/Bipedalism#Evolution_of_human_bipedalism

The problem you’re gonna run in to is that a lot of these problems we might have evolved towards bipedalism to fix are just things primates sorta suck at but other animals have better solutions for, like defense or travel, or they’re things that require a ton of specialized primate abilities to make into a good solution (like we were good at reaching for branches. Which may work for koalas but not sheep).

The one thing I could see working is introducing a very deadly predator in the food chain that’s proportionally more intimidated by large-sized creatures. It is a cheap way to make yourself appear significantly bigger.

Also based on that picture your animal has fingers, low facial prognathism, a butt, ability to use tools, non-rhiperian nose, and maybe a chin and maybe permanently enlarged breasts which are all pretty unusual physical traits it’ll take a lot more working to evolve if you wanna use them.

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