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A common feature in fantasy are half-human creatures such as centaurs, satyrs and merpeople. As much as the worldbuilding community wants these to be real, the evolution and anatomy don't match.


The Problem

Centaurs; can't graze without bending down, removing the horses' advantage of running away whenever it needs to. Satyrs; the digitigrade legs can't realistically evolve in a bipedal matter, the purpose of digitigrade legs is to provide a quadruped with faster sprint speed at the cost of balance, hence needing to be quadruped. Merpeople; the classical merperson lacks the form to serve its functions, it is not fast enough under water to survive efficiently enough to evolve sapience. Half humanity is to complex to work, never mind being able to interbreed with humans.

A Possible Solution?

What if we simply lower our standards? How animalistic can I make a sub species of human without; (A) making them a separate species and (B) making them non-interbreedable with humans? How could they evolve? The features I need to evolve are:

  1. Markings, like a tiger except they are simply lines of melanin marking the skin along the arms and legs.
  2. Enlarged irises, Around twice the circumference of our irises
  3. Pointed ears, like the stereotypical elf
  4. Longer canines (keyword: not sharper but longer)

Edit Their offspring does not need to be fertile, they can be sterile (like horses and donkeys)

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  • $\begingroup$ The real question is "why would they evolve these features?" What environment do they live in and what advantages do they get over either real animals of ordinary humans? $\endgroup$ – Thucydides Jan 22 '16 at 18:18
  • $\begingroup$ @Thucydides You don't necessarily need advantages. Mutations in a bloodline that don't really prevent reproduction could be sufficient. $\endgroup$ – The Anathema Jan 22 '16 at 19:57
  • $\begingroup$ Centaurs could evolve to eat fruit/other things high on trees, ostriches successfully evolved digitigrade bipedalism, and octopode are intelligent, slow moving sea creatures. Merpeople could evolve to fill a similar niche. That being said, all of those are sufficiently inhuman that producing offspring would definitely be impossible, unless magic. $\endgroup$ – ckersch May 27 '16 at 23:10
  • $\begingroup$ @ckersch, ostriches use their necks as counterbalance and most octopuses use very efficient camouflage and mass breeding $\endgroup$ – TrEs-2b May 28 '16 at 0:47
  • $\begingroup$ I don't see how either of those makes digitigrade humanoids or merpeople impossible, though. However, thinking more about satyrs, hooved digitigrade creatures might be insufficiently stable. $\endgroup$ – ckersch May 28 '16 at 0:52
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To interbreed with a Human, you have to be a Homo. So the traits of beast-men would be limited to traits possible in great apes.

6 limbed centauroids are too far a stretch. Quadrupeds are completely possible especially if they are large-bodied herbivores Gigatopitecus as a model. (they are not homos but much closer than say horses).

The aquatic ape is a mostly wrong but interesting model for human evolution. Long-limbed Bipedal wading folks could be close enough to mate with us. I also think otter-like swimers could be worked out streamlined people with long torsos short limbs and big feet.

to hit your specifics.

1) Markings, like a tiger except they are simply lines of melanin marking the skin along the arms and legs.
Humans are already striped so genes would just have to be turned on. Hunters or prey would benefit as long as it would not be sexually selected against.

2) Enlarged irises, Around twice the circumference of our irises Sexual selection could explain this (and nearly anything) large eyes make more sense for nocturnal animals Tasiers have a diurnal to nocturnal evolutionary path and big freaking eyes. We use the whites of our eye for communication if the irises are increase at their expense we lose some of that. maybe they don't care or want to hide their intentions.

3) Pointed ears, like the stereotypical elf our old friends sexual selection and founder effect. maybe sound based hunting.

4) longer canines, Keyword, not sharper but longer there is a lot of variance in human canines so our friends come in again. There are a lot of things with giant teeth just to impress the ladies some of them fairly closely related to us. I don't think you would even need to defend this.

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    $\begingroup$ "To interbreed with a Human, you have to be a Homo" - best opening sentence, ever. +1 $\endgroup$ – AndreiROM Jan 22 '16 at 20:03
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    $\begingroup$ @AndreiROM I certainly got a chuckle out of it. $\endgroup$ – T.J.L. Jan 22 '16 at 21:10
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Well... you got some stuff not quite right there.

Centaurs by no means need to graze. They may be half-horse, but they are also half-humans. More than anything, they would be hunters - excellent hunters, that hunt with the extra speed from a horse and with powerful spears.

Digitigrade feet are, to some extent, not that far away from high heels. If you look what some Korean dancers can do on heels, It's totally plausible to see a creature naturally evolved to walk like that. Maybe they need some extra reinforcement on their feet, but it's nothing that humans can't already do with training. It just becomes easier with evolution.

Swimmers that use mermaid tails are becoming more and more common. While they are not as efficient swimmers as, let say, sharks, they are far better than regular, "legged" swimmers.

Still, that's not quite right you're looking for, it seems. What I did on one my RPG settings:

  • I don't assume that my humans have exactly the same DNA as today's humans. Their DNA is way more complex and varied, so they can evolve way more different traits than we have today.
  • I don't assume that my humans are limited to the variance we have today regarding physical traits. Since their DNA is different, I have a lot more leeway on what I can do with their appearance and the mutations that they can go through.

  • I don't assume that my humans are so prone to incest-related problems as we are today. That means they can interbreed to select for a given trait more easily than real-life humans.

Those 3 "not-assumptions" make my humans much more malleable, and thus I can do whatever I like with them. A few examples.

  • I have a segregated wolf-people that evolved from a small population where longer canines and a better sense of smell were essential to survival. Those attributes were selected to their descendants and made them more preeminent.

  • I have a human spellcaster tribe that have a huge amount of conductive metals on their skins, creating colorful markings that are useful as heat dissipators, since they are almost metallic.

  • My elves are taller humans with longer ears, fair skin and an acute sense of smell, that helps then scouting during the night-time.

The limit of those and how they affect the capabilities of interbreeding are up to you. Really, you don't need to limit yourself to real-life DNA mechanics - it's a fantasy world anyway.

Just keep consistent, and you're good to go. Your readers will pay way less attention to those details than you think if your world is self-consistent in general.

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A different approach

Your question seems to assume that these creatures evolve to the conditions and then evolve intelligence.

However, if you start with a human ancestor that already has intelligence, then the adaptations for different environments simply permit the intelligent species to move into new ecological niches (e.g. Merfolk).

The environment adaptations help expand their range of habitation but it is their intelligence that permits them to compete in the different environment.

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