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How would I change the evolution of humans to result in horns on humans as a result of reproductive fitness, instead of social fitness. Meaning, NEITHER "Horns are sexy", or some strange infection.

Note:

https://worldbuilding.stackexchange.com/a/14907/6812

A predatory creature with claws and large teeth is unlikely to evolve horns. Unlike herbivores, carnivores have evolved to kill things, and don't tend to need to evolve extra weapons like horns. Furthermore, effectively using horns would require that the creature put energy into growing them, and into growing and sustaining the supporting structure needed to make them useful. Horned creatures need strong necks to support use of their horns, and the neck musculature wouldn't be critical to anything else for a creature evolved to run down and claw up its prey.

However, Hair is Keratin, Nails are Keratin, could hair evolve into horns?

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  • $\begingroup$ The way you've stated the question, basically, means people without horns die before they reproduce, or are rendered otherwise unable to reproduce, by not having horns. Can't really see why that would happen with humans. Small "decorative" horns wouldn't require much restructuring, but would violate your social fitness stricture, as they are simply an appearance thing. I am pretty sure human brains couldn't survive the beating bighorn sheep use their horns for. $\endgroup$ – Seeds May 5 '16 at 21:22
  • $\begingroup$ @Seeds - Just want an evolutionary path for horned humanoids. Deviantion from real evolution can be anywhen. ... That might actually be a different question... sigh Goodnight... $\endgroup$ – Malady May 6 '16 at 2:11
  • $\begingroup$ That path includes the why, mutations can provide the basis, as evidenced by the other animals with horns. In order to keep, and refine, the proto-horns there has to be a reason, otherwise it dies out. $\endgroup$ – Seeds May 6 '16 at 14:34
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    $\begingroup$ Do you want humans to evolve horns, or do you want to explain the evolution of a human-like creature with horns? The former is probably impossible, since humans can do anything a horned animal can do with its horns better using tools. Evolution tends to select against unnecessary structures in the name of energetic efficiency, so even if a horned mutation appeared in humans, the horned humans would probably evolve to not have horns, rather than the other way around. $\endgroup$ – ckersch May 6 '16 at 20:49
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    $\begingroup$ odditycentral.com/pics/the-7-horniest-people-on-earth.html for reference. $\endgroup$ – Mołot Sep 29 '16 at 11:42
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Yes, horns are made of the same protein as hair and nails, so yes, humans (as mammals) have the potential to develop horns.

The way to select for horns is to make them advantageous to survival. That might occur (over many millions of years) if humans, in order to protect territory (food source), were forced to fight for limited supplies primarily by head blows. If this were consistently the case, any mutation which would offer an advantage in head-butting competitions (thicker scalps, harder/thicker cranial vaults, better supportive structures for the brain protecting against concussion, etc.) would be advantageous and horns could reasonably be selected for.

That's the only way I can imagine, and that's basically how evolution works: any mutation that offers a survival advantage is likely to be selected for eventually given enough numbers carrying the mutation.

That explains why a mutation spreads through a population, be it deleterious (one sickle cell gene protects against malaria, but two copies of the mutation produces a disease that shortens lifespan) or beneficial (the continued production of lactase such that people can continue to digest milk after infancy.)

However, since humans have arms (both in the sense of appendages and weapons), such a scenario would be unlikely, but wildly inventive if convincing.

The Evolution of Horn-Like Organs

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  • $\begingroup$ Okay, this is good! ... Hope Andrei reads this... $\endgroup$ – Malady May 6 '16 at 2:16
  • $\begingroup$ But the multiple million year (tens of millions, more likely) process of evolving horns would mean that the creatures that eventually had them would be no more 'human' than our earliest primate ancestors. $\endgroup$ – jamesqf May 6 '16 at 17:54
  • $\begingroup$ @jamesqf - Very true. $\endgroup$ – anongoodnurse May 6 '16 at 17:56
  • $\begingroup$ @jamesqf - Nope. It could arise in an isolated population, which would otherwise be fertile with the larger world. it is only if the horned population became infertile wrt the rest of humanity would your claim be correct - and there is no obvious reason to think that growing horns would imply reproductive changes as well. It's possible in principle, but not required. $\endgroup$ – WhatRoughBeast Feb 2 at 20:36
  • $\begingroup$ And there is a second way a mutation can spread through a population: random spread of a neutral mutation. If a mutation confers neither harm nor benefit, its persistence in the population is purely a matter of chance. And most mutations are neutral, or rather, most non-lethal mutations are neutral. $\endgroup$ – WhatRoughBeast Feb 2 at 20:42
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As anongoodnurse has already mentioned -- since humans have arms, it is highly unlikely that they will develop a weapon and all the necessary modifications just so that they can do their fighting by butting heads.

So, since weapons are pretty much out -- how about sensory equipment?

Unlike hair or nails, horns are NOT entirely dead material! Google "Broken horn" for goats, cows, or any other horned creature -- they bleed, they hurt, and they contain very well-circulated and sensory-rich tissue at the base inside!

So, if you somehow manage to voodoo some kind of evolutionary necessary sense or skill located in this special horn-tissue, you've got a reason for humans developing horns. The ceratin layer around it (the visible horn) could then be either protection or a resonance space. And if humans need this special sense / instrument for mating, like e.g. sensing when a female is receptive or triggering ovulation, you'd even fulfill the 'reproductive fitness' checkbox. One thing they'd never do is fighting with the horns, though (no stags clashing in autumn...)

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  • $\begingroup$ Nice one, you could say these horns are a way to detect objects in the dark. $\endgroup$ – Cem Kalyoncu Sep 29 '16 at 11:50
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It can happen that hair mutates into, basically, nails. It is (thankfully) incredibly rare, and disturbing to look at the victims of this affliction (Google it if you're brave enough).

However, humanity will not suddenly start evolving horns - there is no imperative for this, as well as requiring a tremendous number of changes for them to be "functional" (as you yourself list in your question).

That doesn't leave room for a lot of options:

Change History

Simply make it so that our genetic ancestors developed these, and we kept them. Basically, hand-wave it into your story.

For example:

There once existed 3 species of human: Homo Sapiens, Neanderthals, and -name for horned humans here-. Of the three species only -name for horned humans here- survived into the modern age.

Genetic Alteration

People get bored and start genetically modifying themselves. Something goes wrong (nanovirus, etc.), and everyone is modified to grow horns. People then kept them because women find themselves overwhelmingly attracted to horned men, and men will never willingly lower their chances at being found attractive.

Note: Consider the implications of horns as far as modern military and safety equipment is concerned. If humans grow magnificent horns they will not be able to wear helmets very well. They might also not fit into cars, or fighter jet cockpits, etc.

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  • $\begingroup$ Sorry, my mistake! ... I rephrased to clarify that those explanations are basically what I don't want. $\endgroup$ – Malady May 5 '16 at 18:37
  • $\begingroup$ @Malandy - edited my post. So you're looking for humans, as we are now, to suddenly evolve horns? $\endgroup$ – AndreiROM May 5 '16 at 18:40
  • $\begingroup$ Initally wanted list of changes from real evolution to get horns, like in linked question, but now, I just want an evolutionary path for horned humanoids. Most likely get horns, by having a ram-ish creature that then humanoidizes, while keeping horns... But that can only happen as sexually desirable trait as it's useless otherwise? Deviantion from real evolution can be anywhen. ... That might actually be a different question... sigh Goodnight... $\endgroup$ – Malady May 6 '16 at 2:14
  • $\begingroup$ Okay, anongoodnurse's answer is something like what I'm looking for. $\endgroup$ – Malady May 6 '16 at 2:17
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This could also occur due to benign tumors, as in the case of the Jackalope. No, I'm not joking, I'm actually being serious here. There are cases of rabbits that grow 'horns' due to a disease. The disease isn't harmful to the rabbits, who are able to live quite normally, however the disease is contagious which would explain why a whole civilization would have them.

http://www.isciencetimes.com/articles/5556/20130703/worlds-scariest-rabbit-horns-shope-papilloma-jackalope.htm

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  • $\begingroup$ In the OP - NEITHER "Horns are sexy", or some strange infection, and having this infection splice itself into the germline, also is not allowed. $\endgroup$ – Malady May 7 '16 at 0:20

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