# Reversal of Human Sexual Dimorphism

Suppose that approximately 2 million years ago, a mutation developed in hominids that allowed males (and only males) to access magic-type abilities. For the purpose of this question, consider this to be telekinesis (TK) that's roughly twice as efficient as using muscles, energy wise. It rapidly spreads through the population and is carried through until something similar to modern humans develop.

Details:

• TK does not depend on how in-shape the body is at all, it's an entirely separate system. It does draw from your body's reserves but in a different way.
• It's not efficient for long-ranged travel. You're better off using a horse or your own legs.
• TK is powerful but not a trump card in combat, it has strengths and weaknesses just like any other technique. The best groups at fighting and hunting combine the two, so for example 10 TKs + 10 normals would beat 20 TKs or 20 normals. TK requires enough mental concentration that it's not effective to try and combine it with regular combat techniques, any single individual can only use one or the other.

Presumably this mutation would change evolutionary selectors. Specifically, I'm wondering the following:

Would this reverse the "classic" human sexual dimorphism? Would females develop as larger and stronger without access to TK (or at least stay the same), with males becoming smaller and more efficient? Would it be the same as we are, for different reasons? Or would something else happen entirely?

Edit: The current answers could very well be correct, but I wanted to address my logic specifically since I'm not seeing it considered yet. (So basically, if I am wrong, please tell me why in your answer).

Evolution isn't strictly better/faster/stronger. It's about the fittest, and fittest can also mean things like adaptive and efficient.

TK would act as a replacement for physical muscles in many situations. It seems to me that this would favor smaller males, as they would be better long-distance runners who require less food in times of scarcity, while still being just as capable due to their TK.

Secondarily, due to Comparative Advantage, females would start selecting to be stronger and larger. It's more efficient to have 5 TK/5 normals than just 10 TKs, so primates would have an advantage if females took over the less-efficient big/strong role while males concentrated on their more powerful TK role.

It is also possible that my timeline is too short, and 2 million years isn't enough time for these differences to develop - if that's the case I'd be interested in knowing how long it would have to be.

• So, this telekinesis: what's the range on it? How many things can you manipulate at once? And how accurately? – DaaaahWhoosh Jul 29 '15 at 18:11
• @DaaaahWhoosh: I'm not extremely interested in the exact details, as my plan would be that modern humans have expanded and changed their capabilities. I'm just wondering what kind of impact the TK, as described, would have on evolutionary pressures. – Dan Smolinske Jul 29 '15 at 18:36
• Since nothing in your scenario requires a TK adept to not be as strong as any other male, there's no reason for this trait to reduce male physical strength. – Oldcat Jul 30 '15 at 0:00
• @Oldcat: It makes it redundant, extra physical strength requires additional calories and energy for travel (due to mass). Also presumably someone who spent more time using TK would be better at it than someone who split. – Dan Smolinske Jul 30 '15 at 0:27
• And someone who could fight normally would be way better in using even a bit less TK to get a major combat advantage. Whereas a puny TK adept would be in trouble if a strong fellow got hold of him by surprise even without TK. Still see more potential as a supplement rather than a replacement. – Oldcat Jul 30 '15 at 16:53

It would be the same.

This adaptation (which presumably is a mutation on the Y chromosome, thus only affecting males) makes males even stronger; more dominant. More dominant males has not lead to physically stronger females in our current iteration, so there isn't any reason to believe it would in this case. Males will still have the genes for more physical strength and females will not suddenly gain those genes.

Being physically strong is an advantage, adding telekinesis would only increase that advantage. Increased physical strength alone has been a key factor in survival, even during the "lean times" you speak of. Here is the key though: Physical strength can become fatigued, the same must be true of this telekinetic power. If a physically weak male is fighting a physically strong male with the same telekinetic ability and they both become fatigued, the physically stronger male has the clear advantage in close combat with the physically weaker male. The same is true for defending oneself from animal attack, hunting, or fleeing. The advantage goes to the stronger male, including the ability to get the extra calories required to support the additional burden.

However, if the adaptation was inversely proportional to physical strength, then males would likely become physically weaker in favor of stronger telekinesis. In that case females may fill the role of the physically stronger sex, but only due to the reduction of strength in the male population.

Nothing would change.

I agree with Samuel's answer, but for a different reason: Competition.

Bigger males mean that they would be stronger, and thus better suited for fighting. They might be better at jumping, throwing things, and beating up opponents. If males shrunk in size, they would be less suited for battle. This also applies to hunting. If a person is hunting a mammoth, they do not want to be small. Taller, bigger, stronger males will come out on top against creatures whom they hunt or who are hunting them.

Long-distance runners are, in general, tall and lean (I am an example). Sprinters, on the other hand, tend to be short (though not squat). I don't think that this would favor either group, but it would certainly make it easier for both, as both groups need a lot of calories. So I don't think the adaptation would favor one body type. Taller folks are better at distance; shorter folks are better at sprinting (in some cases). Both have their advantages.

I'm not sure I understand the comparative advantage angle, but then again, I've never really understood economics. If I'm reading it correctly, it means that females will select more efficient males. I'm not knowledgeable in this to correctly gauge the effects in this particular arena.

Regarding overall efficiency, you wrote

Evolution isn't strictly better/faster/stronger. It's about the fittest, and fittest can also mean things like adaptive and efficient.

That's correct. But simply by having TK, the males are already more efficient. Looking at it mathematically (because I can't explain it any other way), say that efficiency is defined as $$\text{efficiency}=\frac{\text{effects}}{\text{cost}}$$ Keeping the "costs" - the size - the same but increasing the "effects" - by adding in TK - should be the same as keeping the "effects" the same - by taking away the extra muscle and size - while reducing the "costs" - by making the male smaller.

• Comparative Advantage is a nice counter-intuitive concept. This is a very simplified example, but say you have 10 men and 10 women, and you need a hunting party of 8 individuals, split 4 TK/4 normal. If you used all men, you'd be wasting half of them, because they're using their less efficient muscles rather than TK. If you instead use a 4 men/4 woman split, then you free up those 4 men to stay behind and use their TK for other purposes. This should lead to an overall increase in efficiency/productivity. – Dan Smolinske Jul 29 '15 at 19:02
• @DanSmolinske It would strongly depend on the "other purposes", though. – HDE 226868 Jul 29 '15 at 19:03
• it's also the case that 2 million years ago mankind was competing against the environment rather than against arbitrary social factors, the direct and unthinking application of force is not the deciding factor in human interaction with external forces, but rather appeal to teamwork and ingenuity, that is, the method of application. Whilst tools might be lacking, the mental capacity to utilize such things as leverage are not. A counterpoint is also that whilst the remote application of force will restrain the evolutionary motive injury, – Giu Piete Mar 7 at 14:03
• injuries aren't exclusively negative, they are learning experiences for body and mind, a genealogical line that has preferred for the reduction of exposure to injury will likely become less capable to withstand it. This holds for individuals as much as lines, as a body of greater mass(that is, an individual that performs little in the way of direct physical effort,) is capable of sustaining without breaking - greater force applied against it. Of course, there are other factors, an individual whose capabilities are not so denuded by age has certain advantages, and the line would also be – Giu Piete Mar 7 at 14:05
• capable of making use of it's physically deformed offspring much better than has been the case in natural history. etc etc etc. As that impacts dimorphism tho, idk that the presumption is correct, that it would only affect males, the introduction of this capability would change the relative value of individuals so much that otherwise good and bad stock would have their values adjusted for mating such that all mutations/variations would have their outcomes adjusted also(not simply those that manifest in males) This is the problem with survival of the fittest, it's trash. – Giu Piete Mar 7 at 14:09

Hmm interesting idea.

So TK is twice as efficient energy wise as muscle power. Lets assume that energy in the form of food is still constrained. Ie large scale agriculture has not been developed.

In this case, men would be less muscular simply because it would be better to put all that energy into TK and be twice as strong.

Would women take over the combat role? Unlikely in my view because women still get pregnant and still are the primary care giver for a child until the age of 2 (average age of weaning in hunter gather societies). Only women can breast feed babies.

Would men become smaller than women? That is a hard one.

I guess it would depend on how linked are the growth and development programs of men and women. ie will short men result in short women? The answer we in our world is yes, because at the moment bigger is better. No so in this world. Hmm... or put it another way, is there enough pressure and time to drive sexual dimorphism the other way.

Its half true I think.

Evolution, as the others here seem to miss, is indeed about the fittest. The fittest =/= the strongest, but the most successful at producing offspring. The most successful at getting offspring are the one's best suited to getting food, are efficiënt when food is scarce and have the most viable reproductive organs that produce strong children as well.

Since TK is superior to muscle power it'll be more efficiënt for males to be smaller and survive with less food, which allows them to get more females as they can offer more access food to them and their children which increases their survival rate compared to large men. Normally the lowered size would mean a lower ability to do things like hunt or build shelterd for themselves and their mates&children. But with efficiënt TK this can be compensated, and a small male with an emphasis on storing energy to burn for TK instead of muscle power would be superior.

Does this mean that females immediately grow stronger? Unlikely. Aside from direct warfare there is little reason for males and females to make use of the "5TK+5normal is stronger than 10TK" mantra. In fact, the extra energy is better suited to growing babies and feeding more children. Large families would outbreed small families where the female has a higher strength, meaning that the small families will slowly die out if only because they are more likely to succumb to famine.

• "it'll be more efficiënt for males to be smaller and survive with less food" That is only true if the food supply is restricted in size. If there is ample or large game a larger male would still be a better provider than a smaller male. You would need to up the caloric usage for TK to make smaller males more efficient. You also have to consider, that if the smaller male ever were better at gathering that the larger male would just take it. Ubiquity of pack animals may also change TK's utility. If TK was significantly stronger than muscles you might also see TK males with multiple partners. – user2259716 Mar 7 at 20:16
• @user2259716 you realize that if the larger male would just take it, his social standing would drop and the likelyhood of multiple small males murdering him increases? And that the large male needs to choose strength or TK, and if he does not kill the small male that one will definitely use TK and win the next bout? But if he does kill, the other males will automatically need to resort to murdering/banishing said male to reduce the chances of being treated similarly. There is no long-term gain in using strength for theft in this case. – Demigan Mar 7 at 21:00
• Also know that since TK would be the superior way to hunt, the big and small male would get more or less the same amount of food home. Since the smaller has a higher chance of remaining undetected and uses less energy the small male would automatically be at an advantage with the same amount of food and have more success surviving during scarcity. You are thinking of these as individuals. But humans work in groups. In the Hunter gatherer societies the strongest were given more food, but in return were expected to face danger first and protect the rest. This ensures group survival. – Demigan Mar 7 at 21:00
• "But if he does kill, the other males will automatically need to resort to murdering/banishing said male to reduce the chances of being treated similarly" Not automatically. Societal aggressors can often exist for long periods of time. It can take years before the group snaps, if ever. Different societies have differing levels of complexity, interplay, and concern for their members. A group of large males would also be able to better exert themselves against a group of smaller males. It does not have to be 1 on 1. A group of aggressors would benefit from being larger. – user2259716 Mar 7 at 21:45
• As to food, it depends on availability and size of game as I said. If there were any larger or mega sized fauna such as Elephants, Dinos, Mammoths, etc larger size would help for fighting and hauling. Meat scarcity could lead to smaller males thriving, but that is 1 scenario, hence why I mentioned how food supply effects this. "will definitely use TK and win the next bout" The small male is never guaranteed to win any bouts. You would need to make being large a detriment to TK attacks. It really depends on the nature of TK. – user2259716 Mar 7 at 21:45

I don't think it would change, but for still different reasons.
I think human sexual dimorphism is probably partly vestigial and partly a result of sexual selection. It doesn't serve a practical purpose anymore, but it is a leftover from an earlier era and is maintained because females tend to be attracted to size and strength in men. We've seen in the animal kingdom lots of useless traits get maintained once females come to find them attractive.