I'm creating a post-apocalyptic world several centuries into the future. After the nuclear war whatever is left of humanity has regressed into the dark age.

Society:

The offspring of those few who survived the radiation and the following nuclear winter live in small tribes. They sometimes cooperate, and sometimes compete with other tribes. Humans are few in number.

Technology:

Technology regressed to the dark ages. The survivors hate technology and blame it for the fall. Anyone caught trying to learn or recover the before-the-fall knowledge is ostracized or put to death. Whatever is left from the destroyed civilization has been overtaken by mother nature like in Life After People. Rotting machines are mostly used for scrap metals by village blacksmiths.

Food:

Food production is limited to small scale horticulture and herding. Hunting and fishing are very important food sources since there are many animals and very few humans.

Radiation

Many places are highly radioactive and must be avoided. Some are mildly radioactive and people pass through them quickly if they are forced to. There are many dangerous mutated animals. Humans who show signs of mutations are killed.

Climate

The climate is much colder, with earth just recovering from an ice age. Humanity must rely on many food sources, since climate is very unpredictable.

In my story the average man is twice as large as average women. While the differences in modern world are 10-15kg, the average 25 year old woman in my setting weighs around 65kg, while the average 25 year old man is around 130kg. Obesity is unheard of. Men are bulky; not like modern body builders but like people who gained their muscles from a lifetime of hard work.

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

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    You might want to check out my old question What evolutionary factors can contribute to large sexual dimorphism in large mammals? – α CVn Oct 9 at 17:28
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    You could explain it through female sexual selection shifting dramatically in favor of muscular men. The time frame is a bit short, but you can exaggerate somewhat. – Kleronomas Oct 9 at 17:50
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    To get humans that big you need a lot of quality food. Scarce food and those dimensions aren't compatible without some serious handwaving. – T. Sar Oct 9 at 19:07
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    I am sorry, but your setting doesn't go together. A world with no technology and small tribes didn't produce anywhere near enough food to sustain common 20th century size humans, let alone bigger ones. The bigger your are (and the more muscular as opposed to fat), the more food you need. The large people would be at premium risk for starvation which would be common in your world - as it was everywhere until agricultural and refrigeration technologies went viral. In 19th cntry Ireland, 1 million people died and 1 million emigrated during ab five year famine -- and that was not tribal culture – Gnudiff Oct 9 at 21:44
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    It might be interesting to note that amongst nearly all extant large mammals, there is a very strong inverse correlation between dimorphism index (mass ratio of adult males to adult females) and monogamy. Gibbons, which are close to 1:1, are purely monogamous. Humans, who are closer to 1.3:1 than the figure you quoted, are only kinda-sorta monogamous. Gorillas, which are very close to the 2:1 you posit, are strictly polygamous; an average of 7 - 10 mates, and one example recorded of 65 mothers and infants for one silverback. – Securiger Oct 10 at 11:46

14 Answers 14

up vote 19 down vote accepted

You need some kind of environment where its :

  1. Advantageous to be a bulky man
  2. Disadvantageous to be a bulky woman.

For example let's assume that your tribe constantly fights with neighboring tribes, mutants, wild animals, giant spiders etc.

Big men are more successful in those battles and win more often, abduct more women and acquire more glory. That allows them to acquire more wives and have more extra-marital dalliances with the ladies. On the other hand thin men tend to die more often, kill less mutants, and hunt less boars. Parents are hesitant to give them their daughters, and few ladies find yoga instructors attractive. So bulky men tend to have more children on average then thinner men.

From an evolutionary point of view, it's advantageous to be a bulky man. Unfortunately, the above would also make women grow in size since bulky fathers tend to have bulky daughters.

So lets use culture for the second constraint.

Since there is so much fighting in your dog eat dogs world, man tend to die in droves and tribal warlord is always short on warriors. So he starts recruiting strong women. Bulky girls are pressed into tribal militia where they practice to use glaive & crossbow.

Unlike their sisters who must obey their fathers then their husbands when they get marry, the women warriors are granted same status as man. That means they have the freedom to go where they want, marry who they want, vote in the tribal council and even have a chance to become tribal chief themselves etc.

As a consequence of their more dangerous occupation bulky girls tend to die more often, they marry later (dalliances for serving women are strictly forbidden) and tend to have fewer children then civilian women.

Under above constraints its disadvantageous to be a bulky women, since you will be pressed into military service.

This cultural innovation makes the tribe stronger. Since our brave amazons are wo-manning the village walls and protecting livestock from two headed wolves, the warlord has more spare men to raid neighboring tribes. Which gives more loot and slaves. Tribe grows prosperous and conquers neighboring tribes. The remaining tribes quickly copy this practice. Now everywhere you go you hear the stories of glorious female warriors who protected their village from raiding parties & fire breathing mutants.

Keep these constraints for multiple generations and evolution will find a way to give you the dimorphism you want. Most likely your humans would be far more affected by the hormones, or smaller couples would have more girls or whatever.

Note:

All of the men must be warriors. Strong woman are recruited if they volunteer or if there is a shortage of men.

Excerpt from Dahomey Amazons

The Amazons were recruited in a number of ways. Sometimes they were volunteers – women who were sick of their bullshit day-to-day lives, poor women seeking battlefield glory, or even the occasional royal concubine who decided she was much more comfortable cutting people in half for the King than she was producing male heirs for him.

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    I like how you used the same environmental constraint to explain both the advantage and the disadvantage forces upon natural selection. That constraint also augers well with the post-apocalyptic setting of this question. – user9824134 Oct 9 at 21:17
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    Two headed wolf, head A: "Woof! Why is human on wall small, human on ground big??" head B: "Intriguing indeed. Shall we have a look in StackWB?" – Nahshon paz Oct 10 at 9:35
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    As a consequence of their more dangerous occupation bulky girls tend to die more often It's weird to not apply the same to men. Either you assume that the male soldiers are also less prevalent among the population due to dying in battle (thus giving slender male civilians the procreating edge); or you somehow assume that all men are forced to be soldiers but only bulky women become soldiers (which is a weird - though not impossible - distinction to make). – Flater Oct 10 at 11:53
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    As a countersuggestion, why not simply have women (regardless of size) as trophy wives for the soldiers, and a cultural appreciation for the slender female physique? Bulky women simply statistically die off as they are the "last picks" of their generation and thus more likely to end up unmarried and childless. Similarly, a cultural appreciation for men with combat prowess incentivizes bulky men to procreate more. – Flater Oct 10 at 11:55
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    "So he starts recruiting strong women." Pffft. It's never happened in the past, and it won't happen in the future. – RonJohn Oct 10 at 13:39

In my story average man is twice larger then average women. While the differences in modern world are 10-15kg, average women in my setting weighs around 65 kg, while man are around 130 kg.

That's B-I-G big.

Obesity is unheard of. Man are bulky, not like modern body builders, but like people who gained their muscles from lifetime of hard work.

That's not what people look like after years of manual labor, and when food is scarce from a "climate (which) is much colder then now".

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

Possibly, if the only human survivors happened to be descended from American professional football players (specifically interior linemen).

Practically, no.

enter image description here

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    But founder effect would make both sexes larger. I need to somehow increase the dimorphism. How about man doing hunting and herding, women doing horticulture? Or fighting (each other, enemy tribes, mutants, dangerous animals) whatever. – NewDawn Oct 9 at 15:56
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    @NewDawn Besides the genetics to give you the potential to be very tall and muscular, you need a lot of food and the time to build your muscles. Hunting, herding and agriculture just doesn't give you that. – RonJohn Oct 9 at 16:09
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    @NewDawn: also worth remembering: Evolution doesn’t care what you do. Just because different sexes are given different roles doesn’t necessarily mean your genetic code is going to care in the long run. – Joe Bloggs Oct 9 at 16:17
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    "That's B-I-G big." No it is not! I'm 192cm, 115kg without being obese. People don't look at me when I walk down the street. And they didn't when I was over 120 either. I pass by bigger people in the city couple times a day... – Mołot Oct 9 at 19:24
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    @Molot Now go to somewhere where people get their muscles from hard work. Think of your average village in a non-industrialised part of the world. I guarantee you'll be the biggest person they've ever seen. And you and all those other people your size can only maintain that size with a diet which is impractical for anyone elsewhere. An entire tribe could support the extra dietary demands of one or two oversized people, but you simply couldn't have everyone that size with a regular diet. – Graham Oct 9 at 21:53

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

First off, there is no such thing as radioactive mutations. A mutation may be caused by radiation, but it will be no different from a change in the same gene caused by chance or engineering. Just as if I commit a typo here that gets corrected, the text is no different if it is edited by you or me.

Now, the change may be part cultural, part selection.

Start with a template where, given a 20th century western lifestyle, healthy people would average 80-100 kilograms. Why? Because these people were selected by evolution after the apocalypse, for whatever reason. Say, if you have to wrestle against bears here and there, or if you have to haul what you've hunted large distances across uneven terrain, then more body mass helps you.

Next, all men are either into phisiculturalism by themselves, or forced into it. They've found a way to produce steroidal anabolizants with lower technology (I've seen anedoctal evidence for it, though I lack a source now, so handwave this away) and they eat and exercise a lot. Meanwhile, women are starved. Thus men acquire more mass than what would be a natural average, while women acquire less.

This food division is outright cruel, but is a thing in undeveloped countries. There is an educational cartoon from India which, in one episode, tries to teach people that giving girls smaller food portions is wrong so thay they drop this cruel custom. As far as I know, the custom has not died out yet.


Alternatively, a random insertion mutation that adds growth-relates genes to the Y chromossome could do the trick. Such mutations may cause a lot more changes than just an increased production of growth hormones - the amount of possible birth defects and reasons for miscarriage could increase. Enlargement of males must be a really decisive factor for human survival for this tradeoff to be selected positively.

  • Regarding the increase in mutations due to an insertion in Y, I believe this is overstated. The average transcribed region of a gene is in the ballpark of 10-15 kbp. The total length of Y in humans is around 58 mbp. Barring a huge untranscribed region accompanying this insertion, with a similar sequence on X, the increase in frequency of X-Y crossover should be negligible. – Rat In A Hat Oct 9 at 17:37
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    That being said, this is the best answer so far. The insertion of a growth factor on Y is the most likely reason for an increase in dimorphism. I would just leave out the bit about the increased miscarriage rate. – Rat In A Hat Oct 9 at 17:41
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    +1 for pointing out that there's no such thing as "radiation mutations" – jdunlop Oct 9 at 17:44
  • @RatInAHat thanks, I have edited that out. I see other risks, though. – Renan Oct 9 at 18:31
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    Good answer. One nit pick: body weight won’t help you if all drinking water is alcoholic, because people need to drink water in a quantity roughly proportional to their body mass. A large person can handle 5 beers better than a small person, but that’s not the right comparison. A 2x size person would need to drink about 2x amount of liquid, and therefore get the same amount of drunk as an average size person. – Alex Oct 9 at 23:01

Just look at gorillas.

Such large differences in size already exist in an extremely close relative of ours. Gorillas have a large degree of sexual dimorphism and already weigh in at about your desired sizes. Adult male gorillas weigh between 136 to 195 kg and adult female gorillas between 68 to 113 kg. So, your post-apocalyptic males and females at 135 and 65 kg respectively are well within the realm of possibility both in size and in sexual dimorphism. In fact, males being roughly double the size of females in primates is not uncommon and can be found in Orangutans, Mandrills, Hamadryas baboons, and Proboscis monkeys.

Now, the question is how did such large sexual dimorphism arise? In primate biology, there is a well-known relationship between the degree to which males compete for access to females and the relative sizes of males and females. When a male’s reproductive success is strongly correlated to its ability to intimidate or fight other males there is a strong evolutionary pressure to increase in size in order to better compete. So, if you want to drive larger male size evolutionarily you want a societal structure in which the largest and strongest men are able to monopolize the reproductive capacity of many women.

Unfortunately, these sorts of evolutionary forces will be hard pressed to produce an appreciable change in such a short time span. I anticipate thousands of years at a minimum would be necessary to naturally grow humans to such sizes.

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    The size increase could occur much faster that. There are already lots of male humans larger than 130kg. If these were the only males able to compete for the breading rights, then in 1 or 2 generations the average male will be that large. Howeve, this would also result in females getting comparatively larger as well. The degree of dimorphism would take much longer to occur. – user1751825 Oct 10 at 1:14
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    Agreeing with @user1751825. It is theorized by a lot of evolutionary biologists that trait selection isn't a steady-state thing, but rather progresses at a speed proportional to how intense the selective pressure on that particular trait is. That being said, its quite possible even bow-and-arrow tech would be sufficient to mitigate against physical size advantages in fending off other males, and military use of horses (who can more easily carry small riders) might even select against size in males. – T.E.D. Oct 10 at 12:07
  • The problematic bit here is strongly selecting for male size in ways that do nto contribute to female size - so you have to have both strong upward pressure on male size and strong downward pressure on female, and then you only have those size-related genes that are in some way gender-specific to work with (until/unless others mutate). Basic levels of "males compete with violence for mates" alongside "don't get too large, because food" is going to be slow on this one, especially since size isn't the only contributing factor to being able to win fights like that. Skill/speed/etc also apply. – Ben Barden Oct 10 at 19:01
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    This is the best response so far, imho. @T.E.D I see how you reason about "even bow and arrow tech" as a way to counteract the relationship that mike outlines... however I do not agree with it at all. First of all: all weapons of war from the dark ages needed stamina and strength more than anything... (excluding possibly assinations by daggers and poison). The notion we have from popular media that you can have a strapping thin boy with a boy kill a knight is just wrong. The feared wielders of the english longbow where the most muscular of them all :) – Stefan Karlsson Oct 10 at 21:36
  • oh, and can we all acknowledge, as an interesting side-note: the difference between fullgrown bonobo males and females are about 8 kg. They do not differe much from chimpanzees in size, but are still sometimes called the "pygme chimpanzee", no doubt given the name by alpha-males of the homo sapiens. – Stefan Karlsson Oct 10 at 21:43

I suggest reducing both sizes. In a world where food isn't marketed, manufactured to be addictive, or even plentiful, it might not be unreasonable to have the average woman more like 45 kg. You could then more believably make the average man something like 80 kg.

In a world with very limited technology, there would be a multitude of advantages to men to be big and strong. If women were dependent on men for protection and food, they could well end up mostly petite. Remember that childhood malnourishment stunts growth, and being shorter is a huge head start in being lighter.

130kg is a really large man, and 65kg is not particularly small for a woman. Most 130kg men are very fat rather than very bulky. Even Arnold Schwarzenegger only weighed around 120kg as a bodybuilder.

So let's assume unusually small women and unusually bulky men: Women average 50kg and men average 100kg. This could be achieved without any genetic changes at all, if we assume that all men go through a strict training regime to become bulky, and all women are underfed and small. Girls would be fed as little as possible from birth, and boys would always be fed well and encouraged to fight and be physically competitive. Girls would stay small because they don't get enough nutrition to grow big (they would also be short). Boys who are genetically not able to grow large would often not survive the training regimen.

There would still be some small men and some large women, possibly because their families treated them differently from the norm. But the averages could make men twice as heavy as women.

  • Contrary to his earlier claims, Arnold Schwarzenegger isn't particularly tall. This is why he wasn't all that heavy. – user1751825 Oct 10 at 22:28

It's not very hard to make males big and tall, like Richard Kiel. Just introduce some genetic anomaly which would make males generate much more Growth hormone and you'll have it.

But if you want your males to be very stocky without being fat, that would require a more significant change to human metabolism. To stay at 130 kgs weight, regularly tall (say, 185cm tall) human needs to eat a lot (and it has to be a very nutritious diet) and rest a lot too. For hunters gatherers, such height/weight combination would be detrimental. With more handwaving, another genetic modification may preserve muscle mass (today it quickly goes down with a lack of exercise or less nutritious diet), but as I said, this would be pretty bad for survival.

Men and Women live apart

The most dimorphic (in terms of size) mammals are those that have males and females living apart. Sperm Whales have males in the order of 3 times greater mass than females, male polar bears are about twice the mass of females. In both cases, the males and the females live apart.

Living alone, being able to deal with the dangers in the wild, is easier if you have the bulk to fend off any of the nasty creatures the radiation has created. Killing one of those beasts should also provide food enough for a single male to live off for quite a while. Men would live in the more dangerous regions (for some reason -- perhaps to go to Valhalla or something) while women would live in more favourable regions, where there's no real danger.

  • Counterexample - the anglerfish. The male is on the order of 1/1000 the mass of the female, and latches on to the first female it finds, with most non-reproductive functions (including digestive) withering away. Sort of the ultimate antithesis of living apart. – WhatRoughBeast Oct 10 at 2:59
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    It's not a counterexample, considering he said "the most dimorphic mammals". – Masclins Oct 10 at 6:48
  • @WhatRoughBeast I specifically mentioned mammals because of the anglerfish. – Clearer Oct 10 at 18:07

They may have Gigantism due to mutations caused by the radiation. There are some mutations that can cause Gigantism from early age, resulting in a much bigger and bulkier body in adulthood. If the anterior pituitary gland is damaged by a tumor, the body basically never stops growing.

But this mutation is not limited to men, so you would have at least some women with the mutation and some men without it. Or you need to come up with a clever explaination why women with this mutation and men without it don't survive, thereby driving evolution towards your goal.

There are several problems with this, though. Excess im Human Growth Hormone (which causes Gigantism) often leads to muscle weakness rather than strong muscles. It's also accompanied by Acromegaly, causing joint pain, high blood pressure, impaired vision, Diabetes and reduced sexual function.

One of the most popular people with Gigantism is "André the Giant", who was a professional wrestler.

enter image description here

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    Inconceivable!! – A C Oct 10 at 4:04

My answer is cannibalism.

That solves part of the protein problem. It also explains why big women and weak guys don't make it.

Men fight and eat each other in semi-ritualised hand to hand combat to try to take over the harem of the other guy.

When famine strikes they keep up their strength by eating the women, starting with the biggest.

Ok, that may be a very different story from the one you want to write. But these conditions would over time lead to strong dimorphism.

Sexual dimorphism as you've described could easily occur within a few generations, by males having to fight for access to the females. Weaker, smaller males will quickly be killed off, and larger stronger males will be the only ones able to pass on their genes to the next generation.

Being larger would provide no reproductive advantage for females though, so they would stay pretty much the same size.

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    That’s not how genetics works. The genes for ‘bigger, stronger’ are not necessarily linked to sex, so it’s more likely you’d just see both sexes increasing in size if there’s no advantage/disadvantage to the females being larger. – Joe Bloggs Oct 9 at 16:19
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    Related to this answer, my old question What evolutionary factors can contribute to large sexual dimorphism in large mammals? – α CVn Oct 9 at 17:28
  • @JoeBloggs Yes, of course. I hadn't considered this. There would also need to be some reproductive advantage for smaller women. If larger women were considered less desirable in this society, so were less likely to reproduce, this could happen. However this would take a lot longer than a few generations. – user1751825 Oct 9 at 20:44
  • @JoeBloggs There is always a disadvantage to growing larger as you have to consume more food. This answer describes precisely why evolutionary biologists think males are often larger than females in many mammals, including many primate species. – Mike Nichols Oct 10 at 0:49

While the question is already closed I'd still like to give my version of an answer.

The early days after the apocalypse radiation was rampant. Many people died of radiation sickness. Many were born mutated or disabled. And as you say those born with mutations now are killed off. But in the early days after the apocalypse people gained mutations that they didn't even realize. A mutation that causes the person affected to grow to almost unnatural sizes. Bulky and muscular as you describe.

This mutation affects everyone though. Not just men. But why are women small then? This comes down to X and Y chromosomes. Men have a Y chromosome, which is just an X chromosome with a bunch of information chopped out. This is why color blindness is much more common in men than in women.

While a woman can have a recessive color blindness gene, they can also have a dominant gene that causes this gene to stay hidden and not actually give the person color blindness. But this dominant gene exists on that part of the chromosome that is missing in men. A man just needs this 1 recessive gene in order to be color blind.

Our mutation can work the same way. Women have this dominant gene that offsets the effect of this mutation the majority of the time. But since men don't even have this part of the chromosome the recessive gene affects all men that have it equally.

How would all people in this society get this gene? You only really need it to happen in a few people. And with the majority of humanity being wiped out there's a fairly small gene pool that this gene needs to float through. From here natural selection could favor the men with these genes. The larger men are better able to protect themselves and their families and localized societies. So this gene is preferred as the men with this gene do not die out. And they protect their small community which consists of people with this gene so they don't die out.

You can use this to create some legendary female warrior types from cultural legend and even some current characters because they have this same bulky build as the men in your society.

Malnutrition in childhood is detrimental to height. Perhaps women are thought of lesser importance than men in this society, and always eat last, whatever scraps are left at a table.

Man need to be strong, so they get best picks of food, and eat to the fullest, so they grow big and strong.

That causes the difference in height. Environmental variables are as important as genetics.

I think there are a number of links in this chain.

New food

The most obvious problem is that muscle requires protein. Lots of it, in a form which the body can take up easily. We don't currently have any way of doing this. Hunting has a good protein return, but hunts often don't return a win, and hunting anyway favours wiry hunters with either short-distance speed or long-distance endurance. Even in the animal kingdom, there are no big, bulky hunters. The closest you get are bears, and they're omnivorous scavengers.

So, we need your mutated environment to have come up with a new food crop. It has to have an insanely high protein content, grow prolifically enough that farmers can get high yields without having to work too hard (otherwise we'll be favouring endurance again), and keep well so that it'll tide them over winter.

New environment

Next, we need to encourage muscle-building. Farming is pretty good for this - there's a reason that armies tried to pressgang farmers rather than city kids.

There's a better option though. Back in the day, sailors were the ultimate tough guys. They weren't the scrawny ship rats from Pirates of the Caribbean - every last one was built like an Olympic gymnast, because spending all day climbing rigging will build that upper body strength.

So, let's suppose it's not too safe to live on the ground. Humans are now semi-arboreal - not as well adapted yet as an orangutan, but kind of starting on the way there. Even without major adaptation though, just spending most of your life climbing will give you the kind of muscles you need. Perhaps our new food source grows in the trees? Or perhaps we farm the trees and live in them?

We're only semi-arboreal though. We haven't yet lost our ability to stand upright, and we haven't yet got long enough arms to knuckle-walk, so our legs are still useful.

As an interesting related point, orangutans have a high sexual dimorphism.

Earlier birth

Humans have already evolved for birth to take place well before the child is a viable individual. To be as capable as the average lamb at birth, we'd need more like 3 years gestation. Earlier birth allows for smaller hips, and means the mother is not grossly encumbered by an oversized baby. Many other animals follow the same path, with differing development times. A radically smaller mother could not support a substantially bigger male child - but if the child is born at maybe 7 months then it wouldn't matter so much.

If (male) children get bigger, evolutionary pressures ensure this will just happen. If women don't give birth earlier, they and their son will die in childbirth. Women who give birth "prematurely" will become the new normal.

And with early childbirth, women can get away with being smaller than they currently are. Maybe they'd average 150cm and 50kgs - which only needs the men to be up to 100kgs, which is totally doable.

It's a man's world

Why would women become smaller though? Absent any health or dietary reasons, we're looking at sexual selection.

Suppose the birth rates become skewed, so we only get maybe 1 boy born for every 4-5 girls. This could result in a meerkat-like society where only one chosen female breeds but their relatives help parent their offspring. If men tend to choose the smallest, prettiest woman as their wife, then we'll have sexual selection in favour of small, pretty women.

Why might people follow that model, instead of the harem model? Maybe quality over quantity - raising a few children well and reliably is more successful than giving birth to many children who mostly die. Maybe a matriarchy develops to keep other women in line.

  • Lots of adult males are already more than 130kg of lean mass. For this to become average would just require a couple of generations of selective breeding. – user1751825 Oct 10 at 1:18
  • @user1751825 Lots by numbers, but vanishingly few by percentage. People with dwarfism are outliers on one end of the scale, and your 130kg guys who are that size naturally with no real need for exercise are the outliers at the other end. It needs a lot more than a couple of generations for that, although with enough generations you can start to skew it. Changing lifestyle can help build muscle though, hence my suggestion for environment. And like I said, muscle has a diet cost which can't be met naturally with current foods. – Graham Oct 10 at 6:54
  • Outliers would quickly become the norm, if all other regular males were prevented from reproducing. Consider a society entirely sired by the big Icelandic strongmen. In 100 years, the population of the planet might be comparatively tiny, but the average size of males might be > 130kg and 190cm tall. – user1751825 Oct 10 at 7:07
  • Sorry, but you have too many misconceptions there. First you need to stop everyone else reproducing, in an anarchic world. Then you need strength traits to breed true, which is pretty marginal at the moment. Then you need those traits to give a consistent difference, which even in dogs with 100% selective breeding takes dozens of generations. Then you need children to survive birth, and humans are already pretty marginal there without science to help. And then you need food to sustain these huge bodies. World building needs all this taken care of, not just "let's these guys have babies". – Graham Oct 10 at 8:31
  • @Graham interesting ideas and some good logic (+1), but big males and becoming semi-arboreal seem contradictory - if you look at rock climbers they are wiry types, not Arnie. Squared-cubed means that while a bigger body has greater lifting capacity, it is less able to lift itself, which is critical for climbing around trees without letting go or breaking the branches you are supporting yourself on. (Other problem is that the timeframe is just too short for this evolution to occur given the human reproductive rate, but that is an OP constraint that applies to all answers.) – KerrAvon2055 Oct 10 at 11:10

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