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A group of behaviorally modern humans was cut off from the rest of the earth-like planet. At the time of the separation, humankind had reached a technology level including cattle ranches, brass, writing, and cities. (Same cataclysm, different ark.) About four millennia later,* they were contacted again, but all of them were (or at least looked and acted) female.

I can think of three ways this might have happened:

Hermaphroditic reproduction

It takes two women (and no man) to reproduce.

Some vertebrates are sequential hermaphrodites, beginning as one sex and becoming the other later in life. These include clownfish (cue the "Finding Nemo is doing it wrong" nitpickers) and frogs, which inspired Ian Malcolm's famous line in Jurassic Park: "Life, uh, finds a way." Among mammals, some species of mole vole (Ellobius) and spiny rats (Tokudaia) have two X chromosomes, with sex determined in some unknown manner.

Males that look like females

Hyenas are male and female, yet females produce huge amounts of T at puberty, and they're dominant over the submissive males. Feminization would invert this, with males becoming feminized with things like gynecomastia. Male lactation isn't unheard of in the real world; it's seen in Dayak fruit bats, occasionally in goats, and even in humans suffering from galactorrhea.

Males hidden away

Instead of diminishing to near nothing, the sexual dimorphism has become so extreme that only females appear in public. It need not be as severe as the difference between male and female anglerfish, but a pile of sex-linked genetic disorders becoming fixed might cause the appearance of a man to become shameful.

So which of these options is the most biologically plausible and why? Or what other mechanism could make all individuals look female to an outsider contacting this race for the first time?

* This would have to be a rapid change to happen within a 4 ka timeframe, but this timeframe is negotiable. A separation after emergence of humanity but long before the Bronze Age is also acceptable, as the separation date of 4 ka before present may be mythical.

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    $\begingroup$ Definitely see this movie. Post-apocalyptic with all females, and made to make sense :) $\endgroup$ – Mołot Feb 15 '16 at 14:34
  • $\begingroup$ Yup, really funny movie, worth to see. $\endgroup$ – Peter M. - stands for Monica Feb 15 '16 at 16:51
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    $\begingroup$ Too many answers explore theoretical possibilities, while parthenogenesis - form of asexual reproduction in which growth and development of embryos occur without fertilization, which currently exist on our planet, is ignored. $\endgroup$ – Peter M. - stands for Monica Feb 15 '16 at 16:57
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    $\begingroup$ One word: parthenogenesis. They are all female. $\endgroup$ – 458 Feb 16 '16 at 3:53

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Once you reach a certain technology level males are essentially obsolete. Testosterone and increased physical prowess is irrelevant when all combat comes down to ranged weaponry and engineering. Beyond contributing DNA we are not at all relevant to reproduction either.

You would not need to advance beyond our current technological level much to be able to take stem cells, convert them into sperm cells, then inseminate an egg with them.

So you have a hypothetical future society where, couples (or singles, or multiples) form a relationship and decide to have children. Sperms are generated from the paternal partners (or taken from DNA banks) mixing their DNA appropriately and selecting for compatibility, desirable attributes, etc. (Of course they always select for female at this point too), the egg is artificially inseminated then implanted.

This results in a biologically human (but heavily selected for intelligence, attractiveness, etc) entirely female population.

It would be theoretically possible to have a male baby but if someone did that it would most likely be considered shocking and the child would cause a stir wherever he went.

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    $\begingroup$ Additional information: Impregnating a woman with artificial sperm generated from the genome of another woman would always result in a girl (no y-chromosome), so there is no moral dilemma about what to do with any male babies. It would even be possible to impregnate a woman with artificial sperm from her own DNA. However, it might not be a good idea because the genetic detorioration would be even worse than brother/sister incest. Another option would be cloning (every daughter would be genetically identical to her mother). This was already done with female mammals, so the technology exists. $\endgroup$ – Philipp Dec 4 '14 at 8:30
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    $\begingroup$ Just because I feel the need to support our half of the race, if you think about it, once you reach a certain technology level, females are essentially obsolete. Consider the Azgard in SG-1, which relied entirely on cloning for procreation. $\endgroup$ – Cort Ammon - Reinstate Monica Dec 6 '14 at 23:18
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    $\begingroup$ @CortAmmon It's easier to make artificial sperm than artificial womb, males become obsolete long before the females (obsolete for reproductive purposes that is, and there's more to humans than baby making). $\endgroup$ – Babika Babaka Aug 25 '15 at 9:52
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    $\begingroup$ @SpaceLizard - "It's easier to make artificial sperm than artificial womb". Are you speaking from experience? If not, please provide a justification. $\endgroup$ – WhatRoughBeast Feb 15 '16 at 17:29
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    $\begingroup$ @WhatRoughBeast Reproduction between two female mice, and artificial sperm has been done with our current technology (2 different experiments, by 2 different labs), and we don't have artificial wombs yet. That's why I assume it's easier. tinyurl.com/z8hjrvt tinyurl.com/jk47v9r $\endgroup$ – Babika Babaka Feb 15 '16 at 19:34
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I would go the way of either parthenogenesis or radical feminism.

Given the dogma:

  • Most of the wars are begun by males
  • Male reproductive hormones also lead to aggressiveness
  • Thus, all males are responsible for wars
  • Thus, if there are no males, world will be better place

You can build all-women society using today tech by freezing the sperm of males and breed only women by using in vitro fertilisation

If you are gone with sperm reserves, you can breed males in controlled area for more sperm

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  • $\begingroup$ I remember seeing a future society like this on 'Tales from the Crypt'. The women tried reintroducing a man. $\endgroup$ – Jesvin Jose Dec 4 '14 at 7:52
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I'd go with religion. If males are redundant for things other than reproduction, ie war is either non-existent or not dependent on physical size and strenght or heavily ritualized, it might make sense for the gender ratio to be skewed widely in favor of females and males to be hidden.

Details could vary widely, but there are some basic considerations that probably would be nearly universal.

The civilization would necessarily be matriarchal. This would imply low importance on farming, hunting, mining, fishing and other forms of sustenance that are typically male due to absences from home and family. Although that is mostly relevant in the early stages, if society is already heavily matriarchal, women will simply do the traditional male jobs. These divisions are more about some things being more convenient for males than women being incapable of doing them, so if there is a high inconvenience factor with using males, matriarchy will be stable enough.

Civilization would probably also be entirely matrilinear, with males used as breeding stock, and traded between different lineages. This is actually relatively "realistic" way of getting practical selective breeding with humans. Only the sons of notable women or men who do well in competition would be interesting for trade. And with "realistic" I mean it would be much more likely to work than the eugenics programs proposed in early 20th century, as selecting based on good performance is fairly close to how selective breeding with animals and plants has worked just fine.

There would be a mechanism for killing off excess males. Either the sons of "low-status" females would be ritually sacrificed soon after birth or males would engage in some form of ritual contest with lethal consequences to losers. Or both. Easiest contest to set up would be ritualized warfare. Either roman style gladiatorial games or aztec style flower wars (or whatever they were called) would work just fine if ramped up enough. Most likely would be starting with sacrifices and then adding ritualized warfare when civilization gets more affluent. Affluence adds both ability to raise "useless people" and demand for entertainment. Gladiatorial combats used to be quite popular.

If most males were killed soon after birth and the rest held in secure locations due to being either valuable breeding stock or valuable future gladiators, the society would for all practical purposes be "all female". For that matter as the sacrifices, gladiatorial combats and even breeding would likely be sacred religious rituals not accessible to non-believers, it would be difficult for outsiders to see evidence of males even existing.

Interestingly, there isn't really any need for a religion like this to be unusually malicious or brutal. (The "bad stuff" is contained and separated from normal life.) So discovering this side of their new friends might be a shock to outsiders.

The difficult part would be explaining how such society is created. But if the initial seed colony has high enough gender ratio, people might simply decide to keep it that way. Especially if there is some external factor (read famine) forcing very difficult decisions. This would actually fit with the lack of farming.

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TimB's answer is the most comprehensive, but I wanted to add another possibility from a book I recently read, 2312 by Kim Stanley Robinson:

A Gender Revolution

In that book, gene therapies and the biological sciences have advanced so far (and frankly it's not that far from today) that people can cosmetically alter their primary and secondary sexual characteristics easily. Many, if not most, people end up experimenting with both fathering and mothering a child at some point in their lives, and many maintain both sexual organs (for example, enlarging the clitoris into a small, functional penis) so they can play either role as the mood strikes them.

It's not hard at all to imagine that a society like that might eventually tip the scales toward people preferring one gender over the other, and as TimB pointed out, it would almost certainly be female. What would result is a society of entirely, or almost entirely, female-looking citizens, but whose reproductive organs weren't the binary "male-or-female" options we (for the most part) have now, so the species could easily continue to reproduce.

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    $\begingroup$ "The Culture" by Iain M Banks has a similar setup, where the inhabitants are genetically modified to the point where they can just choose their appearance/gender/etc and their body will adjust accordingly over time. $\endgroup$ – Tim B Nov 11 '14 at 18:34
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Feminizing pollutants. Currently changing fish female in our oceans. Going to require an advancement in your technology, however.

Wolbachia bacteria that affect humans, rather than butterflies.

Females with XY genes: complete androgen insensitivity, or CAIS. Will result in a lot of 'females' who don't have wombs/ovaries. In theory, this should be bred out of a population by remaining fertile men who throw resistant offspring; and thus will take over the population dynamics.

An aside: If you do keep males, you're going to have to make them much wider-ranging. You can fight inbreeding in 95% female populations (like green turtles, due to climate change) by having males range a lot. Each male gets a ton of mating opportunities, but no local overlap.

http://www.sciencedaily.com/releases/2012/01/120124200106.htm

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    $\begingroup$ You are throwing around a lot of interesting elements without explaining what they are. You could improve your answers by adding explanations or adding links to where they are explained in detail. $\endgroup$ – Philipp Dec 4 '14 at 22:37
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A world almost exactly as described in the original question has been constructed in Herland by Charlotte Gilman, back in 1915.
The only real difference is that in the novel the society is cut off for about 2000 years instead of 4000

High in the mountains of South America, a group are cut off by geologic upheaval, and through social uprising the remaining men are killed. The book describes in great detail their world, seen through the eyes of 3 male visitors, and covers how they reproduce (parthenogenesis), as well as the unique social structure they have developed.

You can read it here http://www.gutenberg.org/ebooks/32

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Funny that no one mentioned real answer, which allows for society of pure females could exist, even today, on our own Earth.

Parthenogenesis - form of asexual reproduction in which growth and development of embryos occur without fertilization.

It occurs naturally in many plants, some invertebrate animal species (including nematodes, water fleas, some scorpions, aphids, some bees, some Phasmida and parasitic wasps) and a few vertebrates (such as some fish, amphibians, reptiles and very rarely birds). This type of reproduction has been induced artificially in a few species including fish and amphibians.

So all you need to do is to invent one mad scientist/biologist who will figure out how to induce parthenogenesis in humans.

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  • $\begingroup$ I wouldn't call your examples "society". $\endgroup$ – MakorDal Feb 15 '16 at 16:42
  • $\begingroup$ It depends on your (and OP's) definition of "society". If society is a group of individuals who live in certain ecosystem, why you would not call such group "society"? What is missing? $\endgroup$ – Peter M. - stands for Monica Feb 15 '16 at 16:50
  • $\begingroup$ @PeterMasiar What is missing from the examples is sapience. Unlike fish, amphibians, and reptiles, humans and other mammals have genomic imprinting, where only the copy from the mother or father is expressed. And wouldn't several generations of parthenogenesis lead to inbreeding depression? $\endgroup$ – Damian Yerrick Feb 15 '16 at 18:36
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    $\begingroup$ @DamianYerrick - if genes from missing parent are not necessary, genomic imprinting (silencing genes from father, IIUC) will be NOT a problem. Inbreeding will also be NOT a problem, because organisms created by parthenogenesis have only a single parent (mother), exact genetic clone of the original. Problem might be only adapting to changing external conditions (which sexual recombination of genes increases). Are not birds sapient enough? Enough for me. $\endgroup$ – Peter M. - stands for Monica Feb 15 '16 at 18:54
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In H.G. Wells' The Time Machine, humanity (at least the above ground portion of it, or the "Eloi") appears to be entirely female. He suggests (explicitly or implicitly, I don't remember) that they reproduce asexually. So there is Wells' idea of most biologically plausible ;-)


Note: Even though this is just anecdotal evidence, I believe that this is a good answer because H.G. Wells is known to stick to science as much as possible -- from Wikipedia: "Wells's earliest specialized training was in biology," for whatever that's worth.

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  • $\begingroup$ This started as a comment, but I realized that it could really be an answer....though from a little bit of an unconventional source. $\endgroup$ – Shokhet Nov 11 '14 at 4:57
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    $\begingroup$ Wells also allowed longer than four thousand years for this to develop (about 800,000 years actually). Certainly such a thing is possible, but I don't think it would happen within the next four thousand years. $\endgroup$ – Brythan Nov 11 '14 at 5:13
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    $\begingroup$ @Brythan Hmm....you make a good point. ( Between you and me, I missed the 4K clause of the question ) $\endgroup$ – Shokhet Nov 11 '14 at 5:15
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Look at how the ovum develops: In addition to the ovum three additional bodies are produced with 23 chromosomes. These are all discarded and die. Extrapolation from lower animals suggests that 1 human child in a billion should be the result of such self-fertilization although no examples of this have ever been discovered.

The result of such a fertilization is either a clone of the woman or what amounts to incest with herself--with the high genetic risk this entails. There are no Y chromosomes around, all the children are female.

To get a purely female civilization: They suffer some major calamity. All the men are killed, the women are pushed to the brink of starvation. In their desperation they discover that a certain plant is actually edible, after things return to normal it is discovered that consumption of that plant greatly increases the rate of such self-fertilization. (Nobody else has discovered this because the plant is known to be poison--but it actually is only poisonous to men.)

You get an all-female society that eat the plant in question when they want a child.

This requires a plant with some very particular properties, it doesn't require technology. I think it fits what you're looking for.

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I don't think that your first two options are practical. All the examples of female creatures developing functional male organs occur pretty far from humans. This would not happen naturally with any likelihood. Also, it's not evident that the result would look female which is what I understand your goal to be.

You could naturally have men evolve to look more womanly if there was some reason for it. Finding a reason might be difficult but is not impossible. The problem is that it would take more than your allotted two hundred generations to accomplish unless your reason was extremely good.

The third option is quite practical in terms of the science. The question is why it would be true. I'm not convinced by your men becoming shameful option. If men's appearance changed it seems unlikely that it would cause them to become hidden. However, if men became rare (possibly through a disease that impacts men more than women), then women might hide their men from the other women to keep them safe.

I would also suggest a fourth option of parthenogenesis. This seems likely to become possible within the next hundred years here, so it could happen in your world. Again, men are struck by some disease that kills them while women are simply carriers. Scientists then have to figure out how to mix the DNA from two women to produce a functioning fetus. The problem with this is that all reproduction would need help in this society. There would be no natural children. Note that cloning is also an option here if you prefer that to parthenogenesis. You could even have both.

To summarize, the two most likely options as I see them require men to be either drastically reduced or eliminated by disease. In the first case, men might be hidden away because they are rare. In the second case, science would have to find a way to reproduce without men.

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I could see it being a very big social movement. If we had gone down a path that didn't put women at a lower place, but raised them up. Many women want to be treated fairly like men (of course!), some act like men or dress like them to make it easier to be treated that way. What if it was reversed?

We already have plenty of men and boys that dress and/or act feminine. (emo?!...) If it was a much larger social push it might be a lot more common thing, men might have electrolysis to remove facial hair etc. Of course this would likely be a 'temporary' social movement but it could be in full swing when contacted again

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Many of the physical features associated with a feminine appearance, like protruding breasts, exist because of men and are in no other way helpful for survival (breasts are helpful, but their current size in humans is massively more than what is biologically necessary for holding milk and feeding young. Also they are present even when a women isn't lactating). Whatever option you choose, your resulting women must also be lesbian ( and find the current-species feminine secondary sexual characteristics attractive) or any option that relies on change over evolutionary time will also remove a lot of what is currently considered a 'feminine' appearance. The technologically-performed gender revolution and the secret male options thus are the ones which are viable. However, you mention that the persons in the society 'act' female. This is not really possible in the revolution scenario as over 4 thousand years of time cultural fragmentation is all but certain, and what gender ("acting feminine") means is largely a cultural construct. 4000 years of time passed with the technologies you are talking about and ample space and, consequently, resources should result in populations spread over most of the globe.

In order for a process to end up with the results you want, we need a society that:

lasts 4000 years or eliminated all other differing societies in the past

breeds and has bred women to look 'feminine'

Culturally enforces gender roles for women, despite having no (visible) men.

Of the options mentioned, I think the hidden males option is the most likely to work, but the implementation you suggest is unfeasible as it would result in male mate choice not being a selection factor in breeding. Culture is a very strong force, and a cultural ban against looking male in public would have the same effect.

Humans also all like the look of attractive women, whereas only females like the look of attractive men (Homosexuality notwithstanding). Thus a culture that encourages men to look and act like women so we can all get along is fairly plausible.

Alternatively, women have been historically oppressed by men in many societies and some humans in behaviorally modern society seem to seek to restore the former, strictly patriarchal, gender paradigm. Given the cataclysm you are describing it is possible that such a former minority might take control and lead the society into a sort of anti-Feminism. Provided the male leadership secures the means of production and technological research, they could probably keep the female population under control indefinitely, assuming they aren't opposed to infanticide to keep the gender ratios optimal and kill off any males with pesky egalitarian notions. One of the horrors of modern technology is that it allows power to be concentrated easily in the hands of a few, so while such a distopia would not be possible in the Stone Age, it would not be terribly difficult with Post-Industrial Revolution technologies. Because the males in such a society would originally represent a much smaller percentage of the population than the female and would need to maintain such a ratio in order to keep the male population from benefiting from female choice, and because such a society needs constant technological advancement in order to stay incontestably ahead of the females' attempts at rebellion, and because only males can be trusted to serve as scientists, political leaders, military brass, and other positions capable of overthrowing the patriarchy, and because direct interaction allows the possibility of assassination or lynching, the males of such a distopia would not be publicly visible.

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  • $\begingroup$ I don't really think that one can say "only males can be trusted to serve as scientists, political leaders, military brass". This is really sexist. Females are as much apt as any of those as any male, principally on a more advanced world where muscular power means nothing since almost all combat is ranged and most work can be automated. Really, this answer is really disrespectful regarding to females. $\endgroup$ – T. Sar - Reinstate Monica Mar 5 '15 at 11:23
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    $\begingroup$ @ThalesPereira While I can see where you are coming from I think you misunderstood the answer here. In the theoretical world being proposed with a small elite of males ruling the world then in that case the males would potentially only trust other males in the "sensitive" jobs. The answer says nothing about ability, just about a theoretical highly sexist and patriarchal social setup. $\endgroup$ – Tim B Mar 5 '15 at 11:50
  • $\begingroup$ @TimB Maybe you are right... I will give it some more thinking. $\endgroup$ – T. Sar - Reinstate Monica Mar 5 '15 at 12:02
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It has been suggested that the male-selecting Y-Chromosome is gradually diminishing in size, and that in the future, it will disappear (It has happened in some species, but it is debatable if it could happen in humans).

One possible, and potentially most likely outcome for humans in which the Y-chromosome has disappeared and no other gender-selection mechanism replaces it is for hermaphroditism - there would be one gender with both a penis and a vagina. With a only a little bit of evolution, one undifferentiated foetal gonad could descend and could become a single testis, while the other remains internal and becomes an ovary, or perhaps (and less likely), the foetal gonads could each split in two, the two lower halves becoming two testes and the two upper halves becoming two ovaries.

The appearance of a hermaphroditic human would most likely be essentially feminine, with a penis, as mate selection would likely be based on potential for motherhood, and there is little evolutionary pressure to diverge from the human female ideals that evolved in two-gendered humans.

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  • $\begingroup$ If there is a part of the masculinisation process that suppresses breast development, separate from the part which determines the formation of genitals, then the result could be males with a largely female body plan. Note trans-sexualism: it is surprisingly common for a person to be born with a female brain in a male body or vice versa. Presumably the masculinisation of brain and body are separate pathways. One or the other can independently go awry. $\endgroup$ – nigel222 Feb 15 '16 at 18:24
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Look up whiptail lizards.

These are all-female lizards that evolved from regular, two-sexed lizards, and experienced a mutation that allowed them to reproduce through parthenogenesis. However, they retain vestigial sex drives, and perform 'mock mating' that stimulates ovulation, typically switching between male and female roles.

Yes, they are literally an all-female lesbian lizard species.

Parthenogenesis has its downsides (not much genetic variation, so they are basically all clones descended from a single mutant) but in a sufficiently stable environment and with a particularly robust founding individual it could work. And if it could work in lizards, there's no reason it couldn't happen in humans*, especially if there's a genetic bottleneck involved.

If there are a few mutations that occur over time, leading to differences in, say, hair color or personality, and some social mechanism celebrating diversity and ensuring that each population has members from each 'stock', you can even avoid the issue of them all looking identical - although there will still only be a handful of 'types', with each 'type' being genetic clones of each other.

*EDIT: Actually, this isn't true. Although mammalian females have all the chromosomes needed to produce another female, mammalian egg cells are missing certain proteins that are transmitted by the sperm, so even if, through some mutation or another, the egg self-fertilized and activated, the embryo would never develop. You could probably pull it off with sufficiently advanced genetic engineering, though.

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  • $\begingroup$ Well... regarding your last point, do you have any references to it? I'm not a biologist, but as far as I know, the only difference between the genetic code carried by the sperm is the potential Y chromosome, needed for the development of males. Could you add the source of that information regarding those missing proteins? $\endgroup$ – T. Sar - Reinstate Monica Feb 16 '16 at 10:38
  • $\begingroup$ From the Wikipedia page on parthenogenesis. "During oocyte development, high metaphase promoting factor (MPF) activity causes mammalian oocytes to arrest at the metaphase II stage until fertilization by a sperm." It also mentions cases where the necessary processes are mimicked artificially, which has resulted in successful development. I'm not sure if there have been any recorded cases of a mutant mammal, naturally occurring or otherwise, that could induce parthenogenesis naturally, but it shouldn't be impossible to make one. $\endgroup$ – IndigoFenix Feb 21 '16 at 11:40
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A biological warfare accident, retrovirus mutation, nanotechnology accident, bioengineering, or GMO accident could result in making human Y chromosomes inviable.

Or GMO effects might result in Y chromosome effects being moves to other genes, which might (?) result in female traits developing even in men.

Anything that results in men having female hormones in their system, and/or a lack of male hormones, will have some female rather than male traits occur in men, including breasts, no male-pattern baldness, and reduced facial/body hair, as well as altered behaviour (e.g. aggression and libido), and a wide range of other effects. You can get a lot of details on this by reading up on real-world transgendered people receiving hormones of the other gender.

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The genders simply slowly drift into appearing more similar even though under the hood the remain basically the same. Do to cultural preference or genetic engineering, female breasts reduce in size and their hips narrow slightly. Males become shorter on average and slimmer, while baldness and body hair all but disappear.

Long hair becomes fashionable to both genders as does androgynous dress - probably what we currently consider female; both because that's the direction you're looking for, as well as masking the person's silhouette.

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