Sexual dimorphism is a strange thing.  Some creatures don’t have it at all and even careful observation cannot tell the difference between the sexes.  Some creatures only have one or two key features that distinguish the sexes but otherwise follow the same basic size and shape.  Other creatures will have such extreme differences that at first glance it is difficult to even tell they are the same species.  Humanity is an especially extreme case where the different sexes don’t even typically occupy the same habitat.

Of course, there is enough overlap between male and female ranges for reproduction to occur.  Women are aquatic creatures while men are terrestrial.  Women will beach themselves for the purpose of reproduction and to hand off male children to the terrestrial men.  Similarly, men will sometimes swim in the shallows and take boats even further from the shore.  However, these are exceptions to a lifestyle that typically has the populations fully separated.

This does leave the species as a whole vulnerable to a catastrophe.  Since as a species they are vulnerable to fluctuations in two different habitats and are completely reliant on the edge effect between them, a slight change in one may spell the end for the species.  However, the species as a whole is highly adaptable through higher intelligence than is typical for the planet.  This has allowed the species to thrive despite a narrow range of favorable living conditions.

Early Evolution

Humans separated from other apes approximately 5 million years ago.  At the time, the species exhibited low sexual dimorphism.  Due to being primarily frugivorous, their vision was highly adapted for color sensitivity.  This allowed them to easily spot ripe fruit among other brush.  Utilizing this visual ability, the females at the time were adapted to demonstrate fertility through engorged vulva that were colored similarly to the fruit they ate.  This adaptation is observed in many other primates and is not an abnormal feature for the clade.

This changed with a climate shift.  The fruit that the early humans relied on dropped in population dramatically, so their diet shifted from benign primarily frugivorous and an opportunistic carnivore, to the opposite.  As they took on a more and more carnivorous diet, they began to adapt to life as a predator species.  A key form that this took was their eyesight being less sensitive to colors and more sensitive to movement.

This, of course, radically changed the sexual selection pressures on the female features.  Their vulva lost its coloration while various features that targeted motion-based vision competed.  The feature that rose to prominence was advanced musculature in the vulva allowing controlled movement.  During the transitionary period, this was used in conjunction with the coloration to attract mates.  However, as the coloration was slowly lost the musculature for complex movement of the vulva became more advanced.

Now, the sexual selection pressures applied to the females encouraged greater complexity with the females using them to perform “dances” in order to attract mates.  This eventually led to the development of small tendrils that became prehensile to allow for even more complex “dances” incorporating small objects.  Over the next few hundred thousand years, these tendrils grew larger and stronger as the species unknowingly approached a new threshold.

Eventually, these tendrils became large and strong enough that they could aid in childbirth.  Now, instead of just being used to attract mates, the tendrils were incorporated into both infant and maternal mortality rates.  Stronger and more dextrous tendrils were heavily associated with better reproductive success regardless of how well the female was able to attract a mate, turning them from a selected for but ultimately decorative feature, to a critical survival feature.

At around the same time, the tendrils became strong and large enough to effectively block off the vaginal passage to unwanted entry.  This placed a greater amount of sexual selection power in the hands of the females.  Instead of simply trying to attract their desired mate, they had an unabridged power to reject unwanted mates.  In some species, such an advancement has resulted in a sexual arms race, but further radical shifts would come to humans before the males could develop any new adaptations.

Taking to the Sea

Several hundred thousand years later, another climate shift struck.  This one drove predators from inland towards the East African Rift Valley that humans were endemic to at the time.  While these predators struggled to take on fully grown humans, their children were vulnerable to them.  So, mothers began fleeing into the sea to protect their children.

These women soon made a home in the reefs and lagoons offshore.  For a time, they relied on the men hunting on land to bring them food.  However, after a few thousand years the sea life adapted to the new climate.  An influx of new crustaceans, mollusks, and fish species recolonized the habitat that the women were living in.  Now, instead of simply hiding in the water, the women could hunt for their own food while protecting their children.

Here, the tendrils provided a new advantage.  They could be used for both rapid locomotion in the water and could be used offensively to catch prey.  This caused a level of specialization in the form of the tendrils.  After a few hundred generations, the specialized morphology became apparent.  

The women had half a dozen bulky and powerful tendrils that were used for propelling them through the water or grabbing onto submerged rocks for rapid direction changes.  They also had four tendrils that were both thinner and longer.  These tendrils were much more dexterous and used for hunting prey.  They had keratin tips that functioned as claws which were evolved from modified pubic hair (the only significant hair structure left on the women at this point).  Finally, there remained a cluster of short and softer tendrils which were used for childbirth and carrying children while swimming.

Of course, during the same period of time, the legs atrophied.  At first, simply becoming smaller and then later adapting into a fin structure to aid steering in the water.  These newly adapted legs were awkward on land and would no longer allow the women to walk.  Instead, when beached they would move through a combination of an ungainly waddle on their bulky tendrils and shimmying on their bellies.

This adaption of the legs did have another benefit.  With the pelvis no longer being needed for walking, it first broadened and then later opened completely to become a smaller structure supporting the new fins.  This meant that the pelvis no long formed a chokepoint obstructive during childbirth, drastically reducing infant mortality.  It also meant that the gestation period could lengthen allowing children to be more developed at birth.  This resulted in girls being able to swim as soon as they were born and boys only needing a brief period of breastfeeding before being weaned and sent to live on land.

By the time the population of predators which had caused them to flee to the sea in the first place was reduced enough to allow children to safely live on land again, the women were no longer able to live on land.

Early Modern Humans

As humans became the dominant species in their area both on land and at sea, they began to spread.  At first, expeditions were treated as temporary by both men and women with the intent to return to the small range of beaches the species had evolved at.  However, a few chance encounters made it clear to both sexes that their other half was spreading as well.  After that point, it became a habit for both of them to check the nearby land or water as they explored to see if their counterparts were there.  In a few cases, a group of explorers simply camped out near a beach for years in the hopes that the opposite sex would come eventually.

Of course, both would soon encounter either islands or bodies of water that the other did not have access to.  A few times, women would try seeding islands with boys so they could eventually have men there.  These experiments were ultimately failures as a lack of instruction on how to survive on land meant they developed into little more than feral beasts that needed to be cared for by the women instead of an independent society.  A few men had similar ideas with taking girls to lakes but were promptly drowned to death by the adult women who saw them trying.

It took the early development of technology to overcome this difficulty.  Early boats allowed men to begin sailing out to sea.  They were quickly guided by the women to various islands or other isolated territories that the women had been aware of for generations but did not yet have men on them.  With this teamwork, it was a very short period of time between the invention of the first boat and humans fully colonizing most of the world’s islands.

Taking inspiration from the boats, some men constructed sleds to carry adult women across the land.  It took some convincing, but eventually, the men convinced some of the more adventurous women to make the trek.  Women using sleds to travel between isolated lakes and island seas was much less common than men sailing between islands, but it would happen occasionally keeping the populations at least vaguely in touch with each other.  Soon, humans were truly spread throughout the globe.  

Not long after the spread, both men and women discovered the concept of domestication and began growing crops and keeping livestock.  Men had an easier time developing technologies due to being able to utilize fire, but women were not shy about developing technologies that did not require metallurgy or fire control.  They also were quite willing to use tools made by the men.

Modern Society

In modern times, human culture has been built around this separation between the sexes.  Men and women live in completely different societies and it is common for a male nation to treat a neighboring female nation as a completely foreign power.  However, even during times of strife, reproductive beaches are treated as sacred neutral territory.  If a third party attacked such a beach, it would be taken as a grave assault by both the male and female nations.

It isn’t uncommon for them to speak different languages.  After all, the men and the women spend the bulk of their lives separated.  Young boys will be handed off to be cared for by the men at a young age before they had a chance to learn language from the women.  Meanwhile, the girls would sometimes not even see a man or a boy older than an infant until their adolescence.  Furthermore, the needs for communication are different on land and underwater.  Several phonemes that are common in languages of one sex are seen as unpronounceable by the other sex.

When communicating between a man and a woman who are fluent in the other language, it usually results in a pidgin that has each using terms in their native tongue and simply understanding the other one with the occasional common term.  However, not everyone develops fluency so the average conversation involves a lot of pantomiming to get their point across.  In more crass situations, words may not be used at all.

Because of the separated nature of their societies, it is common for men to not know who the father of a boy is when he is delivered to their society.  Instead, a communal approach to child-rearing is taken.  Women typically have a somewhat better idea of how they are related to each other but will have gaps in their knowledge when it comes to patrilineal tracing.  As such, societies have little in the way of methods to avoid inbreeding.  Even when consciously aware of the risk, the best approaches are usually to avoid breeding at the beach they were born at and to rely on a sufficiently large population.

Advanced technology has made it far easier for men and women to travel through each other’s worlds.  Men can easily sail boats and ships through female territory as well as using SCUBA and other diving gear to easily swim among the women.  Women can ride in vehicles much faster and more comfortable than the early sleds to traverse male territory.  Of equal importance is the fact that modern plumbing has made access to water much easier, which is critical for the skincare of a people used to being submerged most of the time.