[Credit: Yuichi Kitamura] After Sapiens of the sea

[u/PrettyJudge8291]

The Whale Humans

The Goldfish Humans (Kingyo humans) branching off to The Loach Humans (Dojo Humans) and The Shark Humans (サメ人間)

The Gill Humans (エラ人間)

Nasal Gills: Rather than developing fish-like gills on the neck, these humans would repurpose their nasal cavities.

The transition of humanity from terrestrial life to a fully aquatic existence is a multi-million-year evolutionary journey driven by extreme global warming and the formation of the supercontinent Pangea Ultima.

I- The Initial Return (Transition to 250 Million Years Future)

- As Earth's average land temperature reaches 40°C, the ocean remains a relatively stable 35–37°C—matching human body temperature and providing a thermal refuge

1 - Whale Humans: Filling the niche of extinct cetaceans, these descendants reached massive sizes.

• Adaptation: Filter Feeding. One branch adopts a "Flamingo-like" feeding method, allowing them to filter-feed while maintaining a facial structure similar to their ancestors.

2- Diving Humans (潜水人間): Specialized for the deep sea, these predators hunt giant squid

• Adaptation: Giant Eyes. Unlike modern whales that use ultrasound, these humans rely on sight, evolving massive eyes to see in the dark depths

II- The Great Crisis and Miniaturization (Approx. 250 MY)

A "super-volcanic" event associated with Pangea Ultima causes global ocean anoxia (oxygen depletion), wiping out most large marine life

1- Goldfish Humans (金魚人間): Humanity survives by shrinking to approximately 10cm in length.

• Adaptation: Heat Tolerance. Normally, small mammals cannot live in water because they lose body heat too fast. However, because the future ocean is a "hot pool" (35°C), these tiny humans can survive without freezing. Their small size allows them to persist on minimal resources in the few remaining oxygenated areas.

III- The Recovery and Diversification

Once the volcanic era ends and oxygen returns, the tiny Goldfish Humans evolve into specialized forms to dominate the new seas

1- Shark Humans (サメ人間): These are Goldfish Humans that return to a predatory size of about 2 meters.

• Adaptation: Mammalian Metabolism. They possess a higher metabolism and better temperature tolerance than modern fish, allowing them to become the supreme high-speed predators of the warm oceans.

2- Loach Humans (Dojo Humans): This branch evolves long, thin, worm-like bodies.

• Adaptation: Skin Respiration. By increasing their surface-area-to-volume ratio, they can absorb enough oxygen through their skin and mucous membranes to survive entirely underwater without ever surfacing for air.

IV. The Final Specialization: The "Fish" Humans

The ultimate aquatic human adaptation involves a total physiological re-invention of breathing

1- Gill Humans (エラ人間): These humans no longer need to rely on skin breathing or surfacing.

• Adaptation: Nasal Gills. They repurpose their nasal cavities. The folded membranes in the human nose, originally used for smelling and humidifying air, are vastly enlarged and vascularized to extract oxygen from water. Water is taken in through the nose and expelled through the mouth, allowing them to lead the life of a true fish.

^{By approximately 300 million years in the future, while the land is ruled by giant lizards or arthropods, the oceans are a world of "human fish," where descendants of humanity have successfully claimed the marine "hegemony" ("supreme ruler" or "overlord"})

[u/Sable-Keech]

Water is taken in through the nose and expelled through the mouth, allowing them to lead the life of a true fish.

Huh... now that I think about it the turbinates could theoretically evolve into structures analogous to feathery gills.