All written history has been lost before a great calamity, a biblical level flood, which has covered all the world's landmasses in devastating amounts of water. Whatever borders there were, whatever alliances, hatreds, all was thrown away in the face of human survival. As the waters continued to rise, swift construction of glass domes commenced. Many of these domes were faulty, and, unfortunately, all animals and man inside were lost.
Those that did survive were forced to look at alternative methods of survival, as the aquatic wildlife became hostile, ramming the glass domes, many more lives being lost as some of these settlements were crushed under the pressure of the ocean. Testing began, an attempt to mix human and aquatic dna, a concept once thought of, and refused for "Actions against God" were now a necessity, if humans wanted any chance of survival.

There were many failures as testing proceeded, unwanted mutations, complications converting water to oxygen through gills, among many other issues. Fast forward many generations, when only a few civilization domes were left, and the first hybrid human successfully survived, a child of German decent, dna mixed with a smooth hound as a developing embryo, an act of desperation, which became the basis of all future development.
Not long after, a dome which housed mainly refugees from Hawaii and Jamaica broke ground with similar tests using Moon Jellyfish dna. This, however, had complications, and was altered by mixing different breeds of Jellyfish dna.
Off the coast of Asia, Korean, Chinese, and Japanese scientists achieved the same results with Manta Rays. Africa, however, achieved, possibly, the most interesting results. Taking to legends of the fallen Atlantis, they sought to recreate the Mermaid....and succeeded.

Hundreds of thousands of years later, and the glass domed civilizations lay empty. All hints of their former humanity now a distant memory. The different aquatic humanoids lived, fairly peacefully, all sharing a similar origin story. The oceans rose, and life returned to the sea. This lasted some time, till, as it does, history began to repeat itself. The oceans receded, and giant landmasses broke the surf. This development was the driving force between two different events, a global war for oceanic supremacy, as well as creatures returning to the surface, and their development.
The Triton were the first race to build an army, claiming the territory in the southern half of the world, The Jewahi claimed the western oceans, The Sharkish, the north, and the Mantay, the east. The first to rise against the Triton were the Jewahi, defending their seas ferociously. As they were pushed back, they seeked refuge with the Mantay, who had been fighting against Sharkish missionaries. Together, they defended Mantay lands from both fronts.
However, the Triton quickly proved their superiority, merging the religious Sharkish into their ranks, leading an assault on the Mantay, giving the Jewahi a proposition they had no choice but to accept. Turning away from their Mantay benifactors, they closed their eyes as innocent Mantay were slaughtered.

Now with only pockets of resistance, the Triton placed their banners the ocean around. Mantay are rare, due to their race never finding a true home after the aggression against their people. Sharkish happily travel with packs of Triton, shepherds of flocks of fish, eating their choice before the Triton distribute whats left to the rest of the ocean's inhabitants. The Jewahi, due to their agreement, and shame, never crowned another queen, and became the working class.
Still, even with reduced numbers in the ocean, food was scarce, and eyes turned to the large landmasses that had sprouted up and out of the ocean. Slowly, but surely, life expanded it's reach to the shores. However, what greeted them was unexpected. Civilization was already growing on the continents. While the oceanic dwellers fought, and died, the creatures that had found land before them had evolved naturally, some even expanding out into the stars.