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It just seems like it would be a really cool thing to have gills and be able to populate the oceans in the same way we populate the land. We could have houses and shops and vehicles, andgo on walks/swims and just kind of live underwater.
Start a whole new second species of human here on earth maybe, Who knows?
Yeah, evolving lungs ended up clearing the way to make use of the much more plentiful oxygen in the air compared to what is dissolved in water. Amphibians and reptiles have pretty low metabolisms, but birds and mammals basically evolved endothermy (aka warm bloodedness), probably in support of much higher muscular power output. Ectotherms (aka cold blooded animals) have metabolisms that are correlated to temperature, which means they can't exert themselves as well when it's cold. Endothermy allowed animals to be warm all the time, and therefore use higher muscular power output in any environment, especially sustained.
That means mammals and birds were able to cover more distance, and survive in places where reptiles and amphibians can't, and all the advantages that carries.