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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?
Human embryos do have immature gills, but they are reused to create ears, jaw, tonsils, thymus, parathyroids and the large arteries in the neck and upper chest. We could add extra pairs and try to turn them into actual gills, although that would require removing the aortic arch and forcing all blood through the gills. Connecting them to the pulmonary system is not possible, as lungs and gills need different pressures (that's the whole reason we have two circuits). Maybe we could connect them in parallel to the aorta? That would only work as a backup, but with an adequate vasomotor system could do a nice job!