FireFox wrote:
Could we add some sort of gills to humans that would allow us to breathe underwater?
You mean like surgically attaching them? They would have to be extremely large to accommodate the oxygen needs of a mammal that size (there's a reason gilled animals are pretty much exclusively cold blooded) and there's not nearly enough room at the back of the mouth to place them so probably not.
FireFox wrote:
Could lungs and gills coexist or must an organism have either one or the other?
Amphibians have both so its possible but gills tend to be hard to keep from drying out.
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One pill makes you larger
And one pill makes you small
And the ones that mother gives you
Don't do anything at all
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"White Rabbit" - Jefferson Airplane