Why didn’t whales develop gills?

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If evolution is real, why didn’t whales develop gills?

Many reasons, but one of them is gills aren’t really that great.


Gills

Gills allow an animal, in this case a fish, to exchange blood gasses with water. This is great, but water has low oxygen content, only 1 mg/L, or 0.1%. By contrast, air has about 22% oxygen which works out to approximately 280 mg/L. As such air allows the creature to access hundreds of times more oxygen, which whales need for their higher metabolic rate.

That’s the reason why whales are larger and more active than fish. They can afford to be, because they can access atmospheric oxygen, fish can’t. Gills that would meaningfully improve on that on a whale would be larger than the animal itself, so gills have no function for a whale anyhow.

This is on top of how a whale developing gills would be irreducibly complex. Animals don’t just spontaneously develop something that’s useful in a final form, every step of the way would have to be better than what came before and that just doesn’t work for gills on a whale in any concievable way.

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