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> An amazing adaptation to fighting off anoxia on deep dives:
> 
> http://nautil.us/issue/94/evolving/we-didnt-evolve-for-this?mc_cid=cc0dd336e4&mc_eid=6d09ed0f17
> 
> "When a Weddell seal, native to Antarctica, plummets 400 meters beneath the ice on one of its hour-long dives, an ensemble of adaptations come together to keep it alive. The seal’s heart rate slows. At this pace, it will burn through its deep reserve of oxygen—provided by extra-large volumes of blood and hemoglobin—more slowly. The seal’s muscles free massive stores of trapped oxygen from another protein, called myoglobin. If oxygen levels become deficient in its tissues, causing hypoxia, cells can use the high levels of the sugar glycogen stored in its heart and brain to begin anaerobic metabolism, creating energy without oxygen. The seal’s extra-large liver also holds its own store of oxygen-rich red blood cells, like a backup scuba tank. And as oxygen levels plummet well below levels that would leave a human diver unconscious, fine control of the veins that oxygenate the seal’s brain cells allow it to swim on unaffected. Together, these systems ensure that the seal survives these intensely hypoxic events again and again, dive after dive, for the many decades of its life."
> 
> ***
> 
> The human body was never built to survive such extreme oxygen restrictions. That fact becomes especially stark when you compare humans to diving marine mammals. 
> 
> Comment: Deep diving  humans have a small set of changes like the seal, bu t haven.t had the centuries of adaptation time the seals had.