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We Didn’t Evolve for This

A lesson from the animal kingdom on why COVID-19 is so deadly to humans.

When a Weddell seal, native to Antarctica, plummets 400 meters beneath the ice on one of its hour-long dives, an ensemble of adaptations…By Claudia Geib

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.

NO AIR, NO PROBLEM: Marine mammals, like this seal, comparative anatomist Chris McKnight says, “are this wonderful model, because they live a life that to us just seems like a continuous physiological assault.” Lacking oxygen for long stretches isn’t a problem when you’ve evolved traits to meet that challenge.Steve Rupp

For the past year, humans all over the world have been facing an intensely hypoxic event of their own: COVID-19. The difference? 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. 

That is what researchers did in a new paper published in Comparative Biochemistry and Physiology.1 They examined what the extraordinary diving abilities of marine mammals could reveal about what humans face when they contract SARS-CoV-2. The answer they found was, overall, grim: The human body has virtually none of the safeguards that protect a marine mammal when oxygen levels get low.

“Marine mammals have shown us that it takes a lot of coordination, from a lot of tissues, to provide all of that protection in an extreme situation,” said co-author Terrie Williams, a professor at the University of California, Santa Cruz. “And the only thing that we humans can do is make sure that we’re not going to be in a situation where oxygen becomes compromising. Unfortunately, that is exactly what this disease does.”

The human body was never built to survive such extreme oxygen restrictions.

The ways that COVID-19 robs human tissues of oxygen is one of its defining characteristics. The virus invades the lining of the lungs, including the alveoli, tiny air sacs that capture oxygen from each breath and quickly pass it into the bloodstream. As the immune system tries to fight off the virus, the lungs and those air sacs become inflamed and fill with fluid, crippling their ability to transport oxygen into the blood. A recent study of patients critically ill from COVID-19 suggests that respiratory failure due to the virus “can be managed similarly to hypoxic respiratory failure” caused by other diseases.2

Many COVID-19 patients also experience abnormal blood clotting, something that scientists and doctors still struggle to explain.3 This clotting can be serious enough to block oxygen from reaching the brain, causing ischemic stroke.4 Brain and heart cells, unlike other kinds of cells, can survive only minutes starved of oxygen before they die.

This cascade of hypoxia reminded Williams—who studies the physiology of both diving mammals and human athletes—of the ways that marine mammals had previously illuminated human health issues. Her research into the hearts of bottlenose dolphins and Weddell seals helped explain a spate of sudden deaths among triathletes as they entered the water at the start of a race: The sudden slap of cold water triggered an instinctive slowing of their hearts, just as they sped up for exercise.5 These findings helped change the format of some races.

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When it comes to COVID-19, however, applying the lessons of deep-diving mammals is not quite as simple. “Marine mammals are this wonderful model, because they live a life that to us just seems like a continuous physiological assault,” says Chris McKnight, a research fellow in comparative anatomy at the Scottish Oceans Institute of St. Andrews University, who was not involved in the paper. “But that comes with its complexity. They’ve had quite a long time to develop those optimal evolutionary traits.”

Even so, some researchers are looking into how those traits evolved to develop treatments. In particular, McKnight pointed to research going on at Duke University’s Cancer Institute. Duke scientists are studying why marine mammals don’t show inflammation in their lungs and other organs when denied oxygen. This protects them from the troubles that human COVID-19 patients have in transporting oxygen to the blood.

Jason Somarelli, an assistant professor at Duke and researcher on this project, explained in an email that his team is studying whether whales may have lost some genes through evolution that allow them to decouple hypoxia and inflammation. If that’s right, it might be possible to one day develop a drug that could artificially turn the same genes off in humans.

“It’s all possible, but part of getting the translation into treatment right is to encourage the biomedical community to pick up the idea that marine mammals may hold keys,” McKnight said. “I wouldn’t imagine there are a huge amount of human biomedical folks whose first stop would be marine mammals as a good place to look.”

To Williams, the lesson from marine mammals is one of caution: They show us just how much evolutionary protection is needed to protect the body from hypoxia, and how few concomitant safeguards humans have. She sees her paper as another way of flagging just how vital it is that people avoid contracting COVID-19 in the first place.


Claudia Geib is a science journalist and editor based on Cape Cod. Her work covers marine and environmental science, wildlife, and how humans connect with the natural world.


References

1. Williams, T.M. & Davis, R.W. Physiological resiliency in diving mammals: Insights on hypoxia protection using the Krogh principle to understand COVID-19 symptoms. Comparative Biochemistry and Physiology 253, 110849 (2020).

2. Hernandez-Romieu, A.C., et al. Timing of intubation and mortality among critically ill coronavirus disease patients: A single-center cohort study. Critical Care Medicine 48, e1045-e1053 (2020).

3. Galiatsatos, P. & Brodsky, R. What does COVID do to your blood? Hopkinsmedicine.org (2020).

4. Szelenberger, R., Saluk-Bijak, J., & Bijak, M. Ischemic stroke among the symptoms caused by the COVID-19 infection. Journal of Clinical Medicine 9, 2688 (2020).

5. Williams, T.M., et al. Exercise at depth alters bradycardia and incidence of cardiac anomalies in deep-diving marine mammals. Nature Communications 6, 6055 (2015).


Lead image: eldar nurkovic / Shutterstock

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