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Charles Darwin’s five-year tour of the Southern hemisphere on the HMS Beagle let him see the natural world in a new light, helping him develop his theory of evolution by natural selection. But it didn’t provide any insights into a much more personal mystery: what made him so ill.

Darwin spent his entire life searching for relief from an endless litany of recurring symptoms. And while his condition improved at least somewhat towards the end of his life, he never figured out what made him so sick so often. That puzzle has fascinated scientists and doctors in the decades since.

They’ve combed over the symptoms and have come up with more than 40 different diagnoses. And while there’s still no consensus, there are a couple plausible answers, including several that suggest he got an unfortunate genetic hand dealt to him by his mother. Darwin kept extensive health journals, applying the same meticulous note-taking he used in science to his own health.

The detailed documentation of his symptoms has made diagnosing his condition a common challenge in the medical community. But Darwin suffered from such a wide range of problems that it seems almost impossible to pinpoint a single, root cause. For example, he was said to have a “weak stomach” because he was so quick to puke.

And he had bouts of fatigue that went way beyond exhaustion—he’d spend days “most terribly knocked up,” as he phrased it. But at various times in his life, he also experienced headaches, severe skin conditions, chest pains, trembling and numbness in his hands, dizziness, shivering, visual disturbances, backaches, psychological symptoms, and more. One of the earliest guesses was that his famous voyage was to blame, as he may have contracted.

Chagas disease—a parasitic infection spread by bug bite. But the idea has been strongly rebutted because no one else on the HMS Beagle got it, and. Darwin didn't show the classic physical signs of disease, like an enlarged esophagus or colon, in any of the doctor examinations he received throughout his life.

Also, Darwin experienced some of his symptoms before he set sail. Other doctors have proposed that his troubles were psychosomatic: physical symptoms arising from mental illness. He did have anxiety and depression, and some of his episodes coincided with periods of stress.

But many didn’t, and most experts say mental illness alone can’t explain all of his symptoms, especially given the severity and sheer number of problems he had. One diagnosis that fits pretty well is a poorly understood condition called CVS, which stands for a medical condition called , not the pharmacy chain. This is exactly as unpleasant as it sounds: sufferers experience periodically recurring bouts of severe nausea and vomiting, which is definitely something “weak stomached”.

Darwin was all too familiar with. CVS patients have a higher than normal chance of anxiety disorders and depression, migraines, and certain nervous system disorders that might explain Darwin’s mysterious episodes of numbness, partial paralysis, and distorted vision. They also tend to have severe motion sickness, which describes Darwin’s experience on the.

Beagle to a tee. It’s hard to believe the poor guy spent a year and a half of that five year voyage at sea. Another point in CVS’s favor is the fact that patients often seek out water during an episode—either drinking it in excess or spending long periods of time in baths or showers.

Doctors aren’t sure why this helps, but more than half of patients report it eases their symptoms. And Darwin, too, seemed relieved by water: he noted improvement when treated with a popular remedy at the time called the “water cure,” which involved cold foot baths, scrubbing with wet towels, and cold compresses. The exact cause of cyclic vomiting syndrome is unknown, but it has been linked to abnormalities in mitochondrial DNA—which would also explain why his doctors didn’t diagnose it.

Mitochondria produce the universal fuel of cells. That means mitochondrial diseases can affect almost any organ in your body and can cause an enormous array of symptoms. But the first mitochondrial disease wasn’t identified until the 1950s, more than seven decades after Darwin’s death, so his doctors couldn’t have suspected one might be to blame.

And there are unique features to these diseases make them really hard to diagnose without specific genetic tests. You can’t simply track them in families unless you know what you're looking for, for example, because they don’t obey the classical rules of inheritance. That’s because mitochondria only come from your biological mother—they’re they’re passed down through eggs.

And they have their own genome which encodes for many of their essential parts, so abnormalities in mitochondrial DNA can cause these cellular powerhouses to falter. But each cell in your body can have hundreds to thousands of them, some of which might be normal and some that could be abnormal. Since it’s possible to have different ratios of sick and healthy mitochondria in different cells, the same disease can produce a wide range of symptoms in a single person.

They can even present differently in identical twins, let alone different people. If Darwin’s doctors had known about these conditions, they might have looked to see if any of his family on his mother’s side exhibited signs. Interestingly enough, Darwin’s mother and her brother both also suffered from illnesses with long lists of symptoms.

And his mother’s youngest sister died when she was only eight from an illness that strongly resembles another mitochondrial disorder called MELAS syndrome, which stands for ‘mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes.’ If mitochondrial diseases did run in the family on his mother’s side, it would make sense that Darwin could have inherited one. And MELAS might fit the bill even better than CVS, because the stroke-like episodes in its name would explain the periodic episodes of partial paralysis, memory loss, and difficulty or inability to speak that Darwin began to experience in his 50s. MELAS also causes a build-up of lactic acid in the bloodstream which can cause severe abdominal pain, vomiting, and fatigue, all of which Darwin frequently experienced.

Of course, we may never really solve the mystery of Darwin’s illness. And it may be that more than one condition was to blame, which would mean doctors are mistakenly shoehorning too many symptoms into one diagnosis. The only way to know for sure if any of these hypotheses is right would be to exhume his body for tests—which seems a bit extreme.

Thanks for watching this episode of SciShow, a Complexly production. If you want to learn more about Darwin’s transformative theory and how he developed it, you might like the episodes on our sister channel, Crash Course, on evolution. [ OUTRO ].