Excerpted from Andy Warhol Was a Hoarder: Inside the Minds of History’s Great Personalities, by Claudia Kalb. National Geographic Books. Copyright © 2016.

June 1858 was an especially trying month for Charles Darwin, both personally and professionally. The great scientist and devoted father, then 49 years old, was coping with two gravely sick children at home—one of whom would soon die. And at the same time, he received a letter from Alfred Russel Wallace, a respected fellow scientist whose keen observations about native wildlife might have been welcome reading under other circumstances. This particular dispatch, however, contained far more than Wallace’s notes about Ternate, the remote island in the Dutch East Indies he was visiting. Instead, in a concise essay, Wallace laid out an argument for evolution that was shockingly similar to the theory Darwin had been crafting—but not yet published—for almost two decades.

Wallace’s paper hit Darwin with volcanic force. There was no mistaking the similarities in the two men’s ideas about natural selection or the reality that Darwin, who had worked tirelessly to perfect his arguments, might be beaten to the punch on his life’s work. “I never saw a more striking coincidence,” Darwin wrote to his mentor, the esteemed scientist Charles Lyell. An exceptionally ethical man, Darwin felt compelled to act honorably and informed Lyell that he would forward Wallace’s paper to a journal for publication, knowing full well what that would mean. “So all my originality,” he wrote to Lyell, “whatever it may amount to, will be smashed.”

These colliding events at home and at work would have been stressful for any human being. But the impact on Darwin was especially complicated, because he was a man of chronically bad health who struggled with a long list of afflictions, including heart palpitations, stomachaches and headaches. We know from his letters, his autobiography, his methodical health journal and the observations of family and friends that his trials and achievements were often paired with pain, immobilization and isolation. And yet doctors could find nothing intrinsically wrong with him.

So what made Darwin so sick? Since his death in 1882, biographers, historians, physicians and mental health experts have weighed in with dozens of hypotheses. Is it possible that Charles Darwin was battling an infectious tropical bug, picked up on his famous travels aboard the Beagle? Was it irritable bowel syndrome or cyclical vomiting syndrome? Or were Darwin’s lifelong symptoms psychosomatic—physical manifestations of ongoing mental stress?

The list of proposed diagnoses is so divergent you may as well be comparing a monarch butterfly to a great ape. But one key aspect stands out: Darwin was a worrier. He fretted about his children, about his work, about his deadlines, about his reputation and, almost always, about what ailed him. Darwin, it could be argued, suffered from anxiety, one of the most common conditions on the planet. The revered scientist, the man who boldly proposed that “man is descended from a hairy quadruped furnished with a tail and pointed ears,” was altogether very human. Sometimes, like the rest of us, he was one big bundle of nerves.

Sea Changes
Darwin’s father, Robert Waring Darwin was a prominent and well-respected physician, as well as a successful financier, whom Darwin described in largely glowing terms. By contrast, his mother remained little more than a fleeting memory. Prone to intestinal upset and headaches, Susannah Darwin began experiencing severe stomach pains in July 1817, when Charles was just eight years old. She died just a few days later, possibly from an abdominal infection.

Studies have shown that the loss of a parent in early childhood can significantly increase the risk of both depression and anxiety later in life. And Darwin was profoundly affected by the way his mother died—quickly and inexplicably—according to Janet Browne, author of a highly acclaimed two-volume biography of Darwin. Darwin worried incessantly that he or his children had inherited a weak constitution from his mother’s side of the family, and he knew from her experience that sickness could quickly turn deadly.

His anxiety about his health surged in the months leading up to the journey that launched his scientific career. People with anxiety often worry excessively about what might happen and anticipate the worst. Before the Beagle set sail on December 27, 1831, Darwin was nervous about what he might encounter on the ship. He also felt uneasy about being away for such an extended period. And he began to exhibit a fear about his symptoms—hypochondria is rooted in anxiety—as well as an obsession about his health that would reappear throughout his life. “I was also troubled with palpitations and pain about the heart,” Darwin wrote, “and like many a young ignorant man, especially one with a smattering of medical knowledge, was convinced that I had heart disease.”

During the 40,000-mile journey from England to South America, Australia and Africa, Darwin experienced rough seas and navigational mishaps, and struggled with occasional bouts of fever, intestinal distress, a swollen knee, occasional boils and headaches and severe seasickness. He spent much of his time at sea nibbling on raisins (his father’s prescription), lying in his hammock and retching. The young adventurer was mostly in good physical form, however, while exploring on land, which composed the bulk of the journey. Along the way, Darwin collected some 10,000 specimens—plants, fossils, rocks, animals—and shipped them home for analysis.

The Beagle journey, scheduled to last two years, stretched to five. When Darwin returned to England in October 1836, he was a changed man. There is overwhelming evidence that he was sick, often severely so, for much of the latter half of his life. He referred to his ill health repeatedly in his correspondence and painstakingly recorded his symptoms in a health diary whose entries included specific complaints (“boil under arm,” “slight fit of flatulence”) as well as overall ratings (“goodish” and “poorish” to “well very” and “well barely.”) Darwin’s extensive list of woes featured fatigue, dizziness, eczema, boils, muscle weakness, cold fingers and toes, black spots and even hysterical crying. But his overwhelming complaint was abdominal distress, with ongoing bouts of nausea, vomiting and flatulence.

Worried Unwell
Over the years, Darwin’s symptoms have spurred researchers to propose an alphabet soup of diagnoses: agoraphobia, anxiety, appendicitis, arsenic poisoning, barnacle preservative allergy, brucellosis (bacterial infection), Chagas’ disease (infection resulting from a tropical bug bite), Crohn’s disease, cyclical vomiting syndrome, depression, gastritis, gout, hepatitis, hypochondria, irritable bowel syndrome, lactose intolerance, malaria, Ménière’s disease (inner ear disorder), mitochondrial disease (genetic disorder inherited from maternal lineage), neurasthenia (nervous disorder), obsessive-compulsive disorder, panic disorder, paroxysmal tachycardia (rapid heartbeat), peptic ulcer, pigeon allergy, pyroluria (blood disorder) and social anxiety.

What makes Darwin’s case so intriguing is that it exemplifies the powerful interplay between body and mind—the reality that fatigue and vomiting can signify an intestinal infection in one case and emotional upheaval in another. Or both at the very same time. Even without a confirmed diagnosis, though, it is clear that Darwin struggled with a sensitive disposition. Stress of any kind is a common trigger for anxiety, and by the time he was 28 years old and living in London, Darwin had taken on a weighty workload—including finishing a detailed account of the Beagle voyage and scribbling out his earliest musings about evolution in a series of notebooks about the “transmutation of species.” He also began to struggle with poor health.

In 1844, Darwin completed a 189-page draft of his theory, with instructions for Emma, his wife, to publish it if he died. But 14 years later, when Alfred Russel Wallace’s correspondence arrived at his doorstep, Darwin still had not finalized his work. The cause for “Darwin’s delay,” as it has been dubbed, has been scrutinized and debated for years. He was of course a very busy man. And he wanted to be right. Well aware of half-baked speculations about evolution that had already been published, including an account by his grandfather Erasmus, Darwin’s account needed to be as tight as an Anchor Bend knot. There is little doubt that Darwin struggled with feelings of anxiety about publishing his theory, especially early on.

It was competition that would finally push him to act. Charles Lyell, who had received Darwin’s letter with Alfred Russel Wallace’s correspondence, presented both scientists’ theories jointly at a scientific meeting in July 1858—one month after Wallace’s initial communication had arrived. It was a respectable solution that gave Darwin time to move forward honorably. Over the next year, he worked tirelessly to finalize his 500-page account. He also felt sick. As far back as 1838, Darwin had drawn a connection between his brain and his gut. Now, as he raced to the finish line in 1859, Darwin again attributed his physical distress to the strain of completing his “abstract,” as he called it, and he longed for it all to be over.

A Panic Disorder?
Just as allergies manifest in different ways—hives, rashes, watery eyes, sneezing—anxiety disorders can appear in multiple guises, often in the same individual. The DSM lists 11 types of anxiety disorders, including panic disorder and agoraphobia as well as selective mutism (anxiety so severe a child cannot speak in school or other social settings) and the more common generalized anxiety disorder (pervasive and chronic worry about a variety of everyday issues).

Darwin had a well-known aversion to blood (which caused him to abandon medical school as a young man); today it would likely fall into the category of a specific phobia. He was also apprehensive about public speaking, a classic feature of what is now classified as social anxiety disorder and characterized by extreme worry about being scrutinized and humiliated. In a letter to his son Willy, Darwin wrote that when he was required to read papers in his role as secretary of the Geological Society, “I was so nervous at first, I somehow could see nothing all around me, but the paper, & I felt as if my body was gone, & only my head left.” At another time, he noted that speaking for just a few minutes at a scientific society meeting “brought on 24 hours [of] vomiting.”

In a report published in the Journal of the American Medical Association in 1997, two medical doctors concluded that Darwin suffered from panic disorder, an anxiety disorder marked by recurrent panic attacks, characterized by a sudden onset of fear accompanied by severe physical symptoms that can erupt unexpectedly. Darwin experienced a range of features associated with panic disorder, the researchers wrote, including palpitations, shortness of breath, crying, insomnia, abdominal distress and feelings of imminent death. In the midst of a panic attack, it is not uncommon for people to worry that they are having a heart attack and are likely to die. The doctors also described Darwin’s own accounts from his health journals—waking during the night feeling fearful and experiencing “swimming of the head,” trembling hands, and “attacks of sickness”—as experiences consistent with the condition. Darwin’s illness “followed a waxing and waning course typical of the disorder,” the authors wrote. “His attacks caused him great distress and interfered with his work and social life.”

The doctors theorized that Darwin also suffered from agoraphobia, a Greek and Latin term that translates as “fear of open spaces.” The two conditions are often linked (until recently they fell under one diagnosis in the DSM), because people who have repeated panic attacks tend to avoid the places and situations that made them fearful in the first place—a bridge, a crowded theater, an elevator, or being alone. In Darwin’s case, he actively shied away from public appearances, scientific meetings and social interaction, because participating in these activities had a deleterious effect on his health. Soon after his marriage in 1839, Darwin moved from London to Downe, where he put stringent limitations on the time he spent out of his home.

A series of landmark studies suggest that key features of anxiety, which runs in families, are detectable in the earliest days of life. In 1989, Harvard psychologist Jerome Kagan launched a long-term study in which he observed how a group of four-month-old babies responded to unfamiliar people and toys. Some of the babies remained calm and relatively unfazed, but about 20 percent had strong reactions—they cried, they pumped their arms and legs, they arched their backs. Kagan and his colleagues followed those babies for two decades and discovered that the highly reactive infants were more likely to be shy, cautious, and anxious children and adults.

Baby Darwin might have made an interesting research subject. In his youth, he exhibited physical symptoms—trembling, chills, shivering, and intestinal upset—in response to both pleasant and unpleasant events, according to Colp, the Columbia psychiatrist. He left a dog show early after seeing one of the animals react to its owner’s reprimand, telling a friend, “I can’t stand this any longer; how those poor dogs must have been licked.” After shooting his first bird, Darwin later recalled, his hands were trembling so much with excitement that he could barely reload his gun. Listening to music prompted such intense enjoyment “so that my backbone would sometimes shiver,” he wrote in his autobiography. Like the highly reactive babies in the Harvard study, Darwin responded to stimuli in a very physical way and may have been primed for anxiety from an early age.

In Search of Relief
Darwin would have been in excellent company today. Anxiety disorders are the most ubiquitous of mental health conditions, affecting some 40 million Americans. The top psychiatric medication prescribed in the country is the anti-anxiety drug alprazolam, which goes by the brand name Xanax. In 2012 alone, 49 million prescriptions were written.

Throughout the course of his illness, Darwin consulted numerous doctors—even Queen Victoria’s own physician—and submitted himself to a wide array of treatments, including ice packs placed on his spine, mercury pills, antacids, bismuth (the active ingredient in Pepto-Bismol), lemons, codeine and electrical stimulation of his abdomen. His favorite therapy, at least initially, was the Victorian “water cure,” which required spending several months at a spa, where Darwin sweated next to hot lamps, had his body rubbed with cold towels, soaked his feet in cold baths, and had wet compresses pressed on his stomach. The treatment also required getting up early, eating moderately, avoiding sugar, drinking water and walking. Back home, he kept up as best he could, taking frigid showers, even in the winter, and cutting back on his wife’s sweet puddings. Nothing worked for long.

It’s entirely possible that he suffered from other illnesses as well, including Chagas’ disease, irritable bowel syndrome, or cyclical vomiting, any of which could have been exacerbated by stress. Still, anxiety seemed to infuse his very being, entwining itself with whatever else may have been coursing through his brain and his body.

In the end, On the Origin of Species did not set off quite as great an uproar as Darwin may have imagined, in part because he dedicated a chapter to “Difficulties on Theory,” which anticipated and addressed the concerns of critics. But there was plenty of public debate, which Darwin managed to avoid as an alliance of supporters stepped forward to defend his work. His health problems persisted after publication of his celebrated dissertation, but during the last decade of his life—as he turned his attention to far less contentious topics—his symptoms subsided, and he finally found relief. His last book, one of his most popular, was about earthworms.

In the early months of 1882, Darwin experienced chest pains. Emma, his constant protector, attended to him and kept him company. On April 19, he died at age 73 of what doctors called “angina pectoris syncope,” or heart disease, after reportedly telling Emma that he was not afraid to die. By the end of his life, this kind, modest and brilliant scientist had become an intellectual celebrity. He expected to be buried in the churchyard in his hometown, next to two of his children. But in one of history’s great ironies, the man who overturned religious doctrine with barnacles and pigeons and apes found his final resting place in a velvet-draped coffin at the illustrious Westminster Abbey.

For Darwin, nothing was ever simple.