100 Million Years of Food (21 page)

As Ethne Barnes has catalogued in a marvelous book,
Diseases and Human Evolution
, the invention of agriculture was a godsend for microbes. Your average bacterium, virus, protozoan, or other parasite wants nothing more than affordable housing, opportunities for romance and sex, and plentiful food. To be sure, prior to the Agricultural Revolution, the life of hunter-gatherers had been no disease-free picnic. As previously mentioned, a diet of raw or undercooked meat was a good way for our ancestors to pick up a friendly tapeworm, which, although it conveniently prevented other tapeworms from colonizing the gut, did have the worrying ability to grow up to fifty feet long. Fortunately, due to their long evolutionary history with us and other mammals, tapeworms rarely cause significant health problems and live in mostly peaceful relationships with their hosts.
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On the other hand, the larvae of the parasitic worm
Trichinella spiralis
(which causes trichinosis) are passed on when their hosts are eaten raw or undercooked by carnivorous animals, and they don't mind surging into muscle tissues to overwhelm host immune systems, thereby crippling their hosts, either directly or through the effects of toxins. Once the host is dead, they sit around and wait for a carnivore to eat their hosts and digest them. If lucky enough to get inside the stomach of the new host, the digestive juices break down the larvae's coverings. This cues the larvae to dig a hole through the intestines and burrow into flesh, becoming adults and giving birth to a new generation of parasites.

The tapeworm and
Trichinella
that our hunter-gatherer ancestors had to deal with, however, were a cakewalk compared to the infectious diseases in store for our agricultural ancestors. To begin with, these were parasites that had previously lurked in the surrounding environment but now multiplied due to poor sanitation, close quarters, large populations of humans and domesticated animals, and human-made habitats. Malaria, filariae, yellow fever, and dengue fever were carried by mosquitoes that hung around like tiny thugs in shaded areas around houses and animal shelters, laying their eggs in convenient urban puddles and reservoirs. The diseases they bore came from wild animals, particularly primates. The protozoan that causes malaria was originally a parasite of macaque monkeys and chimpanzees. The yellow fever flavivirus was a longtime resident in West African monkeys, while the closely related dengue flavivirus spread from monkeys to people living in Asia, and perhaps competed with yellow fever.
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From the viewpoint of parasites, a dead host is usually not a congenial one, so malaria and dengue fever, both long-established diseases, tended to result in low death rates, albeit with painful and dangerous episodes of fever and liver damage. Humans evolved genetic adaptations to deal with the dangers of malaria.
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Yellow fever, on the other hand, moved relatively recently out of West Africa with the slave trade that began in the seventeenth century and delivered high mortality rates of up to 80 percent in newly exposed populations. Dengue fever has existed for an intermediate period and has a maximum mortality rate of 50 percent.
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A few years ago, after returning to Vietnam from a motorcycle trip through Thailand and Laos, I came down with fever, chills, and diarrhea—dengue fever—in Hanoi. I checked myself into the Vietnam-Cuba Friendship Hospital, then spent a few miserable nights in the crowded malaria-dengue ward lying on a metal bed with another man's feet in my face. The open-pit toilet in the adjoining room reeked, but I was so weak that I couldn't even hobble there without fainting and had to make use of a bedpan—humiliating when you're crammed in a ward with a dozen patients and their numerous relations. To add insult to injury, a would-be-thief snuck in among the billowing mosquito nets during the middle of the night, aiming for a wallet under a man's pillow, and I had to shoo the intruder out of the ward. It was a miserable, debilitating experience, but one that woke me up to the real dangers of parasitic diseases and the challenging conditions that most people in the world have had to contend with, including our ancestors.

Aside from mosquito-borne diseases, other parasites and parasitic diseases that humans picked up during the transition from nomadism to often filthy and crowded sedentism include smallpox, originally harbored among monkeys; plague, from rats; typhus, from rats, lice, and fleas; schistosome flukes, borne by snails in slow-moving waters; sleeping sickness, transmitted by tsetse flies; leishmaniasis, or river blindness, from sandflies; Chagas disease, carried by ticks; and rickettsioses, from ticks, mites, and chiggers. Other pathogens have unknown ancestry: bacteria like the ones that cause leprosy, rubella, whooping cough, diphtheria, chicken pox, syphilis, cholera, poliomyelitis, meningococcal meningitis, and hepatitis, as well as shigella,
E. coli
, and streptococcal and staphylococcal bacteria; viruses like the ones responsible for viral encephalitis; and fungi like
Candida albicans
.

Then there were the parasites that we caught from our newly domesticated animal companions. During my undergrad years when I went backpacking through South America I disdained tourist restaurants and preferred street food. One night in Ecuador, I bought barbecued pork from a street-side stand. The slab of meat, hung from the roof, was lit with a single naked bulb, and a horde of flies buzzed through the smoke. I watched the serving lady dip my fork and dish into a bucket of gray water mottled with scum before the utensils were handed to me. For some unknown reason, I still ate the pork.

Two days later, I set off into the Ecuadorian rainforest, following a path that, judging from a crude map, I guessed would lead to somewhere interesting. I had strong legs, a sturdy backpack, a tent and stove, and several packets of dried noodles. After about an hour of trudging over roots and slipping farther and farther away from civilization, I was beset with a strong thirst and a headache pounded at my temples. I dropped to my knees. Damn, time to backtrack. I made it to a hotel at the edge of the jungle, where I boiled a bowl of noodle soup in the hotel kitchen, then fainted. When I awoke, I found that I had smacked my head into the bowl of soup, sending its cheap contents all over the table and floor. Hotel staff helped me get back to my room on the second floor, where I swooned through a high fever that afternoon.

The next day, I took a bus and then a cab to the closest town and found a hotel. I lay in bed, racked with fever and diarrhea. The following morning I took a cab to the state hospital. I blurted to reception, “I have malaria!” and was rushed to a bed and administered an emergency dose of mefloquine, the antimalarial medication that I had been taking on a regular basis. The preventative dosage was once a week, but the doctors now gave me three or four pills in quick succession. The doctor leaned over me and asked in clear English, “Do you know anyone in this town?”

“Someone named Maria. She owns a restaurant,” I responded weakly.

The medicine caused me to be extremely nauseated. I couldn't eat any food for a few days and wobbled unsteadily to the bathroom. It also induced other effects. I became absolutely convinced that the male nurses were going to kill me. The only question was whether it would be accomplished through dropping a stone on me, knifing me, or slipping poison into my medication. At the same time, I was absolutely convinced that one of the student nurses had fallen in love with me (they were all teenagers) and thought I could hear her voice mingled with her friends', chattering about the Canadian down the hallway. When I closed my eyes, I saw dismembered heads marching against a black backdrop, climbing into a tree.

Somehow the hospital staff managed to track down Maria, a restaurant owner whom I had briefly met and chatted with on my way into the jungle. Maria used to work for a citrus company in California and had retired. She spent half her year in California and the other half running her hotel/restaurant/farm on the edge of the Ecuadorian rainforest. “I am the happiest person in the world!” she had declared to me, and I believed it. Maria contributed to the local economy by providing work for the local youth. Now she came into my hospital room bearing a fragrant baked spring chicken, slaughtered that morning for my meal, along with coleslaw and fries—classic American comfort food. I was extremely touched by her generosity and thoughtfulness.

“Don't you worry, they'll take very good care of you here,” she remarked.

I hadn't been able to touch any of the hospital food prior to Maria's visit—mostly rice and beans—but something in Maria's magical baked chicken roused my appetite. I devoured her meal. She sat back, watching me with a smile. The next day, I was able to get up and walk around the hospital. A man in my ward, amazed at my rapid recovery, gestured at the thick book that I was leafing through—the
Lonely Planet
guide to South America.

“Is it the Bible?” he asked.

I didn't have the heart to tell him the truth; besides, it was my bible, in a way. I told him I was interested in religion, so he came to my bedside and prayed fervently, thanking God on behalf of the idiot atheist. When I checked out of the hospital, I tried to pay for my stay, but it was a state hospital where all were admitted for free, even foreigners. I shoved some bills into a doctor's hands and asked him to do some good with it. I dropped by Maria's restaurant. She beamed at me but also would not take any money, stating simply, “Do something good for someone else in return.”

When I got back to Canada and told my doctor about my presumed bout of malaria, he said that it was much more likely to have been food poisoning caused by typhoid; malaria would have produced more severe symptoms and taken far more time to recover from. Typhoid is caused by
Salmonella typhi
and likely arose from our post–Agricultural Revolution association with domestic animal poop and dirty water, which allowed the bacterium to eventually transition from living in domestic animals to our guts.
Salmonella typhi
are crafty enough to evade detection by the immune system while nesting within our cells. When they burst out, the immune system goes berserk, prompting the bacterial horde to pump out endotoxins. This probably triggered the misery that I had experienced in Ecuador: severe headache, weakness, aches and pains, fever. I was fortunate to escape from the clutches of typhoid relatively lightly; 10 percent of people afflicted with typhoid fever die.
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Infectious diseases like typhoid and influenza, and parasites like hookworms and pinworms, arose from comingling with unspecified domesticated animals, but others have been traced to specific ancestry. Dogs are perhaps our longest-running animal associates—we go back at least ten thousand years together—providing us with watchful sentinels, hunting assistance, and possibly dog stews and steaks (as still practiced in many parts of the world, particularly Asia), but also rabies, whipworms, and our first introduction to the deadly scourge of measles. Cows gave us their milk, their meat, and their tuberculosis, as well as the much-feared anthrax. Goats furnished us with milk and brucellosis, also known as Mediterranean Disease, characterized by fevers, chills, weakness, headaches, depression, and weight loss; Napoleon Bonaparte suffered from symptoms consistent with brucellosis, and the diagnosis was confirmed in tissue samples taken from his body. Poultry gave us drumsticks and the mumps. Cats dispatched bothersome rodent pests but also infected us with the protozoan disease toxoplasmosis. Perhaps one in two Americans carries the protozoan parasite
Toxoplasma gondii
, which is usually harmless but can inflame lymph glands in the neck and cause low fever and fatigue, or, in people with compromised immune systems, severe damage to heart, muscle, and brain tissue. Horses were difficult to tame, but after humans domesticated them around three thousand years ago, they returned the favor by bequeathing us the common cold. Pigs gave us heavenly barbecue, along with the giant roundworm
Ascaris lumbricoides
.
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Roughly one out of every four people in the world is infected with
Ascaris lumbricoides
. This parasite begins its bizarre life cycle in the small intestine of an infected person, where the hardworking mother worm, up to a foot long, pumps out 200,000 to 240,000 eggs per day. Her progeny exit the body along with feces and, if they reach soil, take about one month to develop to the infectious stage. If an egg is unwittingly swallowed (children at two institutions in Jamaica were found to have ingested on average nine to twenty giant roundworm eggs a year), it hatches in the small intestine of the new host.

Now, if the giant roundworm larvae were content to just grow up, mate, and produce eggs in the small intestine, then infections with these parasites would be more benign than they actually are. Instead, like college kids with a yen to backpack through Europe or Asia, the larvae head out into the great unknown expanses of the human body for a sightseeing tour. First, they burrow through the lining of the small intestine and enter the circulatory or lymphatic system. The young parasites journey to the lungs, spend around two weeks fattening up in the lung capillaries, break into the alveoli air sacs, wander through the lower respiratory tract, shimmy up the larynx, and are then coughed up and swallowed by the host. This puts them right smack back where they started, in the small intestine. So why go through so much trouble? After all, from a biological viewpoint, all that effort of traveling around the human body is wasted, like calling a moving company and having them transport your belongings around the country for a few months before moving back into the same home.
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The likeliest explanation is that the small intestine is a pretty harsh place to live, even for a parasitic worm. You're immersed 24/7 in a scalding bath of gastric acid, bile, and digestive enzymes; the intestine walls are always trying to push you farther downstream; the mucous membranes that you're trying to hang on to have a tendency to slough off like mountain avalanches; a barrage of bulky human food pummels you; and oxygen levels are chronically low. By comparison, life in the tissues is a Mediterranean resort. The immune system can't hammer an invader ensconced in lung tissue the way it can blitz intestinal pathogens, because lung tissue is especially sensitive to inflammatory attacks. Migrations of parasitic worms out of the small intestines is a change of scenery that provides a chance to grow larger and hence more reproductively robust.
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