The Great Shadow: A History of How Sickness Shapes What We Do, Think, Believe, and Buy

Within a single lifetime, farming accelerated forward at break-neck pace. Wild landscapes were transformed into gardens. Meadows of wild cereal gave way to cultivated plots of lentils, peas, and rye. Vegetable food abounded. Fertility rates skyrocketed. "Civilization," writes biologist Peter Ungar, "was just around the corner."

But there were serpents in those gardens.

Our bones and teeth began to change. We grew shorter and frailer. The new crops were high in calories, but as our diet grew less and less varied, we grew deficient in certain vital nutrients. Grit and hulls from the grain pitted our molars; bacteria that had once lived harmlessly in our mouths crept through the breaches. Toothless by our forties, we could no longer nourish ourselves properly into old age.

The death rate rose at end of life, but also at its beginning. Tiny settlements—Çatalhöyük, north of the Mediterranean; Abu Hureyra, on the banks of the Euphrates; Jericho, between the eastern Mediterranean shore and what is now the Dead Sea—exploded into towns. But in the graveyards of these most ancient towns, skeletons of children began to crowd the rest. As many as one-third of the dead were younger than seven.

Granted, there were more children, period. Farmers have larger families than nomads; women can tend crops while heavily pregnant, but it's a lot harder to chase down a wildebeest in that condition. And dependent children are more easily fed when you're not always on the move.

But children, always more vulnerable than adults to illness, were the canary in the coal mine. Those graves point to a new and disturbing reality.

We were getting sicker.

Hunters and gatherers got sick, of course.

Malaria was probably with us from our earliest days on earth (the malaria parasites that infest primates are practically identical to ours, suggesting that we shared them from before the moment that the family trees diverged). Not long after came tetanus, possibly sleeping sickness, the swelling and paralysis of trichinosis. But these sicknesses struck individuals, not groups, and came in discrete and isolated instances. They were not epidemic; they did not cluster; they did not move through the small roving bands. The suffering was always unexpected, and the sudden distress of one member of the group implied nothing for the rest.

In this newly sedentary world, though, different illnesses slithered quietly into villages and moved sideways through them. Bacteria prospered, strengthened, and mutated in the growing heaps of feces and food scraps. Hookworms, roundworms, and other intestinal parasites, once left behind as hunters moved on, cycled back into the vulnerable children who played in the dirt. Pools and reservoirs, dug out to hold water for crops, bred mosquitoes and nurtured parasites: Entamoeba histolytica, bringing amoebic dysentery; Aedes aegypti, the mosquito that carries dengue fever and prefers to live in man-made vessels; schistosomes, bearing snail fever and spreading into the intestines of anyone who dipped water from contaminated wells, stunting growth and damaging livers and kidneys; typhoid.

Suffering rose from the newly broken ground. Waste piles, spread on the fields as fertilizer, coated crops with the parasites and spirochetes that brought yaws, boils, stomach upsets, fevers, diarrhea. Farmers clearing new fields disturbed the mites carrying scrub typhus: from them came headache and cough, rash and flux, encephalitis and lung failure.

Those tiny innocent house mice and sparrows, scampering underfoot and roosting in the eaves, multiplied into colonies and shed viruses, fungi, and bacteria. And they were only the advance guard. Hand in hand with farming came the domestication of animals: the ultimate game changer.

Wolves had already been tamed by hunters, back during the Ice Age, and at some hard-to-pinpoint moment, the race of wolves had spawned the dog: Canis lupus familiaris, a smaller-skulled, brown-eyed divergence from the C. lupus family tree. By the waning of the Ice Age, at least some of these had joined human households. The oldest known C. lupus familiaris skeleton was found, buried with its masters, in a Bolling-Allerod grave.

Like mice and sparrows, dogs carried diseases that would extend delicate fingers into Homo sapiens (leptospirosis and brucella, both causing fever and malaise). But the illnesses that would change our history came from larger beasts. By 8000 BCE, two millennia into the farming revolution, villagers in the Zagros region of modern Iran had become the first to tame, breed, and nurture wild sheep and goats. Slowly, these skinny, hardy mountain beasts were transformed into plumper, more placid providers of milk, wool, and meat. A little more than a millennium later, wild cattle and bison joined them in domestic enclosures; pigs, shortly after that.

These wild beasts had been carrying various smaller forms of life within them since before antiquity began. Now, the new concentrations of livestock and people gave those microorganisms a whole new venue within which to play, change, and spread. They danced from cow to cow, cow to pig, pig to sheep, goat to cow—and, occasionally, from beast to human hand, throat, eye, lung.

The rinderpest virus (watering eyes and noses, diarrhea, lesions, death) had hitchhiked along with bovine herds for centuries, battling to survive. Now cows, like people, were staying put. In stagnant pools of water and jostling enclosures, rinderpest spread with new ease. For the moment, it merely killed the herds when it exploded outward. But it met man, and liked him; measles was gestating.

We can't blame M. tuberculosis entirely on cows (an occasional human skeleton shows traces of the infection from before cows came to live with man), but the new concentration of milk-giving bovines within ancient villages opened up a new feasting table for the bacteria.

Anthrax spores, long carried by animals, also found humans delightful. Hand in hand with livestock domestication came a massive expansion of B. anthracis; at this exact moment the bacteria suddenly diversified into multiple forms, each one striving to last longer, to tolerate worse conditions, to spread more rapidly.