Pathogenesis: A History of the World in Eight Plagues

by Jonathan Kennedy

Over the last two decades, scientists have discovered several more species of humans that were alive at the same time as Homo sapiens. Denisovans split from Neanderthals not long after they’d ventured out of Africa and went on to occupy the eastern part of Eurasia.

Anatomically, Denisovans would have looked similar to Neanderthals although they appear to have had much bigger teeth, and they carried a number of gene mutations, including one that affected red blood cells and allowed them to live comfortably at high altitudes.

Homo floresiensis lived on the Indonesian island of Flores. They are colloquially referred to as Hobbits on account of their height—they stood just over a meter tall—and disproportionately long feet.

Within a few thousand years, Homo sapiens burst out of Africa and quickly spread across the world—from western Europe all the way to Australia. At the same time, all other species of human vanished from the face of the earth.

recent research demonstrates that Neanderthals were the first human species to create cave art,

About a decade ago, researchers managed to extract DNA from Neanderthal bones and sequence the genome.[51] When they compared their findings to the Homo sapiens genome, they realized that anyone alive today whose ancestors are Europeans, Asians or Native Americans has inherited about 2 percent of their genes from Neanderthals.

providing incontrovertible evidence that the two species not only met, but had sex and reproduced. Neanderthal males coupled up with Homo sapiens females and vice versa.

Homo sapiens evolved to better cope with the various challenges that they encountered in Africa. This is how Homo sapiens and Neanderthals became two separate species.

Ice Age Europe would have been a forbidding environment for Homo sapiens, having evolved in Africa.

Without vaccines, our immune systems are the only defense against infectious diseases.

Homo sapiens could have overcome the invisible barrier created by Neanderthal diseases through Darwinian evolution. Sooner or later, genetic mutations would have occurred that allowed Homo sapiens’ immune systems to mount an effective response to Neanderthal pathogens.

As a result, many of the Neanderthal gene variants that remain in our genome relate specifically to our immune response.

many introgressed Denisovan gene variants carried are involved in immune-related processes, suggesting that these genes facilitated Homo sapiens’ adaptation to pathogens that they encountered as they pushed into Eastern Eurasia.

Tibetans carry a Denisovan gene mutation that affects red blood cells, making it possible to live comfortably on a 13,000-feet- or 4,000-meter-high plateau where the air contains 40 percent less oxygen than at sea level.

The Inuit of northern Canada and parts of Greenland and Alaska have retained Denisovan genes that influence the storage of fats, which helps them to thrive in an exceptionally cold climate.

around 500 years later, the stones were transported and raised. The “bluestones,” which weigh between 2 and 5 tons, came from Preseli Hills in west Wales, over 140 miles away.

Stonehenge wasn’t built by the earliest people to permanently inhabit the British Isles. It was constructed by farmers who originated in Anatolia and arrived in northwestern Europe about 6,000 years ago and almost completely replaced the genetically distinct hunter-gatherer population who had lived there since the end of the Ice Age.

Farming allows women’s bodies to recover faster from the strain of childrearing because they get to eat calorie-rich cereals and dairy products rather than low-calorie game, seafood and plants, and expend much less energy on carrying their infants.

almost everywhere that humans adopted settled agriculture, early farmers were less healthy than hunter-gatherers. Their skeletons were shorter and more likely to show signs of anemia due to iron deficiency and enamel defects as a result of lack of vitamins A, C and D, calcium and phosphorus.

Malnourishment weakens the immune system, which was a problem because humans’ exposure to pathogens increased markedly after the adoption of agriculture.

In contrast to the spread of Neolithic European Farmers a few millennia before, ancient DNA analysis suggests that the Steppe Herders may have used violence as they surged across the continent.

Ninety percent of the migrants were male, which indicates that the westward spread of Western Steppe Herders could have involved an invasion led by warriors with the aid of cutting-edge bronze weapons, horses and wagons.

Rather, the impact of the migration of Steppe Herders looks much more similar to the European colonialization of the Americas after 1492. In the decades after the Spanish arrived they managed to conquer vast and sophisticated empires, sometimes with just a few dozen men, as Old World pathogens raced ahead of them and literally decimated the Native American population.

The remains belonged to farmers who inhabited the northernmost parts of Europe. They lived in small settlements consisting of dispersed farmsteads, so it is remarkable to find a mass grave with that number of people. When the sample was analyzed in a laboratory, it became clear that the ancestry of the people in the tomb was about half Neolithic European Farmers and half the pre-existing hunter-gatherer population. But their cause of death remained unknown.

The researchers concluded that an epidemic must have swept through southern Sweden almost 5,000 years ago, killing large numbers of people, including those buried in the mass grave.

a “Neolithic Black Death.” But this devastating epidemic differed from the fourteenth-century Black Death in one crucial respect. Yersinia pestis did not evolve into a flea-borne bubonic plague until the beginning of the first millennium BCE.[70] Prior to that it would have been transmitted by sneezing and coughing and infected the lungs. According to the World Health Organization (WHO), pneumonic plague kills almost all infected people if it is left untreated, compared to between 30 and 60 percent for bubonic plague.

outbreaks of infectious disease were seen as “collective calamities, and judgments on a community.”

half-dead men tumbled in the streets and around all the springs in their craving for water.”

They chanced upon a mass grave that dated to the exact years of the plague, and when archeologists investigated further they noted that upper layers of corpses had been thrown in far more haphazardly than those at the bottom, pointing to “a sense of mounting panic in the city.”

Even the lauded baths were more of a danger than a benefit to public health. In Rome’s bigger institutions thousands upon thousands of people soaked in the same water every day. Bathers didn’t use soap, preferring to cover themselves in olive oil and then scrape it off with an instrument called a strigil. Contemporary writers complained about the water being dirty and contaminated with human excrement. In other words, Roman baths created an ideal environment for waterborne diseases to spread.

Galen’s description of the symptoms, which include a black rash that covered the whole body. The most likely candidate is smallpox. Galen’s discussion of the causes and cures of the disease reveals the poor state of Roman medicine.

by an excess of the humor called black bile, and suggested treatments including mountain cows’ milk, dirt from Armenia, and boys’ urine.

Most Romans understood this devastating outbreak of infectious disease as an act of divine anger.

At the same time, paganism suffered a startling collapse.

The Christian faith skyrocketed because it provided a more appealing and assuring guide to life and death than paganism during the devastating pandemics that struck the Roman Empire in the second and third centuries CE.

One major point of difference between the old and new faith was the issue of what happens when we die. Jesus promised everlasting life in paradise, whereas “pagans believed in an unattractive existence in the underworld.”

Alas, the Black Death was anything but an aberration. The pestilence struck again and again over the next couple of centuries as Yersinia pestis was reimported into Europe from the mountains of Central Asia, where it continued to circulate in wild rodent populations.

It was the recurrent nature of the plague that made it so destructive and ended up causing such profound social, political and economic change. A second outbreak in 1361 killed perhaps 20 percent of the population, but among young people who hadn’t been alive a decade earlier—and therefore hadn’t developed immunity—the mortality rate was no different from that of the first epidemic.

The Italian city-states were the first communities to take action to protect themselves from the plague. Starting in the 1370s, all ships wanting to enter Venice had to wait on the nearby island of San Lazzaro until the health magistrates granted the crew permission to disembark. Over time, the waiting period became standardized at forty days—the word quarantine is derived from quaranta, the Italian for forty.

quarantine was deemed to be an effective public health intervention and it was adopted throughout western Europe.

In medieval Europe, the Jesus sect was no longer an insurgent movement of principled, self-sacrificing martyrs. The Church had grown into an extremely wealthy, powerful organization and expected Christians to pay the tithe—10 percent of their income. So many people had left property to the Church when they died that by the Middle Ages it owned about a third of all cultivated land in western Europe.

the feudal system leads to economic stagnation because it isn’t in the interests of either feudal serfs or their lords to maximize profits.[70] The serfs’ priority was to produce enough food to survive. As a result, they adopted a risk-averse subsistence strategy. Their strips of land were spread across different fields, reducing the danger that the whole harvest would be damaged by disease, bad weather or marauding animals.

Serfs were the lords’ main source of income. Lords would sell surplus crops and spend the proceeds on strengthening their military capacity—for example, building castles, employing soldiers and buying weapons.

the Spanish laid siege to Tenochtitlan. It took seventy-five days for the Mexica capital to fall. The besiegers destroyed large parts of the city and slaughtered tens of thousands of its inhabitants, including most of the nobility.

Smallpox was only just the beginning. Over the next few decades, the indigenous people of Mesoamerica were afflicted by deadly epidemics again and again. Measles arrived in the early 1530s. A disease the Mexica referred to as cocoliztli (from the Nahuatl word for “pestilence”) killed up to 80 percent of the region’s inhabitants in 1545, making it the deadliest epidemic in recorded history.[21] It returned again between 1576 and 1578.

The first recorded influenza epidemic occurred in 1558, slaying another third of the population.

people who weren’t killed by infectious diseases died from famine, as crops rotted in the fields because there was no one to harvest them. The malnourished survivors were susceptible to whatever pathogens would arrive next from Spain. The scale of the cumulative devastation is hard to imagine: the indigenous population of Mesoamerica was about 20 million when Cortés arrived but had fallen to 1.5 million a century later.

When this gruesome affliction first strikes, many seemingly healthy people flee in fear. But as the disease has an incubation period of up to two weeks, the refugees all too often carry the virus with them. In this way, smallpox raced ahead of the Spanish and devastated whole communities.

The first epidemic struck the Inca Empire in 1524, plunging the biggest, most sophisticated society in the Americas into disarray. The virus killed between 30 percent and 50 percent of the population, including the emperor, Huayna Capac, his designated heir and most of the court.

This led to a war of succession between two of Huayna Capac’s other sons in which the incumbent,