How germs made history

In this exclusive article, author Dr Jonathan Kennedy takes a look at some of the claims he makes in his fascinating new book Pathogenesis: How Germs Made History, explaining how germs and microbes shaped history and the world as we know it.

A portrait of Louis Pasteur in his laboratory. Credit: Albert Edelfelt 

Antonie van Leeuwenhoek has probably come as close as anyone in history to experiencing what it feels like to fall down a rabbit hole or step through a looking glass and find oneself in an unfamiliar world of fantastical creatures. In the second half of the 17th century, the Dutch haberdasher began to use the lenses he developed for inspecting cloth to gaze upon the natural world. He was enchanted by what he saw: everything from a drop of water to the plaque on our teeth is alive with what he called “animalcules”.

It took another two hundred years for scientists to understand what these microbes did. Louis Pasteur’s experiments demonstrated that bacteria were responsible for a variety of processes including the fermentation of grapes, souring of milk and putrefaction of meat. He also proved that illnesses and infections were not caused by gods, black magic, imbalanced humours, bad smells or inauspicious planetary alignments. Rather, we become sick when invisible pathogens in the environment enter our bodies.

We are still discovering more about the microbial world. Our bodies host something in the region of 40 trillion bacteria—slightly more than human cells. There are ten times this number of viruses. Together, they weigh between one and two kilos. The majority of these microbes help our bodies to digest foods—but they also do much more than this.

A recent study published in Nature Microbiology revealed that bacteria in our guts are able to produce neurotransmitters—chemical messengers—like dopamine and serotonin that play a key role in regulating human moods. Participants in the study who suffered from depression were found to be lacking in two particular types of gut bacteria.

Microbes don’t just have a big influence on our bodies and minds. They have also had a massive impact on history. Let’s begin with the extinction of the Neanderthals about 40,000 years ago.

In Sapiens, Yuval Noah Harari argues that our ancestors survived because we were more intelligent than Neanderthals and outcompeted them. Harari isn’t the first person to make this argument. The idea that we are defined by our cognitive abilities is implicit in the name we give ourselves—Homo sapiens literally means “wise humans”. One 19th-century naturalist even suggested calling Neanderthals Homo stupidus, although it didn’t catch on.

In the last few years, archaeologists have gathered a mass of evidence to show that Neanderthals were not brutish cavemen. They talked, painted caved walls, used medicinal plants to treat maladies and even sailed between Greek islands. As far as we can tell, Neanderthals were just as smart as us.

Infectious diseases provide a much more convincing explanation for the extinction of the Neanderthals.

Homo sapiens lived in Africa and Neanderthals in Western Eurasia. They had been separated from each other for at least half a million years. When they began to meet again, each species carried pathogens that had an innocuous impact on them because they had evolved some level of immunity, but that were devastating for the other species.

As Africa is closer to the equator than Europe, it receives more of the sun’s energy. This means vegetation is thicker, more animals live on that vegetation, and more microbes live on the animals. As a result, Homo sapiens would have carried more and more deadly infectious diseases. This is why they survived but Neanderthals didn’t.

A mapped illustration of the spread of pathogens throughout history. Credit: Jonathan Kennedy

The real golden age for microbes began when humans started to experiment with agriculture about 12,000 years ago. In early farming settlements, our ancestors lived in proximity to animals and each other for the first time. This created ideal conditions for infectious diseases to emerge and spread. Unsurprisingly, many of the most infamous pathogens—including the plague, tuberculosis, polio, smallpox and measles—emerged around this time.

Infectious diseases had an enormous impact on the ancient world. The Roman Empire was hit by several devastating pandemics in the 2nd and 3rd centuries. This caused political chaos and contributed to the decline of the Western Empire, but these plagues also helped the emergence of Christianity as an Empire-wide religion.

When pandemics struck, pagans—who didn’t have a developed concept of the afterlife—fled to save themselves. But followers of Jesus stayed behind in the cities to tend to the sick in the belief that suffering and charitable acts in this life would guarantee their entry to heaven. Basic nursing would have saved up to two-thirds of their patients, which created the best recruitment material any religion could ask for: “miracles”.

We have all heard of the Black Death, but you might not be aware of that if it hadn’t been for this devastating pandemic we’d probably still be living in the Middle Ages.

In the middle of the 14th century, the plague killed over half of Europe’s population in five years. The demographic collapse plunged the feudal system into crisis. Serfs struggled to take advantage of the new scarcity of agricultural labour and glut of agricultural land to win better terms.

The lords resisted for decades. They used parliament to pass laws that kept serfs’ obligations as they were before the Black Death. When this failed, unsettled by the increasing wealth of the lower orders, they enacted legislation to limit the clothes they could wear and food they could eat.

But ultimately, the feudal lords were unable to withstand the pressure from below. By the mid-fifteenth century, most English peasants had won their freedom and a new system emerged in which lords rented out their land at market-determined rates.

The most entrepreneurial peasants became commercial farmers. They adopted the latest technology and grew the most lucrative crops. Innovation and specialisation led to the astonishing growth in output. This transformed society. Farming not only fed the rapidly growing urban population. Food became so plentiful and cheap that people had extra money to spend on things like textiles, which encouraged the emergence of industrialisation.

Credit: gorodenkoff

We are still dealing with the epidemiological consequences of the Industrial Revolution today. Technological advances such as the development of vaccines and antibiotics brought some respite. But unprecedented population density, encroachment on animal habits, factory farming, and long-distance travel have created a new golden age for infectious diseases.

The Covid-19 pandemic wasn’t an aberration. It is just another episode in the age-old struggle between humans and microbes. Just as infectious diseases have had an enormous impact on our species’ past, they are also very likely to have a big impact on the future.

Pathogenesis: How Germs Made History is out now on Torva Books.

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