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A Short History of Disease
By Sean Martin
Oldcastle BooksCopyright © 2015 Sean Martin
All rights reserved.
There was once a time when there was no disease. Life spans were much longer than those we enjoy today, there was no suffering, and people possessed magical powers. They could fly, go to heaven at will, and understood the language of animals.
This is the myth of the golden age, found in cultures the world over. The oldest stories predate Eden: Sumerian cuneiform tablets speak of Dilmun, 'a place where sickness, violence and aging are unknown.' When the sun-god Utu and Enki, lord of soil and earth, brought water, Dilmun flowered and became a beautiful garden. Another pre-Edenic tale is the ancient Persian story of Yima, the first human. During his time, 'there was neither heat nor cold, neither old age nor death, nor disease'. Yima built a beautiful garden, the most widespread image for paradise. This is no coincidence, as Richard Heinberg noted: 'The word paradise itself comes from the Avestan (Old Iranian) word Pairidaeza, meaning a walled or enclosed garden.'
But then disaster struck. Myths of the fall are as widespread as those of the golden age. In Eden, the Serpent tempted Eve to eat the fruit from the Tree of the Knowledge of Good and Evil. In Persia — one of the few stories not to attribute the loss of paradise to the actions of a woman — the Fall was brought about when Yima refused to do the bidding of Ahura Mazda, the Zoroastrian god. Divine displeasure resulted in shorter life spans, pain, toil, conflict, and disease. We have been living in this world ever since.
If paradise in mythology was a garden, in reality, it was probably a beach. Bacteria found at the Strelley Pool Chert in Pilbara, Western Australia, is thought to be around 3.4 billion years old, making it the oldest known form of life yet discovered. At that stage, the Earth was dominated by ceaseless volcanic activity, the continents still in the process of forming, the skies a thick cloud.
We could think of it as an age without disease, but it was also an age without life, or at least life as we know it. We would have found it impossible to breathe, as there was at that stage of Earth's evolution no oxygen: life at Strelley Pool was sulphur-based. The bacteria probably resembled the extremophile bacteria that can be found today in sulphurous caves, acid lakes and in rocks far underground. Rather than being the heavenly arbours of Dilmun, Persia or Eden, the Earth when Strelley Pool Chert was home to the first bacterial life probably more resembled an apocalypse from a painting by John Martin.
Bacteria are not only the original form of life on Earth, but also far and away the most successful and abundant. They had the planet all to themselves for at least a billion years. When the Earth had cooled sufficiently, a type of bacteria known as cyanobacteria began to photosynthesise. That is, they were able to use sunlight to convert carbon dioxide and water into carbohydrates. Oxygen is the byproduct of this process. As Earth's atmosphere began to fill with oxygen, the bacteria slowly began to use it as another energy source.
Around two billion years ago some photosynthetic cyanobacteria invaded other primitive single-celled organisms to form the first plant cells, which were better able to generate energy because they possessed chloroplasts, filaments that were devoted to photosynthesis. Microbial organisms called alpha-proteobacteria amalgamated with other microbes to form mitochondria. These new organisms were eukaryotes, meaning essentially that they were a larger, more advanced form of microbial life. But it is from them that all other life evolved, being powered by chloroplasts and mitochondria.
There were other simple life forms. Single-celled protozoa belong to this category of early life forms. Among their number was thought to be the plasmodium that causes malaria. However, at this very early stage of Earth's history, there were no life forms in which it could cause what we know as malaria; it was simply a microorganism going about its business. Then, as now, that meant finding somewhere to live, generating energy and reproducing. Left to their own devices, most bacteria can reproduce themselves every twenty or so minutes. In one day, a single bacterium can produce a colony of over four sextillion. That stupefyingly large number has twenty-one zeroes after it. Another way of expressing this would be to say that, in a single day, one humble bacterium can produce more of itself than there are vertebrate life forms on the planet.
Bacteria sustain life on Earth. They are in the soil, the air and the water. Each gram of soil contains between one and ten billion bacterial cells, each millilitre of seawater around a million. They are in nature's engine room, constantly transforming matter into energy, constantly purifying, taking in and giving out. In the human body, bacteria outnumber cells by about ten to one (that's roughly 100 quadrillion bacteria to 10 quadrillion cells). Most of them can be found in the gut, and aid the digestion of food, or our immune systems. Others live on our skin, in our mouths, and in places you can't mention in polite conversation.
Another early, very simple and very small life form was the virus. Unlike a bacterium, a virus can't live on its own (it's what's called an obligate parasite). It must of necessity invade a cell and use its host's energy before it can come to life. Once it has done so, a virus will turn the cell it's living in into a production line, spewing out thousands of copies of itself. The virus does not do this out of spite, it's not trying to cause disease, it's simply doing what it's doing. But viral replication nearly always weakens and destroys the cell it's living in, and once the cells start to die off, the organ or organs affected will start to weaken too.
No one knows exactly when viruses first appeared, and to ask why is perhaps to ask the wrong question. It's possible some were the result of imperfect bacterial cell division (although such imperfect offspring usually result in mutated bacteria, rather than viruses). The eminent virologist Dorothy Crawford has dubbed viruses 'rogue pieces of genetic material' which have broken free and found a way to reproduce inside cells. In their natural hosts, viruses can often be benign, only causing disease when they infect a new host. Bats, for example, can carry many viruses that are completely benign to them, but when the viruses make the species jump to humans, they can cause some of the worst diseases currently known, such as Marburg virus disease. We could think of viruses as the microbial equivalent of the Asteroid Belt or Oort Cloud, objects that were too small — or too far away — to become part of larger bodies like planets when the Solar System was forming, and have remained 'free agents' (albeit bound by gravity) ever since. Viruses have always acted to keep life forms in check when any given life form threatens to become overabundant, whether it's human, animal or blooms of algae in the sea.
Out of all the microbes currently known — there are around a million — only 1,415 are known to cause disease in humans. Many are helpful, such as the bacteria that help us digest food, and those that decompose matter and return it to the soil (including human corpses). Some viruses even produce things that we deem beneficial, or pleasing: the bands of colour in variegated tulips, for instance, are caused by a virus. Some bacteria are not naturally harmful to humans, but are only made so by a type of virus called a bacteriophage, or phage for short. The bacteria that cause cholera and diphtheria, for instance (Vibrio cholerae and Corynebacterium diphtheriae), would be harmless were it not for their resident phages 'switching on' the disease.
And then, around five hundred and fifty million years ago, the socalled Cambrian explosion happened: the first vertebrate life crawled out of the sea and onto dry land.
* * *
Mediaeval philosophers were fond of referring to the Book of Nature. If you could but read the Book of Nature, they argued, you would attain wisdom. In some senses, they were right: scientists researching the very early history of life on earth have the fossil record to consult. The fossil chapters from the Book of Nature tell us much about the earliest things to have lived on Earth, such as the Strelley Pool bacteria, or the weird and wonderful extinct life forms preserved in the Burgess Shale in the Canadian Rockies.
Human fossils are relative newcomers. Fossils of our ancestor homo erectus, from around 1.5 million years ago, show evidence of yaws. This is a tropical disease of the bones and skin that produces unsightly swellings and lesions, and leaves skeletal traces. (Yaws is also related to syphilis, although not transmitted by sexual contact. But more of that later.) Paleopathology can tell us what diseases leave traces in the bone: various forms of dental decay, osteoarthritis, rheumatoid arthritis, osteomyelitis, tuberculosis, leprosy, venereal syphilis, poliomyelitis, fungal bone infections, osteoporosis, rickets, scurvy, thyroid disease, diabetes and anaemia. Tumours can also leave traces, and bones of course will leave clear evidence of trauma (breakages) and disorders of growth and development. Some of these diseases are also found in animal remains. Arthritis, for instance, is found in the remains of cave bears.
Paleopathology has its limitations, however, as Charlotte Roberts and Keith Manchester note. Although a disease may appear to have been present in a body, it is not necessarily the cause of death. Bones can be fragile, and may not survive the process of excavation and examination, so the cause of death is often guesswork. A skeleton may not be representative of the community from which it came — the person could have been a relative newcomer, for instance — and total access to a complete cemetery is the exception, rather than the rule. Furthermore, if a person or community had not developed immunity to a disease due to surviving a previous occurrence, acute infective disease is 'likely to have killed people very quickly in antiquity, especially if the individual had had no previous exposure or experience of the invading organism. Therefore, no evidence of abnormal bone change developed.'
In other words, you die before your bones know what's hit you. Roberts and Manchester point out that 'Many diseases also only affect the soft tissues and therefore would not be visible on the skeleton. It is therefore quite possible that skeletons from the younger (nonadult) members of a cemetery population were victims of an acute, or soft-tissue, disease because frequently they do not have any signs of abnormal bone change. Additionally, their immune systems may not have been fully developed to defend against disease.' Despite these and other limitations, paleopathology can provide vital clues for trying to reconstruct what diseases our ancestors suffered from. 'What can be indicated are the disease processes an individual may have been suffering from in life and whether the disease was active or not at the time of death.'
Human bones aren't the only thing to bear traces of disease. Coprolites, or fossilised faeces, also tell us something about prehistoric disease. From fossilised poo, a general state of health can be deduced. They can tell us whether the person had been infested with parasites such as worms, for instance. As Arno Karlen notes, paleoparasitologists, who study coprolites, prove that 'one man's mess is another's treasure'.
When groups of homo erectus moved from Africa to Italy, around half a million years ago, yaws made the trip with them. As to why early humans left Africa, there was probably no one reason. Huntergatherer communities could have been following game; they could just have easily been escaping other tropical diseases. Dorothy Crawford notes that sleeping sickness could have been a major problem for early peoples in Africa, as it's endemic to the continent's tsetse fly belt, and is always fatal if not treated. We know that the tsetse fly was active around a million years ago, as fossilised specimens attest. (The fly could also have been responsible for a sizeable extinction of prehistoric horses in North America.) Starting with a headache, sleeping sickness progresses to attack the lymph nodes, produces a rash on the skin and makes the joints ache. When the trypanosome (the protozoan that causes the disease) enters the brain, lethargy, coma and death aren't long in following.
Hunter-gatherer peoples would have been generally healthy, albeit with a fairly short life span of around 30 years. Many of the diseases that affected them would have been 'wear and tear' diseases like arthritis and rheumatism. (Although some of these conditions could have been hereditary, too.) Comprised of several large family groups — perhaps no more than fifty people — hunter-gatherers would have led a life dictated by the seasons and the movements of animals. They hunted, trapped, fished and stalked their prey. This search for food was constant. But there was a balancing act between hunter-gatherer and the environment. As Dorothy Crawford has pointed out, 'On average, hunter-gatherers required around one square mile of foraging area per person, so the number of people in a band was critical: past a certain tipping point further increase would be self-defeating'. It's thought that, if a group got too large, it would practise infanticide, or split into two.
But human populations grew, communities behaving unwittingly like bacteria in ceaselessly splitting into more and more groups, and following more and more big game for food. It did not end well, either for the hunter-gatherers, or the big game. Early humans are thought to have 'exterminate[d] up to 90 per cent of the larger species' between 50,000 years ago in Africa, 20,000 years ago in Europe and Asia and around 11,000 years ago in the Americas. This theory, known as the overkill theory, holds that mastodons, giant sloths and sabre-toothed tigers were all hunted to extinction in order to feed a growing human population. Following them into the cooking pot were gomphotheres, four species of mammoth, ground sloths, glyptodonts, giant armadillos, giant beavers, giant peccaries, the stag moose and the dwarf antelope, brush and woodland musk oxen, the American camel and the American lion, short-faced bears, the dire wolf and the dirk-toothed cat. Australia lost the diprotodon, the 'one-ton wombat', while New Zealand said goodbye to the moa, a flightless bird whose biggest specimen was larger than an ostrich.
The diminishing number of animals to eat, and the threat of diseases like sleeping sickness, would have driven hunter-gatherers further afield. At this point, hunter-gatherers would have increased what is known as the human disease burden. There have been several shifts in this, and they have all caused irrevocable shifts in the pattern of disease.
The earliest human ancestors were apes, who lived largely in the trees. Some diseases would have been either endemic to life in that environment, or at least more common than they would have been on the ground. Certain birds, mammals and insects spend their entire lives in the canopy, and never come down to ground level. Other species, in contrast, require the shade and moisture of life on the ground. All creatures, regardless of what level of the forest they lived in, would have had their own viruses, parasites and diseases. As Arno Karlen notes, 'Changing its niche by only a few meters can radically alter a species' prey, predators and microbes.' Karlen also speculates that it could have been diseases acquired in the trees that may have first forced our ancestors to come down to ground level. Ancestral forms of viral diseases like polio and meningitis could have 'left our arboreal ancestors too crippled to swing through the branches, and enough survivors squeaked out a marginal adaptation to the forest floor to launch a new species.'
When our ancestors shifted their habitat down to the ground, they naturally also made themselves vulnerable to the new diseases that existed there, such as parasitic worms from animal droppings. And the flies that plagued those animals would have given the new ground-based human ancestors sleeping sickness. This was the first shift in disease burden, but over time, our ancestors — and their diseases — adapted to each other. Changes in diet could have led to a second shift: it is probable that our very early ancestors were herbivores, but life on the ground presented them with new opportunities in the shape of animals.
Eating meat, or an increase in the amount of meat in the diet, caused the second shift in the disease burden. Humans were now in more regular contact with animals in order to kill, butcher and eat them. This would have made them susceptible to animal diseases, which were able to make the species jump from animals to humans, known as zoonoses, or zoonotic diseases. This would be particularly true if the animal caught was itself sick; sick animals being slower and easier to catch than healthy ones. However, zoonoses don't enter the human story in quantity at this time, as we'll see.
Excerpted from A Short History of Disease by Sean Martin. Copyright © 2015 Sean Martin. Excerpted by permission of Oldcastle Books.
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Table of Contents
ContentsIntroduction: Definitions, Origins,
Chapter One: Prehistory,
Chapter Two: Antiquity,
Chapter Three: The Dark and Middle Ages,
Chapter Four: The New World,
Chapter Five: Early Modern to 1900,
Chapter Six: The Twentieth Century,
Chapter Seven: New Diseases,
Glossary of Diseases,