How do we learn about health? From disease, unfortunately! And there are intricate connections between large-scale disease and human history. As morbid as it sounds, I’m enjoying learning about such things as I read Kyle Harper’s Plagues Upon the Earth. I don’t know why I’m drawn to reading such things during Covid. Best not to ponder that question for too long. Instead, on to the book!
The historical approach taken by Harper is anchored in two recent scientific advances: molecular phylogenetics and sequencing ancient DNA. In his book, Harper refers to these as “tree thinking” and “time travel”. The four major parts of the book are Fire, Farms, Frontiers, and Fossils. A memorable way to remember the key parts of the book. I’m only halfway through so I can tell you that Fire relates to the hunter-gatherer epoch and that Farms is about the rise of agriculture. Harper is a professor of classics, so he also includes archaeological evidence and historical written evidence where available. Here are some highlights of what I’ve learned.
Human diseases take a new turn as we start to live in more crowded areas: villages, towns, cities, metropoli. Or perhaps I should say that the disease vectors evolve to take advantage of this situation. The main four vectors are viruses, bacteria, worms and protozoa. While we have focused a lot of spillover from other animals thanks to Covid, it was interesting to learn that compared to other primates, we humans are the biggest carriers. Reverse spillover happens more often – we infect chimps more than they infect us!
One chapter is focused on Black Death, the plague recorded as sweeping Eurasia and Northern Africa in the late fourteenth century. The nasty offender is the bacterium Yersinia pestis. I had known about the rats, in particular the black rat Rattus rattus. But what I didn’t know is how well adapted rats are to living alongside congregating humans in cities supported by agriculture and trade. I also learned about the blood-sucking fleas. Turns out that Y. pestis doesn’t care about humans – it’s adapted to the flea-rat combo – and that’s why humans died indiscriminately. We’re a dead-end for the bacteria. Literally. I also learned that the Justinian (Roman) plague some eight centuries before the Black Death was likely also due to an earlier form of Y. pestis. Ugh.
When I was young, growing up in the tropics, worms were a concern. My mother was constantly telling me to wear shoes when I was running around outside. I didn’t like socks and shoes. They made my feet hot! They felt heavy, and I felt less fleet-footed. Reading about helminth-caused diseases and the life-cycle of these worms inside one’s body made me shudder. I’m glad I didn’t contract worms. When I was nine, my mother put me on a regimen of deworming medication because I seemed so skinny even though I ate voraciously. The thought was that the worms inside me might be eating my food and that’s why I wasn’t putting on any weight.
The section on tuberculosis was particularly interesting. Harper calls TB the Ultimate Human Disease. It’s been around for a long time. Harper writes: “TB is a respiratory disease characterized by patience. It possesses the remarkable ability to modulate the human immune responses and cause chronic infection. It featured prominently in the classical medical literature of China, India, and Greece… thrived throughout antiquity and the Middle Ages. In modern Europe, it was the white plague…” TB was a slower killer. It was known as the “wasting disease”. While we see the bacterial lineage in humans and animals, we’re the likely ones who spread the disease based on the genomic data. You can learn about human migration and trade routes from the spread of TB strains.
The grand-scale historical scope of the book singles out how humans are different from any other animal on Earth. With our ability to extract energy from nature (fire!), our growing numbers and spread as the alpha-predator, our dependence on vast monocultural farms as a primary food source, our settling into dense urban areas, we have directly caused remarkable changes to the evolutionary history of parasites upon the earth. They continuously adapt to us, spread with us, and some will kill us. Perhaps to the other animals, we are the great plague upon the earth.
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