Homo sapiens is in a fairly unique situation when it comes to our immunology. We seem to get sick extremely easily compared to other mammals, and epidemics are more common in our species than almost any other (except, strangely, domesticated animals). But fungal and parasitic infections are less common in us than in other mammals. What gives?
We’ll have to look back a long, long time, back to the evolution of the first human species (yes, there were multiple), to get our answer. Though there’s plenty of fascinating insights into our behavior that this historical analysis can provide, let’s focus only on the relevant stuff. Early humans, homo sapiens included, were mostly nomadic hunter-gatherers who lived in small groups of up to around 150. Up until then, our role in the world’s (or at least East Africa’s) ecology was small, comparable to the other great apes. It was the cognitive revolution 70,000 years ago which really set us apart from every other animal species on earth.
We’ll skip over the potential causes and other interesting parts of the cognitive revolution (do some research on your own; we highly recommend Sapiens: A Graphic History by Yuval Noah Harari), and focus on the main effect: we conquered the world. Homo Sapiens spread rapidly, migrated to the top of the food chain, killed off the other human species, and reshaped every ecosystem it touched. The important part is that all of this happened incredibly quickly, evolutionarily speaking, meaning homo sapiens are all almost genetically identical to each other, even though geographically our populations are incredibly widespread. That’s Part 1 in our search for the evolutionary cause of pandemics: a disease that can affect one group of homo sapiens can spread just as easily to almost any other group (usually).
Part 2 comes along with the agricultural revolution, when many homo sapiens began to adopt a settled or cyclically nomadic agricultural lifestyle. In Eurasia and Africa especially, the domestication of livestock began a little later too. Population densities grew in those areas. Close proximity with animals and other people, along with generally poorer health and sanitation (it’s true!), created the perfect breeding grounds for strong contagious diseases to rapidly evolve. Infectious bacteria and viruses evolve much much faster than large mammals do: the more hosts they have, and the more immune systems they encounter, the faster natural selection turns them into highly contagious, tough, deadly pests.
Though not all infectious diseases arose during the agricultural revolution, many of the most deadly ones did such as measles, pertussis, smallpox, and influenza. You might notice many of these are native to Eurasia; if you’re interested in the potential causes and historical repercussions of this fact, we highly recommend Guns, Germs, and Steel by Jared Diamond. It should also be noted that this is certainly an oversimplification of the unique evolutionary process which created humans’ unique smorgasbord of plagues, and some studies show that the process was probably more complicated than this.
The final nail in our immunological coffin came in with the advent of “civilization” (whenever that started is highly debatable), by which we mean tightly-packed cities and continent-wide trade. Unprecedented population density, shared water pumps (see the London Cholera outbreaks) and other resources, and historically low sanitation made an even more ideal breeding ground for plagues. They then travelled, and further toughened and evolved along long trade-routes, vastly expanding the breeding pool for diseases. The bubonic plague, for example, moved from Ireland all the way to Japan.
The advent of modern medicine has added many additional tools to our biologically unique fight against disease, such as antibiotics, vaccines, and better hygiene, but the incredible global population spike has also further increased population density, and modern transport allows disease to travel faster than ever before. Epidemics are becoming increasingly common, and some bacteria are evolving defenses against antibiotics faster than we can make new ones.
Since the dawn of our cognition, Homo sapiens has faced unique biological challenges, and overcome each of them. The ones we have the most difficulty overcoming are not the challenges nature throws at us, but the problems we create ourselves. This is why we created MedSoc Talk.