Judging by our natural parasitic load, Homo sapiens is among the dirtiest primates. This observation may be motivated by personal interest: we know more about our parasites than about those of other species because they are important to us, and this knowledge gives a false impression of the number of human parasites. However, there are reasons to believe that the parasitic load on a person is actually extremely high.
The first reason is the restlessness of our species. About 15,000 years ago when the paleo-Indians crossed the Bering Strait in the Bering land bridge connecting Siberia and North America, people are able to live in almost any environment existing on Earth, from tropical jungles and Australian deserts to forested areas of Eurasia in the temperate zone and the Arctic tundra. Human characteristics such as omnivore and adaptability (possible thanks to technology) contributed to its rapid spread around the world.
The only species that has captured so many niches, let alone close contact with animals since their actual domestication began about 12,000 years ago, is the same species that has encountered many parasites. According to some estimates, 80% of the approximately four hundred parasites that call the human body their home are zoonotic, which means that in the past they moved to humans from other species and adapted to their new home. “Homo sapiens is among the species most heavily infested by parasites —” Parasitologists Ashford and Crewe write in their article The Parasites of Homo Sapiens. — In all likelihood, there are few species of parasites that have never had the opportunity to infect humans.”
In the assessment of Ashford and Crewe considered only eukaryotes — organisms whose cells contain a fully developed kernel. I use the term in a broader sense: any organism (whether single-celled, multicellular, or virus) that needs a human body to complete its own life cycle, which can cause a human disease.
The extremely high level of sociality of our species has also increased its parasitic load. Today, some anthropologists argue that, leaving aside our relatively large brains, humans are distinguished from other large primates by another characteristic, such as the ability to cooperate. We can and do work as a team, and that kind of teamwork makes us more effective. However, cooperation also implies compact living, so from the moment agriculture emerged 12,000 years ago (and even earlier), people began to unite into larger communities. Human communities have become more complex, more structured, and in some ways more capable of harnessing and channeling human ingenuity and energy. On the other hand, these communities have become more contagious, more needy and more hostile.
According to one view, the period of human history since the late Paleolithic is characterized by a constant desire for the formation of larger human networks, a steady movement towards globalization, which restrained the spread of diseases caused by the same trend. Pollution of human habitats reached a high level in the West along with rapid urbanization during the industrial revolution in the late XVIII — early XIX centuries. There were well-founded fears that mechanized civilization will drown in the products of its own decomposition. Over time, this concern has led to health reforms, the benefits of which we still enjoy. These improvements helped humanity make the second great epidemiological transition. The first transition occurred when hunters and gatherers began to lead a sedentary lifestyle and cultivate the land. And the third transition is happening before our eyes: microorganisms of the past, which have formed resistance to antibiotics, return to active life. In addition, in the context of this book, the most important thing is that the modern landscape of diseases is characterized by chronic degenerative diseases that have no visible infectious causes.
So how about our excessively large parasitic load? As the author of this book, I am particularly interested in the Paleolithic parasites — the parasites we began to lose during the second epidemiological transition. We have spent a lot of time together with these organisms, and long periods of coevolution contribute to the formation of closely intertwined relationships.
The approximate meaning of the word Paleolithic in ancient Greek is “ancient stone”. The term refers to tool-making, which has improved dramatically in the 3.2 million years since Lucy (our Australopithecus ancestor) lived in East Africa. The Paleolithic period was dominated by disparate groups of 30 to 70 people. Under such circumstances, the approach to parasitism, based on the tactics of scorched earth (multiply as quickly as possible, and to hell with the host!), was doomed to failure. Any parasite that is quickly killing its only host in the person of a man was doomed to extinction.
In this regard, parasites from the Paleolithic era usually expect long-term residence in the host. They tend to have a more” mild ” impact, at least compared to later epidemics. To claim that the constant presence of such organisms over millions of years has invariably affected our immune function would be to misinterpret the depth of our complex relationships. Parasites altered our immune system as well as atmospheric oxygen changed our lungs and land — limbs. This means that our immune system has evolved in many ways just to solve the problem of parasites. They were the dominant feature of the environment in which our evolution took place.