Diseases and Human Evolution

Ethne Barnes

• Publisher: University of New Mexico Press
• Year: 2007
• ISBN:
  9780826330673
• Topic: Medicina
• Subtopic: Malattie e allergie

Urgent interest in new diseases, such as the coronavirus, and the resurgence of older diseases like tuberculosis has fostered questions about the history of human infectious diseases. How did they evolve? Where did they originate? What natural factors have stalled the progression of diseases or made them possible? How does a microorganism become a pathogen? How have infectious diseases changed through time? What can we do to control their occurrence?
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Ethne Barnes offers answers to these questions, using information from history and medicine as well as from anthropology. She focuses on changes in the patterns of human behavior through cultural evolution and how they have affected the development of human diseases.
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Writing in a clear, lively style, Barnes offers general overviews of every variety of disease and their carriers, from insects and worms through rodent vectors to household pets and farm animals. She devotes whole chapters to major infectious diseases such as leprosy, syphilis, smallpox, and influenza. Other chapters concentrate on categories of diseases ("gut bugs, " for example, including cholera, typhus, and salmonella). The final chapters cover diseases that have made headlines in recent years, among them mad cow disease, West Nile virus, and Lyme disease.
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In the tradition of Berton Roueché, Hans Zinsser, and Sherwin Nuland, Ethne Barnes answers questions you never knew you had about the germs that have threatened us throughout human history.

1: Introduction

Recent interest in new diseases, such as HIV/AIDS and Ebola, and the resurgence of older diseases, such as tuberculosis, has raised important questions about the history of human infectious diseases. How did the various infectious diseases evolve? Where did they originate? What factors have hindered or facilitated diseases? How does a microorganism become a disease-causing agent (a pathogen)? Have infectious diseases changed through time? What can we do to prevent diseases from occurring and recurring?

In an attempt to answer these questions, many books and articles have been written over the years, some quite recently, from different viewpoints. Some writers have focused on the historical perspective of major diseases affecting civilization. Others have focused on the evolution of specific diseases from a medical approach. Some discuss the interaction between human behavior and the evolution of certain human diseases. This book brings together the various viewpoints in a multifaceted approach to the issues.

Human beings are unique in that we have evolved not only biologically, like all other creatures of this Earth, but also culturally. Changes in human behavior patterns with cultural evolution throughout the millennia have had the greatest impact on the evolution of human diseases. The study of human beings and their behavior through time, the province of anthropology, gives an objective perspective of how human behavior patterns and attitudes have influenced the evolution of human diseases.

The Meaning of Disease

The very word disease implies discomfort, or lack of “ease,” within the body. Whenever the functioning of the body or any of its parts becomes impaired, disease occurs. Sometimes the impact of disease can be very minimal, while at other times it can be incapacitating and life-threatening. Most human diseases are caused by microorganisms from our environment, and human behavior patterns frequently influence how these microorganisms are spread.

Current perceptions of disease and its causes have roots in earlier belief systems; such perceptions also vary throughout the world. How people view their diseases depends upon how their culture interprets what constitutes a disease, and there are many different cultures with differing ideas about disease.

Modern Western medicine views the physical signs of disease as a biological process with a specific biological cause, while healers in other parts of the world may see the same physical signs of disease as the result of an imbalance within the body or with nature, a supernatural process, invasion by foreign substances, sorcery, or punishment for violation of rules governing human behavior (Murdock 1980). Not all human diseases are universally recognized as diseases. In areas where it is common to find the majority of the community afflicted with the same health problem for generations, people frequently accept the problem as part of their natural existence. This is common among peoples infested with parasites in underdeveloped countries, and among peoples suffering with goiters for lack of iodine in their soils, as in the highlands of Ecuador and Papua New Guinea.

The biological interpretation of disease in modern nations, developed by Western European civilization, rests on the premise of specific etiology (Dixon 1978). The European germ theory developed during the seventeenth century emphasizes specific causes for specific infectious diseases. Cure is obvious: just eliminate the causative agent by using medicines, surgery, radiation, or other intervention measures.

Modern public health measures employed to prevent disease follow the same premise of using tactics designed to attack specific germs that can invade water supplies, food sources, and living and working areas. Indeed, many infectious diseases that frequented our communities in the past have been greatly reduced or vanquished through public health measures and better standards of living. However, overzealous sanitizing of modern environments, water, and food sources can undermine natural human immune responses, leaving the seemingly protected population vulnerable to invading microbes able to sneak past the protective barriers.

The germ theory led to the antibiotic era that began in the early 1950s, dramatically reducing the threat of serious bacterial infections. Unfortunately, overuse and misuse of antibiotics in medical treatment and in animal feeds over the years has caused more harm than good, as the targeted microorganisms have figured out how to resist these drugs.

The “miracle” antibiotic drugs were never effective against viral infections; they could only be used against bacterial infections. The “miracle” drugs were oversold to the public to “prevent” secondary bacterial infections following viral infections, such as the common cold and influenza. This abuse contributed to the overuse of these drugs and their eventual ineffectiveness against life-threatening bacterial infections.

We are now experiencing a comeback from bacterial infections that once were considered defeated by antibiotics. This resurgence of bacterial infections results from their renewed vigor, gained from strains, often more deadly, that developed from antibody resistance.

Meanwhile, viral infections continue to rage unchecked among human populations, with new viruses silently making their way into our communities. Modern medicine is hard-pressed to cope effectively with viral infections. The common cold and influenza viruses continue to plague us, and the more deadly viruses, such as HIV, still challenge human control.

The massive efforts for the past decades to vaccinate whole populations of children against certain viral and bacterial infections, such as diphtheria, pertussis, tetanus, measles, chickenpox, and polio, have created new problems. Some of the targeted microbes are developing strains resistent to the vaccines, strains often more deadly than the originals.

The parasites that cause malaria have also revolted against the efforts of modern medicine by developing resistance to antimalarial drugs. Failed mosquito-control programs have added to this problem. Many of the health campaigns waged to control malaria-carrying mosquitoes have been disrupted by war and governmental neglect in many parts of the world where malaria is rampant, contributing to its resurgence.

Clearly, the theory of specific etiology, based on linear thinking as used in modern medicine, has failed to contain infectious diseases. Current public health measures may not be adequate to keep the new breeds of disease-causing microorganisms from spreading and creating new health hazards throughout the world. In addition, overpopulation of the world, with ever-increasing crowding of poor people in squalor within large cities, provides hotbeds for potential lethal epidemics.

Hippocrates, the father of modern medicine, was unaware of specific disease-causing microorganisms. He interpreted disease as an imbalance within the body and with nature. This early Greek physician, who flourished in the fifth century B.C. following his predecessors from earlier medical schools, recorded their medical knowledge for posterity. Man, as a part of nature, was susceptible to disease whenever disharmony prevailed, upsetting the balance of the essential qualities of the body. Hippocrates divided these essential qualities into four “humors,” and it was the physician’s job to direct treatment to restoring the balance between these humors (Sigerist 1945).

The Greeks were not the only ancient people to believe that disharmony within the body and with nature caused disease. The idea was common throughout ancient Asia. Medical writings spanning China’s three-thousand-year written history focused on the cause of disease as disrupted harmony of the natural forces within the body, with treatment directed in ways to restore the natural equilibrium of these forces.

With the rise of Latin Christianity and the fall of the western Roman empire, religion began to play a key role in the concepts of disease throughout Western Europe. The medical knowledge accumulated from the ancient Greeks and passed on to the Romans became fragmented and mixed with local superstitions in this part of the world. During Western Europe’s Dark Ages, disease was often linked to people’s sins and the wrath of God. Meanwhile, physicians outside Western Europe, in the eastern Mediterranean region, retained the old beliefs of the early Greek physicians, following similar treatment regimes to restore the balance within the person’s body (Sigerist 1945).

The Renaissance in Western Europe brought back some of the ancient Greek rational approaches to the treatment of disease as the new scientific era began to unfold the secrets of the human body and the microbial world. This laid the foundations for the development of modern medicine as we know it today. Robert Koch’s discovery in 1876 of the bacteria causing anthrax led to the idea of specific etiology, or the germ theory, bringing modern science into the realm of medicine.

Today, modern Western medicine has become very technical and specialized, with an emphasis on treating body parts instead of the whole person. Somehow the thread of Hippocrates’ teachings that treated the body as a whole by correcting imbalances of its constituent parts has been lost or overlooked. There is more emphasis on treating the diseased part and less emphasis on treating the person, and efforts are directed toward determining the causative agent of the disease in the affected body part and attacking it with high-tech agents.

Approaches to Understanding Disease

We can gain a better understanding of how human diseases evolved to what they are today by combining several different interpretive approaches. While anthropology provides insights into how human attitudes and behavior patterns influence the development of human diseases, we need to know more about other important factors that govern the evolution and spread of disease agents.

Medical science can tell us how a disease process takes place, how the immune system reacts to pathogens and other mishaps, and why some people are more susceptible to certain pathogens than other people are. The biological sciences provide understanding of the life cycle of microbes and parasites capable of causing disease, and of the mechanisms responsible for the spread of disease agents. The science of ecology deals with the interactions of living organisms and their environment, including human beings as well as microorganisms and other parasites that cause disease. All this information is necessary for the science of epidemiology to analyze disease patterns.

From all these approaches we learn what factors hinder or facilitate diseases, and we gain clues about how diseases evolved and about possible places of origin. To continue our search for more clues to the origin and evolution of human diseases, we need to combine these factors with an evolutionary perspective gained from historical evidence.

History provides many clues not only to diseases of the past, but also to human activities and attitudes responsible for the evolution and spread of diseases. Historical written records provide a major source of this information, but they are limited to a small portion of human history. Anthropological archaeology, besides adding to the historical information through reconstructions of past human behavior patterns and their impact on local environments, also provides important clues of human activities long before writing was invented.

Another branch of anthropology, known as paleopathology, the science of ancient disease, provides factual information from history. Paleopathologists can track through time the various diseases that leave recognizable signs on human skeletons and mummies excavated from archaeological sites. This information adds to our understanding of the evolution of human diseases and the interaction of disease with human cultural evolution.

Coming Together

As human beings have evolved to our present status within nature, so have the multitudes of microorganisms and parasites that have come into contact with us. Most of our microscopic associates create few, if any, serious threats to our bodies. Some have become important contributors to our health. Others act as microscopic predators on specific body tissues, creating disturbing situations leading to disease states that can be crippling or fatal, while some trigger overreaction of host immune responses that can become disabling.

The vast majority of known diseases result from insults on the body created by the microorganisms we know as viruses and bacteria. Some diseases are caused by protozoa and rickettsiae. Some fungi (mycoses) also can invade the human body and cause disease. Microscopic and macroscopic worm infestations can also create disease.

The body’s own response to intrusive environmental agents can create disease by overreaction to a foreign disturbance. Generally, it is the body’s own immune-response mechanisms that determine the course of a disease process, regardless of underlying cause.

Sometimes disorder within the body’s own physiology or structure can lead to a state of disease. Physiological disturbances can be caused by genetic upsets, nutritional imbalances, and environmental agents. Many times disease results from a combination of factors. Friendly microbes can become pathogens with changes in the host. Malnutrition often leads to lowered immune response to infection and poor tolerance of microbes and other parasites.

The human body is constantly adjusting and shifting responses to its internal and external environment in an attempt to maintain harmony within its integrated systems for a balanced state of health. Perhaps the ancient view of disease remains more accurate. Human beings, as a part of nature, constantly strive for harmony with the microbial environment around us and within us. Most of us are unaware that our bodies are engaged in this perpetual struggle to maintain our existence in a reasonably functional state. Dis-harmony creates dis-ease.

Germs, Genes, Geography, and Human Behavior

Many factors influence the development of human diseases. Human behavior has played the major role in the evolution and spread of diseases, and in return, some infectious diseases have had a major impact on the development of civilization. Human manipulation of the environment, beginning with early agriculture, has altered the way microbes and men have interacted for centuries. Every time we rearrange our environment, trouble begins. Our habits of daily living also affect how diseases develop. The spread of most agents of infectious diseases counts on poor hygiene habits and poor sanitation. Recent attempts to control the internal environment of our living, travel, and work spaces with closed air systems have also contributed to the maintenance and spread of many respiratory microbes.

Geography determines where diseases come from and influences what types of diseases will occur within each environment. In the past, most infectious microbes and parasites were confined to their place of origin within specific geographic region...