Chapter 6: THE HUMAN ORGANISM
As similar as we humans are in many ways to other species, we are unique among the earth's life forms in our ability to use language and thought. Having evolved a large and complex brain, our species has a facility to think, imagine, create, and learn from experience that far exceeds that of any other species. We have used this ability to create technologies and literary and artistic works on a vast scale, and to develop a scientific understanding of ourselves and the world.
We are also unique in our profound curiosity about ourselves: How are we put together physically? How were we formed? How do we relate biologically to other life forms and to our ancestors? How are we as individuals like or unlike other humans? How can we stay healthy? Much of the scientific endeavor focuses on such questions.
This chapter presents recommendations for what scientifically literate people should know about themselves as a species. Such knowledge provides a basis for increased awareness of both self and society. The chapter focuses on six major aspects of the human organism: human identity, human development, the basic functions of the body, learning, physical health, and mental health. The recommendations on physical and mental health are included because they help relate the scientific understanding of the human organism to a major area of concernpersonal well-beingcommon to all humans.
In most biological respects, humans are like other living organisms. For instance, they are made up of cells like those of other animals, have much the same chemical composition, have organ systems and physical characteristics like many others, reproduce in a similar way, carry the same kind of genetic information system, and are part of a food web.
Fossil and molecular evidence supports the belief that the human species, no less than others, evolved from other organisms. Evidence continues to accumulate and scientists continue to debate dates and lineage, but the broad outlines of the story are generally accepted. Primatesthe classification of similar organisms that includes humans, monkeys and apes, and several other kinds of mammalsbegan to evolve from other mammals less than 100 million years ago. Several humanlike primate species began appearing and branching about 5 million years ago, but all except one became extinct. The line that survived led to the modern human species.
Like other complex organisms, people vary in size and shape, skin color, body proportions, body hair, facial features, muscle strength, handedness, and so on. But these differences are minor compared to the internal similarity of all humans, as demonstrated by the fact that people from anywhere in the world can physically mix on the basis of reproduction, blood transfusions, and organ transplants. Humans are indeed a single species. Furthermore, as great as cultural differences between groups of people seem to be, their complex languages, technologies, and arts distinguish them from any other species.
Some other species organize themselves sociallymainly by taking on different specialized functions, such as defense, food collection, or reproductionbut they follow relatively fixed patterns that are limited by their genetic inheritance. Humans have a much greater range of social behaviorfrom playing card games to singing choral music, from mastering multiple languages to formulating laws.
One of the most important events in the history of the human species was the turn some 10,000 years ago from hunting and gathering to farming, which made possible rapid increases in population. During that early period of growth, the social inventiveness of the human species began to produce villages and then cities, new economic and political systems, recordkeepingand organized warfare. Recently, the greater efficiency of agriculture and the control of infectious disease has further accelerated growth of the human population, which is now more than five billion.
Just as our species is biological, social, and cultural, so is it technological. Compared with other species, we are nothing special when it comes to speed, agility, strength, stamina, vision, hearing, or the ability to withstand extremes of environmental conditions. A variety of technologies, however, improves our ability to interact with the physical world. In a sense, our inventions have helped us make up for our biological disadvantages. Written records enable us to share and compile great amounts of information. Vehicles allow us to move more rapidly than other animals, to travel in many media (even in space), and to reach remote and inhospitable places. Tools provide us with very delicate control and with prodigious strength and speed. Telescopes, cameras, infrared sensors, microphones, and other instruments extend our visual, auditory, and tactile senses, and increase their sensitivity. Prosthetic devices and chemical and surgical intervention enable people with physical disabilities to function effectively in their environment.
A human develops from a single cell, formed by the fusion of an egg cell and a sperm cell; each contributes half of the cell's genetic information. Ovaries in females produce ripened egg cells, usually one per menstrual cycle; testes in males produce sperm cells in great numbers. Fertilization of an egg cell by a sperm ordinarily occurs after sperm cells are deposited near an egg cell. But fertilization does not always result, because sperm deposit may take place at the time of the female's menstrual cycle when no egg is present, or one of the partners may be unable to produce viable sex cells. Also, contraceptive measures may be used to incapacitate sperm, block their way to the egg, prevent the release of eggs, or prevent the fertilized egg from implanting successfully. Using artificial means to prevent or facilitate pregnancy raises questions of social norms, ethics, religious beliefs, and even politics.
Within a few hours of conception, the fertilized egg divides into two identical cells, each of which soon divides again, and so on, until there are enough to form a small sphere. Within a few days, this sphere embeds itself in the wall of the uterus, where the placenta nourishes the embryo by allowing the transfer of substances between the blood of the mother and that of the developing child. During the first three months of pregnancy, successive generations of cells organize into organs; during the second three months, all organs and body features develop; and during the last three months, further development and growth occur. These patterns of human development are similar to those of other animals with backbones, although the time scale may be very different.
The developing embryo may be at risk as a consequence of its own genetic defects, the mother's poor health or inadequate diet during pregnancy, or her use of alcohol, tobacco, and other drugs. If an infant's development is incomplete when birth occurs, because of either premature birth or poor health care, the infant may not survive. After birth, infants may be at risk as a result of injury during birth or infection during or shortly after the event. The death rate of infants, therefore, varies greatly from place to place, depending on the quality of sanitation, hygiene, prenatal nutrition, and medical care. Even for infants who survive, poor conditions before or after birth may lead to lower physical and mental capacities.
In normal children, mental development is characterized by the regular appearance of a set of abilities at successive stages. These include an enhancement of memory toward the end of the first month, speech sounds by the first birthday, connected speech by the second birthday, the ability to relate concepts and categories by the sixth birthday, and the ability to detect consistency or inconsistency in arguments by adolescence. The development of these increasingly more complex levels of intellectual competence is a function both of increasing brain maturity and of learning experiences. If appropriate kinds of stimulation are not available when the child is in an especially sensitive stage of development, some kinds of further biological and psychological development may be made more difficult or may even fail to occur.
This extraordinarily long period of human developmentcompared to that of other speciesis related to the prominent role of the brain in human evolution. Most species are very limited in their repertory of behavior and depend for survival on predictable responses determined largely by genetic programming; mammals, and especially humans, depend far more on learned behavior. A prolonged childhood provides time and opportunities for the brain to develop into an effective instrument for intelligent living. This comes not only through play and interaction with older children and adults but also through exposure to the words and arts of people from other parts of the world and other times in history. The ability to learn persists throughout life and in some ways may improve as people build a base of ideas and come to understand how they learn best.
Developmental stages occur with somewhat different timing for different individuals, as a function of both differing physiological factors and differing experiences. Transition from one stage to another may be troublesome, particularly when biological changes are dramatic or when they are out of step with social abilities or others' expectations. Different societies place different meaning and importance on developmental stages and on the transitions from one to the next. For example, childhood is defined legally and socially as well as biologically, and its duration and meaning vary in different cultures and historical periods. In the United States, the onset of pubertythe maturation of the body in preparation for reproductionoccurs several years before an age generally considered physically and psychologically appropriate for parenthood and other adult functions.
Whether adults become parents, and (if they do) how many offspring they have, is determined by a wide variety of cultural and personal factors, as well as by biology. Technology has added greatly to the options available to people to control their reproduction. Chemical and mechanical means exist for preventing, detecting, or terminating pregnancies. Through such measures as hormone therapy and artificial insemination, it is also possible to bring about desired pregnancies that otherwise could not happen. The use of these technologies to prevent or facilitate pregnancy, however, is controversial and raises questions of social mores, ethics, religious belief, and even politics.
Aging is a normalbut still poorly understoodprocess in all humans. Its effects vary greatly among individuals. In general, muscles and joints tend to become less flexible, bones and muscles lose some mass, energy levels diminish, and the senses become less acute. For women, one major event in the aging process is menopause; sometime between the ages of 45 and 55, they undergo a major change in their production of sex hormones, with the result that they no longer have menstrual cycles and no longer release eggs.
The aging process in humans is associated not only with changes in the hormonal system but also with disease and injury, diet, mutations arising and accumulating in the cells, wear on tissues such as weightbearing joints, psychological factors, and exposure to harmful substances. The slow accumulation of injurious agents such as deposits in arteries, damage to the lungs from smoking, and radiation damage to the skin, may produce noticeable disease. Sometimes diseases that appear late in life will affect brain function, including memory and personality. In addition, diminished physical capacity and loss of one's accustomed social role can result in anxiety or depression. On the other hand, many old people are able to get along quite well, living out independent and active lives, without prolonged periods of disability.
There appears to be a maximum life span for each species, including humans. Although some humans live more than a hundred years, most do not; the average length of life, including individuals who die in childhood, ranges from as low as 35 in some populations to as high as 75 in most industrialized nations. The high averages are due mostly to low death rates for infants and children but also to better sanitation, diet, and hygiene for most people, and to improved medical care for the old. Life expectancy also varies among different socioeconomic groups and by sex. The most common causes of death differ for various age, ethnic, and economic groups. In the United States, for example, fatal traffic accidents are most common among young males, heart disease causes more deaths in men than women, and infectious diseases and homicides cause more deaths among the poor than among the rich.
The human body is a complex system of cells, most of which are grouped into organ systems that have specialized functions. These systems can best be understood in terms of the essential functions they serve: deriving energy from food, protection against injury, internal coordination, and reproduction.
The continual need for energy engages the senses and skeletal muscles in obtaining food, the digestive system in breaking food down into usable compounds and in disposing of undigested food materials, the lungs in providing oxygen for combustion of food and discharging the carbon dioxide produced, the urinary system for disposing of other dissolved waste products of cell activity, the skin and lungs for getting rid of excess heat (into which most of the energy in food eventually degrades), and the circulatory system for moving all these substances to or from cells where they are needed or produced.
Like all organisms, humans have the means of protecting themselves. Self-protection involves using the senses in detecting danger, the hormone system in stimulating the heart and gaining access to emergency energy supplies, and the muscles in escape or defense. The skin provides a shield against harmful substances and organisms, such as bacteria and parasites. The immune system provides protection against the substances that do gain entrance into the body and against cancerous cells that develop spontaneously in the body. The nervous system plays an especially important role in survival; it makes possible the kind of learning humans need to cope with changes in their environment.
The internal control required for managing and coordinating these complex systems is carried out by the brain and nervous system in conjunction with the hormone-excreting glands. The electrical and chemical signals carried by nerves and hormones integrate the body as a whole. The many cross-influences between the hormones and nerves give rise to a system of coordinated cycles in almost all body functions. Nerves can excite some glands to excrete hormones, some hormones affect brain cells, the brain itself releases hormones that affect human behavior, and hormones are involved in transmitting signals between nerve cells. Certain drugslegal and illegalcan affect the human body and brain by mimicking or blocking the hormones and neurotransmitters produced by the hormonal and nervous systems.
Reproduction ensures continuation of the species. The sexual urge is biologically driven, but how that drive is manifested among humans is determined by psychological and cultural factors. Sense organs and hormones are involved, as well as the internal and external sex organs themselves. The fact that sexual reproduction produces a greater genetic variation by mixing the genes of the parents plays a key role in evolution.
Among living organisms, much behavior is innate in the sense that any member of a species will predictably show certain behavior without having had any particular experiences that led up to it (for example, a toad catching a fly that moves into its visual field). Some of this innate potential for behavior, however, requires that the individual develop in a fairly normal environment of stimuli and experience. In humans, for example, speech will develop in an infant without any special training if the infant can hear and imitate speech in its environment.
The more complex the brain of a species, the more flexible its behavioral repertory is. Differences in the behavior of individuals arise partly from inherited predispositions and partly from differences in their experiences. There is continuing scientific study of the relative roles of inheritance and learning, but it is already clear that behavior results from the interaction of those roles, not just a simple sum of the two. The apparently unique human ability to transmit ideas and practices from one generation to the next, and to invent new ones, has resulted in the virtually unlimited variations in ideas and behavior that are associated with different cultures.
Learning muscle skills occurs mostly through practice. If a person uses the same muscles again and again in much the same way (throwing a ball), the pattern of movement may become automatic and no longer require any conscious attention. The level of skill eventually attained depends on an individual's innate abilities, on the amount of practice, and on the feedback of information and reward. With enough practice, long sequences of behaviors can become virtually automatic (driving a car along a familiar route, for instance). In this case, a person does not have to concentrate on the details of coordinating sight and muscle movements and can also engage in, say, conversation at the same time. In an emergency, full attention can rapidly be focused back on the unusual demands of the task.
Learning usually begins with the sensory systems through which people receive information about their bodies and the physical and social world around them. The way each person perceives or experiences this information depends not only on the stimulus itself but also on the physical context in which the stimulus occurs and on numerous physical, psychological, and social factors in the beholder. The senses do not give people a mirror image of the world but respond selectively to a certain range of stimuli. (The eye, for example, is sensitive to only a small fraction of the electromagnetic spectrum.) Furthermore, the senses selectively filter and code information, giving some stimuli more importance, as when a sleeping parent hears a crying baby, and others less importance, as when a person adapts to and no longer notices an unpleasant odor. Experiences, expectations, motivations, and emotional levels can all affect perceptions.
Much of learning appears to occur by association: If two inputs arrive at the brain at approximately the same time, they are likely to become linked in memory, and one perception will lead to an expectation of the other. Actions as well as perceptions can be associated. At the simplest possible level, behavior that is accompanied or followed by pleasant sensations is likely to occur again, whereas behavior followed by unpleasant sensations is less likely to occur again. Behavior that has pleasant or unpleasant consequences only under special conditions will become more or less likely when those special conditions occur. The strength of learning usually depends on how close the inputs are matched in time and on how often they occur together. However, there can be some subtle effects. For example, a single, highly unpleasant event following a particular behavior may result in the behavior being avoided ever after. On the other hand, rewarding a particular behavior even only every now and then may result in very persistent behavior.
But much of learning is not so mechanical. People tend to learn much from deliberate imitation of others. Nor is all learning merely adding new information or behaviors. Associations are learned not only among perceptions and actions but also among abstract representations of them in memorythat is, among ideas. Human thinking involves the interaction of ideas, and ideas about ideas, and thus can produce many associations internally without further sensory input.
People's ideas can affect learning by changing how they interpret new perceptions and ideas: People are inclined to respond to, or seek, information that supports the ideas they already have and on the other hand to overlook or ignore information that is inconsistent with the ideas. If the conflicting information is not overlooked or ignored, it may provoke a reorganization of thinking that makes sense of the new information, as well as of all previous information. Successive reorganizations of one part or another of people's ideas usually result from being confronted by new information or circumstances. Such reorganization is essential to the process of human maturation and can continue throughout life.
To stay in good operating condition, the human body requires a variety of foods and experiences. The amount of food energy (calories) a person requires varies with body size, age, sex, activity level, and metabolic rate. Beyond just energy, normal body operation requires substances to add to or replace the materials of which it is made: unsaturated fats, trace amounts of a dozen elements whose atoms play key roles, and some traces of substances that human cells cannot synthesizeincluding some amino acids and vitamins. The normal condition of most body systems requires that they perform their adaptive function: For example, muscles must effect movement, bones must bear loads, and the heart must pump blood efficiently. Regular exercise, therefore, is important for maintaining a healthy heart/ lung system, for maintaining muscle tone, and for keeping bones from becoming brittle.
Good health also depends on the avoidance of excessive exposure to substances that interfere with the body's operation. Chief among those that each individual can control are tobacco (implicated in lung cancer, emphysema, and heart disease), addictive drugs (implicated in psychic disorientation and nervous-system disorders), and excessive amounts of alcohol (which has negative effects on the liver, brain, and heart). In addition, the environment may contain dangerous levels of substances (such as lead, some pesticides, and radioactive isotopes) that can be harmful to humans. Therefore, the good health of individuals also depends on people's collective effort to monitor the air, soil, and water and to take steps to keep them safe.
Other organisms also can interfere with the human body's normal operation. Some kinds of bacteria or fungi may infect the body to form colonies in preferred organs or tissues. Viruses invade healthy cells and cause them to synthesize more viruses, usually killing those cells in the process. Infectious disease also may be caused by animal parasites, which may take up residence in the intestines, bloodstream, or tissues.
The body's own first line of defense against infectious agents is to keep them from entering or settling in the body. Protective mechanisms include skin to block them, tears and saliva to carry them out, and stomach and vaginal secretions to kill them. Related means of protecting against invasive organisms include keeping the skin clean, eating properly, avoiding contaminated foods and liquids, and generally avoiding needless exposure to disease.
The body's next line of defense is the immune system. White blood cells act both to surround invaders and to produce specific antibodies that will attack them (or facilitate attack by other white cells). If the individual survives the invasion, some of these antibodies remainalong with the capability of quickly producing many more. For years afterward, or even a lifetime, the immune system will be ready for that type of organism and be able to limit or prevent the disease. A person can "catch a cold" many times because there are many varieties of germs that cause similar symptoms. Allergic reactions are caused by unusually strong immune responses to some environmental substances, such as those found in pollen, on animal hair, or in certain foods. Sometimes the human immune system can malfunction and attack even healthy cells. Some viral diseases, such as AIDS, destroy critical cells of the immune system, leaving the body helpless in dealing with multiple infectious agents and cancerous cells.
Infectious diseases are not the only threat to human health, however. Body parts or systems may develop impaired function for entirely internal reasons. Some faulty operations of body processes are known to be caused by deviant genes. They may have a direct, obvious effect, such as causing easy bleeding, or they may only increase the body's susceptibility to developing particular diseases, such as clogged arteries or mental depression. Such genes may be inherited, or they may result from mutation in one cell or a few cells during an individual's own development. Because one properly functioning gene of a pair may be sufficient to perform the gene's function, many genetic diseases do not appear unless a faulty form of the gene is inherited from both parents (who, for the same reason, may have had no symptoms of the disease themselves).
The fact that most people now live in physical and social settings that are very different from those to which human physiology was adapted long ago is a factor in determining the health of the population in general. One modern "abnormality" in industrialized countries is diet, which once included chiefly raw plant and animal materials but now includes excess amounts of refined sugar, saturated fat, and salt, as well as caffeine, alcohol, nicotine, and other drugs. Lack of exercise is another change from the much more active life-style of prehistory. There are also environmental pollutants and the psychological stress of living in a crowded, hectic, and rapidly changing social environment. On the other hand, new medical techniques, efficient health care delivery systems, improved sanitation, and a fuller public understanding of the nature of disease give today's humans a better chance of staying healthy than their forebears had.
Good mental health involves the interaction of psychological, biological, physiological, social, and cultural systems. It is generally regarded as the ability to cope with the ordinary circumstances people encounter in their personal, professional, and social lives.
Ideas about what constitutes good mental health vary, however, from one culture to another and from one time period to another. Behavior that may be regarded as outright insanity in one culture may be regarded in another as merely eccentricity or even as divine inspiration. In some cultures, people may be classified as mentally ill if they persistently express disagreement with religious or political authorities. Ideas about what constitutes proper treatment for abnormal mental states differ also. Evidence of abnormal thinking that would be deliberately punished in one culture may be treated in other cultures by social involvement, by isolation, by increased social support, by prayers, by extensive interviews, or by medical procedures.
Individuals differ greatly in their ability to cope with stressful environments. Stresses in childhood may be particularly difficult to deal with, and, because they may shape the subsequent experience and thinking of the child, they may have long-lasting effects on a person's psychological health and social adjustment. And people also differ in how well they can cope with psychological disturbance when it occurs. Often, people react to mental distress by denying that they have a psychological problem. Even when people recognize that they do have such a problem, they may not have the money, time, or social support necessary to seek help. Prolonged disturbance of behavior may result in strong reactions from families, work supervisors, and civic authorities that add to the stress on the individual.
Diagnosis and treatment of mental disturbances can be particularly difficult because much of people's mental life is not usually accessible even to them. When we remember someone's name, for example, the name just seems to come to usthe conscious mind has no idea of what the search process was. Similarly, we may experience anger or fear or depression without knowing why. According to some theories of mental disturbance, such feelings may result from exceptionally upsetting thoughts or memories that are blocked from becoming conscious. In treatment based on such theories, clues about troubling unconscious thoughts may be sought in the patient's dreams or slips of the tongue, and the patient is encouraged to talk long and freely to get the ideas out in the open where they can be dealt with.
Some kinds of severe psychological disturbance once thought to be purely spiritual or mental have a basis in biological abnormality. Destruction of brain tissue by tumors or broken blood vessels can produce a variety of behavioral symptoms, depending on which locations in the brain are affected. For example, brain injuries may affect the ability to put words together comprehensibly or to understand the speech of others, or may cause meaningless emotional outbursts. Deficiency or excess of some chemicals produced in the brain may result in hallucinations and chronic depression. The mental deterioration that sometimes occurs in the aged may be caused by actual disease of the brain. Biological abnormality does not necessarily produce the psychological malfunction by itself, but it may make individuals exceptionally vulnerable to other causes of disturbance.
Conversely, intense emotional states have some distinct biochemical effects. Fear and anger cause hormones to be released into the bloodstream that prepare the body for actionfight or flight. Psychological distress may also affect an individual's vulnerability to biological disease. There is some evidence that intense or chronic emotional states can sometimes produce changes in the nervous, visceral, and immune systems. For example, fear, anger, depression, or even just disappointment may lead to the development of headaches, ulcers, and infections. Such effects can make the individual even more vulnerable to psychological stresscreating a vicious circle of malfunction. On the other hand, there is evidence that social contacts and support may improve an individual's ability to resist certain diseases or may minimize their effects.
Copyright © 1989, 1990 by American Association for the Advancement of Science