The Human Animal: A General Survey of Its Composition, Structure, Function, Capacities, and Limitations
History has been made and experienced by a particular kind of animal, the human. We have examined the long sequence that led to the presence of human animals on this planet, and have noted the mammalian, primate, and uniquely sapiens features of them. Humans often forget about this heritage, seeing themselves in almost dualistic terms, as if mind and body were separate entities. But as we have just noted, humans are first and foremost bodies. Their physical nature has a way of asserting itself forcefully, reminding them of what they are, even humbling them. Now, for the record, we will describe the “typical” body of the animal that makes and experiences history—and be reminded of the physical essence that is at the core of its being.
I thought the following data would be a good place to start. These numbers refer to a human weighing 70 kilograms:
The human body consists of ~7 x 1027 atoms arranged in a highly aperiodic physical structure. Although 41 chemical elements are commonly found in the body's construction, CHON comprises 99% of its atoms. Fully 87% of human body atoms are either hydrogen or oxygen.1
If we measure a human body’s composition by the weight of the elements in it, the picture is different, but only slightly so. I thought it would be interesting to look at where the elements out of which we are composed fall on the Periodic Table. The lower the number of an element on the Periodic Table, the more basic and simple in structure it is. I also wanted to note what type of nucleosynthesis had created these elements. With this in mind, here are the elements out of which humans are composed:
Element by Weight Position in the Periodic Table Nucleosynthesis Type
Oxygen 65.0% #8 Stellar
Carbon 18.5% #6 Stellar
Hydrogen 9.5% #1 Primordial
Nitrogen 3.3% #7 Stellar
Calcium 1.5% #6 Stellar
Phosphorus 1.0% #15 Stellar
All Others 1.2% 2 Various Stellar, Primordial
In short, humans, like all other life forms, are basically composed of simple materials. There is nothing particularly exotic about them. And they are indeed “star stuff”. This is their chemical heritage.
In terms of water composition, a human body is about 60% water overall, but water content varies by the type of tissue. Muscle tissue, for example, has a much higher percentage of water than skeletal. Men typically have a higher percentage of water than women, chiefly because of differences in muscle mass. Some 55-60% of the body’s water lies within cells. The water between cells, in plasma, and in cartilage and connective tissue accounts for 35% of the body’s water. Other repositories of water comprise only a minor amount of the body’s total.3
Cells, Tissues, Organs
Humans, like all living entities, are composed of cells. There are varying estimates of the total number of cells in an adult human body, ranging from 1013 (ten trillion) to 1014 (one hundred trillion). The cells that comprise a human body are not a fixed set. They are continually being formed, used, broken down, and replaced, although different kinds of cells are replaced at different rates, and some may never be replaced in a lifetime. (There are differences of opinion on the regeneration of neurons, for example.) In a sense, a human is a constantly shifting and changing mosaic, with parts of him or her being torn down and rebuilt (at different speeds) constantly over the course of a lifetime.
Structurally, all human cells (except for blood cells) have a plasma membrane, the outer wall, which is semi-permeable, allowing various molecules to move in and out of the cell. The cytoskeleton gives a cell its shape, helps take in necessary molecules, and can shift internal cell structures. The cytoplasm is the gel-like substance that fills much of the cell body. It contains nutrients, and helps carry out many internal cell functions, such as moving material throughout the cell and helping to eliminate waste. Cells have structures known as organelles that carry out key functions. Chief among the organelles is the nucleus, a roughly spherical structure surrounded by its own semi-permeable membrane. The nucleus contains the chromosomes, which in turn contain the DNA that determines a human’s genetic make-up. It is in the nucleus that DNA is transcribed to messenger RNA (mRNA). The mRNA is transported out of the nucleus to the ribosomes. Ribosomes are organelles that float in the cytoplasm. Within them transfer RNAs (tRNAs) are used to arrange the sequence of amino acids used to synthesize proteins. This is the translation process. (See pages 220-221.) The mitochondria, as we have previously noted, are the organelles that chemically convert nutrients into energy. The endoplasmic reticulum and the Golgi apparatus are structures that transport proteins out of the cell, and lysosomes and peroxisomes contain digestive enzymes that break down proteins, nucleic acids, and certain kinds of sugars.4 It is this cellular team that carries out the most basic, essential functions of the body.
There are over 200 different kinds of cell in a human body. They come together to form the four separate kinds of tissue found in us. Epithelial tissue lines the surfaces of the skin and the internal structures within the body, providing protection to underlying tissues and forming boundaries between structures. Muscle tissue is capable of contraction and forms not only the musculature itself but also the muscle tissue of the intestines (and other internal organs) and the cardiac muscle of the heart. Connective tissue binds organs and other structures together, and forms the cartilage, bones, tendons, and ligaments. Nervous [or Nerve] tissue consists of the neurons and their supporting structures, such as the glial cells.5 Tissues in various combination form the organs of the body, collections of specialized cells structured to carry out specific tasks.
The human body contains, according to most classification plans, 11 distinct systems. A system is best understood as a set of organs which interact with each other to regulate a particular function of the human animal, and then in turn interact with the other systems, modifying and being modified by them. (The skin is a single organ, and thus atypical in some respects.)
The pitfall in listing these systems and describing their chief functions is that we can easily overlook the fact that all of them are in a continuous relationship with each other. These systems connect to each other through various interfaces, and they cannot truly function in isolation. Further, the body is a chemical factory and the site of constant energy exchanges and feedback loops. A human is alive because of the dynamic nature of his or her body. The systems that comprise it are therefore a system of systems, acting interdependently and synergistically to maintain those functions that will allow a human to stave off complete entropy, otherwise known as death.
THE NERVOUS SYSTEM
The function of a nervous system, as I pointed out earlier, is to provide information to an animal about its external situation and its internal status. Humans have the most “advanced” nervous system of any animal, owing to the highly complex brain that is its most important feature (and which will be the subject of much of Volume Two of this work). The average human brain is generally about three pounds (1.36 kilograms) in weight and contains approximately 100 billion neurons and 900 billion glial (structural) cells. (In recent years the glial cells have been found to be more significant than previously thought.)6 The neurons can form astonishingly numerous and complex interrelationships and combinations with each other.
As we will see in much greater detail in Volume Two, the brain is a collection of structures conserved across many species and some particular structures unique to humans. Brains have structures that regulate such functions as breathing and heartbeat. They have structures that process emotional responses to events in the world outside of, or within, a human. They have specialized regions of neurons that give a human the capacity for symbolism, visualization, sequential reasoning, and problem-solving. The sum total of the thoughts, feelings, urges, and (consciously noted) needs generated by the brain comprises human consciousness, the most debated-on phenomenon in the sciences. In my view, consciousness is purely a byproduct of the brain’s structure and function. I think that the mind is wholly contained within the brain, and exists nowhere else.
The brain and the spinal cord comprise the central nervous system (CNS). The brain and spinal cord are both covered in cerebrospinal fluid that acts as a protective layer. The fluid also fills the brain’s cavities or ventricles. The rest of the nerves in the body comprise the peripheral nervous system. Afferent nerves carry signals from various parts of the body to the brain. Efferent nerves carry signals from the CNS to the parts of the body. [When we say that a body part has a supply of nerve cells in it, we say that it is innervated.] The peripheral nervous system is subdivided into the sensory-somatic nervous system and the autonomic nervous system. In the somatic system, information from the senses is relayed to the brain, chiefly the cerebral cortex, and impulses from the brain are sent to the skeletal muscles of the limbs and trunk, allowing for voluntary movement. In the autonomic nervous system, signals from the body’s internal organs and the blood vessels are sent to regions of the brain that are not involved in conscious awareness, chiefly the hypothalamus, medulla, and pons. These signals trigger automatic responses by the appropriate brain regions that help the internal organs and blood vessels deal with environmental factors and stresses of various kinds.7 The autonomic system is divided into the sympathetic and parasympathetic divisions. Nerve impulses from the sympathetic division cause an excitation (an increase in activity) in a particular organ. The parasympathetic division sends signals that cause an inhibition of activity, a slow-down, in the organ’s function. The heart, to cite one instance, increases its heartbeat when receiving signals from the sympathetic division, and decreases its heartbeat upon receiving signals from the parasympathetic division. Most organs are innervated by both divisions of nerve cells.8 Nerve cells are covered in an insulating sheath known as myelin, which facilitates the transmission of nerve impulses.
The brain processes sensory stimuli from both the world external to the body and the world within it. It does so in such a way that a meaningful picture of reality, a version of the unknowable real reality, is presented to and shaped by human consciousness, in a deeply reciprocal manner that is not yet understood. These stimuli also suggest or compel certain actions on the part of a human (avoidance reactions, fight-or-flight responses, opportunistic behaviors, and so on).
The senses all have dedicated nerve pathways to the brain and particular regions of the brain devoted to processing their signals. The traditional view of the sensory apparatus (vision, hearing, smell, taste, and touch) is now challenged by many researchers, who would include in our list of the senses the sense of where one’s body is in space (proprioception), the sense of time passing, and several others. I am dubious about these schemes, inasmuch as it seems to me that these other alleged senses are sometimes simply elaborate combinations of the existing senses, combinations that are not well understood. In fact, the senses typically act in concert with each other. It is the perception of these combinations of sensory input, combined with memories and unconscious reactions, that forms the basis of that elusive phenomenon called an experience.
The stimuli received by the nervous system, and translated into perception in the human brain, create experiences that are irreducibly fundamental, i.e., they cannot be understood except through experience. One cannot know what it is like to perceive color without having done so. One cannot know what it is like to hear music if one has never done so. One cannot know what it is like to be hurt if one has never felt it. These experiences can be described to those who have never had them, but those who have never had them cannot know them.
The most significant sense is not necessarily vision, as most people might tend to believe. It is touch or feeling, the ability to physically sense the world. Pleasurable experiences, ones that feel good, tend to be reinforced, and painful ones, once experienced, tend to be avoided. In a way, as we go through life, we are receiving regular feedback from the physical environment: Touch this, don’t touch that, experience this, don’t experience that. Our behavior has been shaped to a larger degree than we might know by these constant “reality tests”. It is these tests that make us act carefully, so as to avoid unpleasant experiences.
And as much as humans hate it—and this is by far the most difficult subject for me to discuss of any in the world—pain, an aspect of the sense of touch, serves certain evolutionary purposes. (The nerves that are sensitive to it are called nociceptors.) It is a response by the body to injury or illness. It is the body saying, “I’m hurt (or sick)—do something now”. If we were never able to receive painful stimuli (and there are some humans who cannot), we would incur a multitude of injuries that could avoided by initial, brief suffering. (And any animal species the members of which were incapable of detecting harmful stimuli would be extinct in short order.) Indeed, if we never experienced discomfort, we would never shift position in our sleep, with potentially dire consequences. The problem, of course, is that an injured or sick part of the body keeps telling us it is hurt or sick long after we have perceived that it is. In a sense, pain is an idiot that doesn’t always know when to quit. It exists on a continuum from merely irritating to horrendous, and it is the human fear of it that makes us avoid certain situations, or submit to threats, or take extraordinary steps to relieve it. (Curiously, every part of the body can generate it except the brain, the place where it sensed.) Endorphins, natural pain-killers, are a response the body makes to it, but unfortunately their effects are transient.
Chronic or regular pain is the tragic reality of many. It can force us into inactivity, and drastically limit our focus. It can even drive some to suicide (and in my opinion it is the only justifiable cause for self-destruction). The desire to protect their loved ones from it makes most humans fiercely protective of them. The fear of experiencing it, or the fear of others we care about experiencing it, forms the basis of many of our worst moments. The willingness to face it is one of the tests of bravery. The ability to endure it and carry on is one of the definitions of everyday courage. The wanton infliction of it is one of the definitions of cruelty and barbarism. Medical science in part arose in an effort to ameliorate this form of suffering. As I said, this subject is difficult for me—I almost left it out of the book, but that would have been less than honest of me. In my view, one of the great tasks humans face is finding new ways to treat it, and reduce it to an absolute minimum. (It is entirely natural, but completely useless, to despairingly ask “Why?” when we or a loved one are in pain. The “why” is because we are animals. The focus must be completely on relieving the suffering.) Two of our other great tasks are to get humans to stop using it to punish other humans, and to stop humans from threatening each other with it. On these latter points, given our history, I am not optimistic.
Through deliberate effort, consciousness can modify the sensations we experience. We can direct our attention purposefully elsewhere. Further, we can be distracted by focus on some stimuli at the expense of others, shifting our view of reality and the nature of our experiences. Further, the perception of sensory or somatic events by a human is subject to a wide range of variations (such as tolerance to cold). But however we are perceiving the world, it is the nervous system that reminds us of the physicalness of the human being. It is our tie to the external world, the indispensable link between the real reality that exists beyond our comprehension and the internal reality of the self.
THE CARDIOVASCULAR SYSTEM
It is the cardiovascular system that circulates oxygen, hormones, nutrients, and immune cells to every part of the body and removes carbon dioxide and waste products produced by cells. The center of the system, the heart, has an average size of 13x9x6 centimeters and weighs about 300 grams—only about two-thirds of a pound. The heart lies within a sac called the pericardium. This sac is attached to the sternum and various tissues of the thorax. It is connected as well to the membrane-like covering of the superior vena cava. (The SVC is a large vein that channels deoxygenated blood back to the heart from the head, chest, neck, and arms.) The pericardium is also connected with the membrane that covers the arteries and veins that connect the heart to the lungs. [Arteries carry blood away from the heart; veins carry blood back to it.]
The heart itself has grooves on its exterior that hold blood vessels, most importantly sections of the coronary artery that serves the heart muscle itself. Within the interior of the heart there are two halves divided by a semi-vertical wall of tissue. The two halves in turn each hold two chambers. The upper chambers are known as the atria [singular atrium], the lower chambers the ventricles. The atria receive blood from every part of the body and send it to the ventricles. The ventricles pump blood to every part of the body. The right atrium receives deoxygenated blood through the SVC and its smaller companions, the inferior vena cavae. The inferior vena cavae bring in blood from the abdomen, pelvic areas, and legs. The right atrium does not receive blood from the lungs, however. Blood from the right atrium is sent to the right ventricle, which then pumps it into the pulmonary artery, which branches out and sends blood to the lungs, where carbon dioxide is exchanged for oxygen. The left atrium is connected to four pulmonary (lung) veins, which feed it this oxygenated blood. The very thick-walled left ventricle receives this blood from the left atrium and pumps it out through the largest artery in the body, the aorta. The aorta distributes this blood to every part of the body except the lungs. Coronary vessels circulate blood within the heart itself.9
The heart has three layers. It is the contracting of the middle layer, the myocardium, that drives the heart’s pumping actions. All of this blood flow is regulated by a series of valves that open and close in sequence, preventing backflow. It is the opening and closing of these valves that we hear as the heartbeat.10
The vessels that circulate blood consist of the arteries, arterioles, capillaries, veins, and venules. The system of which they are a part is extraordinarily complex. Astonishingly, if all of the blood vessels in a human body were laid out in a straight line, they would be 60,000 miles (about 96,500 kilometers) in length. The tissues of the body are supplied by 20 major arteries [such as the carotid, thoracic, and femoral] which branch [in fractal-like iterations] first into arterioles and then capillaries. Capillaries finish the job of actually delivering oxygen, nutrients, hormones, and immune cells to the body’s cells. Capillaries collect carbon dioxide and waste and send the blood containing them to larger vessels called venules, which feed into veins.11
The blood that is being circulated by this extraordinary system amounts to a volume of about five liters in the average adult. By content, it is 60% plasma (water, proteins, sugars, and fat) and about 40% red blood cells, platelets, and white blood cells. Blood cells are made in the bone marrow. The extraordinarily tiny red blood cells contain hemoglobin (which gives blood its red color), a protein that binds with oxygen, and the red blood cells carry that oxygen throughout the body. When red blood cells have run their course (within about four months), they are removed from circulation by the liver and spleen. The white blood cells play a part in protecting the body from infection. Platelets help blood clot after wounds have been incurred.12
Not all human blood is identical. There are four broad blood groups in the human population, A, B, AB, and O. The type of blood a human has is determined by two factors. The first is the kinds of antigens that cling to the surface of the red blood cells in the human’s body. Antigens are substances that can trigger an immune response. Some are sugars, some are proteins. Every person’s blood type is characterized by its own antigens, which are ignored by the person’s immune system. Broadly speaking, antigens are divided into A and B categories. The second factor is the kind of antibodies in the plasma. Antibodies are substances that attack invading antigens. People with certain kinds of antigens on their blood cells lack the antibodies that attack that kind of antigen. There is a third major kind of antigen as well, Rh. One either has it (making one Rh+) or not (making one Rh˗). One can be therefore be A˗ or O+, for example. If a person possessing a particular kind of blood receives blood with antigens that his or her own blood does not have, the results can be harmful, even lethal (which is why doctors are so careful when transfusing blood). Different regions of the world are dominated by different blood types. For example, in Asia, Type B is most common. In the Americas, and among Aboriginal Australians, Type O is most often found, and in Central and Eastern Europe Type A is very common. The rarest major blood type is AB.13 It should be noted, by the way, that there are many more, lesser blood groups in the human population (some with as few as 50,000 members) and there are now 32 distinct types that have been identified.14
So the cardiovascular system is the means by which the body’s vital needs are distributed throughout it. It is involved with all other systems, but it is most intimately tied to…
THE RESPIRATORY SYSTEM
The most fundamental need of a human being, the one that must be met above all others, is for oxygen. It is the job of the respiratory system to take in oxygen with the air, exchange it for carbon dioxide, and work in conjunction with the cardiovascular system to distribute that oxygen throughout the body. The central part of the respiratory system is of course the lungs, two sac-like objects situated in the thoracic cavity, each divided into upper, middle, and lower lobes. (To recall the evolution of lungs, see pages 271-272.) The lungs sit on either side of the heart. Air reaches the lungs by being inhaled through the mouth or nose (and entering the sinus cavities as well), traveling past the pharynx, and through the larynx, the organ of speech, which also acts as a protective sphincter to the airways. Air then passes through the trachea. In the thoracic cavity the trachea divides into two passages known as the left and right principal bronchi. [Singular: bronchus] Each of these in turn divides into smaller bronchi, which then branch off into smaller passages.15
The ends of this branching are the hundreds of millions of tiny air sacs in the lungs, known as the alveoli. [Singular: alveolus] Each one is about one-tenth of a millimeter across. The alveoli are lined with surfactant, a lubricant that helps them function. Oxygen passes through the walls of the alveoli into the network of capillaries that surrounds them. Each of these walls is incredibly thin—only about one-tenth of micrometer. The alveoli also collect wastes, which are expelled through exhalation. The breath intake of a young male is about 500 milliliters, the average female breath intake about 450 milliliters. In a minute, the average number of breaths is 12, so young males take in six liters of air a minute. Strenuous exercise can cause as much as 17 liters per minute to be taken in.16
The whole process of breathing is facilitated by the muscles located between the ribs, and by the diaphragm, the thick, muscular wall at the lower end of the chest cavity. The contractions of the diaphragm take air in; relaxations of the diaphragm push air out. Thus respiration is accomplished—and our priority need met.
THE SKELETAL SYSTEM
The skeletal frame of a human consists of 206 distinctly identifiable bones, ranging in size from the femur, the longest bone, to the stapes (or stirrup) bone in the inner ear. The skeleton is the indispensable support framework of the body. (To recall the evolution of the skeleton, see pages 254-255.)
Two experts in the anatomy and physiology of bones have summarized bone function in this manner:
Bones act as essential mechanical components of the musculoskeletal system. They serve to protect and support soft tissues; to anchor muscles, tendons, and ligaments; and as the rigid levers that muscles operate to produce movement. Bones also function as physiologically critical centers for the production of blood cells, as storage facilities for fat, and as reservoirs of important elements such as calcium (essential for blood clotting and muscle contraction).17
These scientists are also careful to point out that bones exhibit variation. They undergo change throughout the growth years. Their average size differs from male to female. Different populations exhibit different average bone sizes, and bones obviously exhibit individual (or idiosyncratic) variation as well. We also forget sometimes that bones are alive. They are not inert pieces of mineral holding up our bodies. They are capable of self-repair. Made out of a composite of collagen and hydroxyapatite, they are also tremendously strong yet light in weight.18
The skeleton can be divided into the following regions: the skull, the vertebral column, the thoracic cage, the upper limb bones, and the lower limb bones. The skull, the vertebrae, and the thoracic cage form the axial skeleton, the center of the system. The limbs form the appendicular skeleton. The teeth are a part of the skeletal system, but they are not bones.
In the skull there are the bones of the cranium and the face. We tend to think of the cranium as being a solid unit, but it represents the fusion of different bones (occipital in the rear, two parietal bones that form the sides and the top, the frontal bone that comprises the forehead, two temporal bones that form part of the sides and the base of the skull, the sphenoid bone toward the front, and the ethmoid bone that forms the root of the nose).19 There are 14 facial bones. We have already noted several of the more prominent bones of the face, such as the mandible, the maxilla, and the zygomatic arches (the cheekbones).
The vertebrae are 26 in number, 24 of which are divided (from top to bottom) into cervical, dorsal, and lumbar types. The sacrum and the coccyx form the bottom two. The vertebrae form a column of support for the cranium and the trunk, and they provide protection for the spinal cord. They are connected to each other by a series of muscles and ligaments. It is the atlas vertebra at the top of the spinal column that attaches to the head. It is capable of rotating (which allows your head to turn) and it pivots on another vertebra called the axis.20 In the thoracic cage lie the sternum and 24 ribs, 12 ribs per side [the same number for both males and females, despite what some might think].21 Above the sternum lies the hyoid bone which, as we saw in the previous chapter, anchors the tongue and is the only bone in the body not connected to any other.
The large upper limb bones begin with the clavicle (collar bone) and scapula (shoulder bone) that form the shoulder girdle. Next, there are the upper arm bone or humerus, and the ulna and radius that form the forearm. In the remarkably complex wrist and hand there 8 bones in the carpus (the wrist itself), 5 metacarpal bones of the hand, and 14 phalanges (three in each finger and two in the thumb). At the top of the lower extremities is the very strong pelvis. The pelvis includes the hip bones. The pelvis supports the spine and rests on the lower limbs. Its configuration differs from male to female, [since the female pelvis has evolved to allow for the presence of a birth canal]. Next is the femur or thigh-bone, the longest, biggest, and strongest bone in the entire skeleton. Below that is the patella (which sits atop the knee-joint), the tibia at the front of the lower leg, also very strong, and the fibula, on the outer side of the leg. The ankle and feet mirror the complexity of the wrist and hands, with 7 bones in the ankle region, or tarsus [the heel bone being the largest one], five metatarsals in the foot, and 14 phalanges (2 in the big toe and 3 in each smaller toe). The tiny bones of the ear round out the skeleton.22
The meeting places of the bones are the joints, also known as articulations. There are three broad categories of them—immovable, slightly movable, and freely movable. Cranial joints are immovable. Vertebral joints are slightly movable. Vertebral joints are lined with cartilage, which is in turn connected to a disc [which acts as something of a shock absorber]. There are several other slightly movable joints in the body. Most joints in the body are freely movable. They come in six different varieties, and allow (by one count) 16 different kinds of movement.23
The skeletal system works most closely in tandem with…
THE MUSCULAR SYSTEM
There is muscle tissue throughout much of the body, but here we are primarily concerned with that muscle attached to the skeleton. The percentage of the body’s weight that is comprised of muscle varies throughout life, ranging from about 22-25% in a newborn to about 42% in a five year-old boy and 40% in a five year-old girl. By 17, boys are about 54% muscle by weight (on average) and girls are about 45%. At all stages of life males tend to have a greater percentage of muscle. After age 60, there tends to be a considerable drop-off in muscle mass.24
Skeletal muscles are controlled by motor neurons found in the central nervous system. Motor neurons control motor units, which consist of the motor neurons themselves and the specific muscle tissue they control. Electrical signals from the motor neurons cause calcium ions in the muscle tissue to be released. This release causes a reaction in the action myofilaments of the muscle, the actual contractile tissue. The cells shorten; the muscle contracts. When calcium ions return to their original positions, the muscle relaxes. The fuel for this reaction is chiefly the adenosine triphosphate (ATP) within the muscle cells. All of this is accomplished in a few thousandths of a second.25
Muscles generally work in antagonistic pairs. When one muscle contracts, the paired muscle relaxes. For example, in the arm, the biceps and triceps are paired. When we bend the arm, the biceps contract, the triceps relax. When we straighten the arm, the triceps contract and the biceps relax.26 The musculature of a human is arranged in 656 muscles distributed throughout the body. There are 327 paired antagonists and two unpaired muscles. These muscles can be divided into three areas: the muscles of the head (which handle such functions as facial expression and chewing), the muscles of the neck (which move the head, the hyoid bone, the larynx, and the upper ribs), and the muscles of the trunk and limbs, the most important of which are involved in breathing and the movement of the vertebrae. Every region of the trunk and extremities can be subdivided by its muscle pairs. One of the largest muscles in the body, one that is heavily involved with the movement of the trunk, is the gluteus maximus. 27
Finally, movement is also made possible by the ligaments and tendons, tissues made of collagen. Ligaments connect bones to other bones; tendons connect muscles to bone. It is the coordinated actions of the musculature, the nervous system, and the skeletal system (supported at all times by the circulatory and respiratory systems) that allows an animal to roam the landscape—including animals of our kind.
THE DIGESTIVE SYSTEM
The digestive system transforms food into forms that a human’s metabolic processes can use. The digestive system is basically the digestive tract, a series of structures that physically break food into progressively smaller units, allowing the nutritionally-valuable parts of it to be absorbed and used in the processes of cellular energy generation. It of course starts with the mouth, where the teeth grind food up and the salivary glands, (controlled by the autonomic nervous system), facilitated by the tongue, begin the act of disintegrating the food. Teeth, as we have seen in some detail, developed to have specialized functions. Human teeth [and the curvature of the rows of teeth and the hard palate in general, the dental arch] are distinctive, as every anthropologist knows. The cheeks, palate, and lips are involved in the initial break-down process as well.28
After being sent by muscles through the pharynx, ground up food travels through the esophagus and into the stomach. (Swallowing, by the way, is an involuntary reflex. Once food reaches a certain point in the throat, it will be swallowed without conscious choice.)29 There is a valve that acts as a gateway between the esophagus and the stomach. The stomach, of course, is highly variable in shape. It is served by a complex network of arteries and nerves, and has multiple layers of muscle. The stomach churns and mixes food, while secreting gastric acids and enzymes to break down food further, reducing it to a semi-liquid form called chyme. Gastric acids help break apart proteins and stimulate the flow of bile and pancreatic juice.30
The stomach is connected by the pyloric valve to the duodenum, the first part of the small intestinal tract. Chyme is periodically released into the duodenum. It takes between two and four hours for the stomach to empty completely. (Fats take the longest time to be processed.) Secretions from the liver (by way of the gallbladder) and the pancreas, feed into the duodenum from the common bile duct and the pancreatic duct. Enzymes released by the pancreas help digest carbohydrates, proteins, triglycerides, and nucleic acids. The liver, the heaviest gland in the body, is divided into two large lobes, left and right. The gall bladder is mostly covered by the liver’s right lobe. The liver secretes bile, which travels through ducts and is temporarily stored in the gall bladder. It has digestive functions in physically breaking down fats (lipids). Bile also is excreted. As it is broken down, one of its products is stercobilin, which gives feces their characteristic brown color.31 In addition to secreting bile, the liver helps metabolize carbohydrates, lipids, and proteins, store vitamins (such as vitamin A and B12), and synthesize vitamin D, among other functions. The liver is truly a crucially important organ.32
It is in the small intestine that the major digestion of carbohydrates, proteins, lipids, and nucleic acids takes place, made possible through the action of a variety of enzymes. The small intestine is divided into the duodenum, jejunum, and the ileum. Peristaltic contractions mix chyme with digestive juices. This action also brings the mixture in contact with the membranes (mucosa) that line the intestine. The inside walls of the small intestine are covered with tiny, finger-like projections called villi. [Singular: villus] Since there are 20-40 villi per square millimeter, the total area of the surface of the small intestine’s interior is very large. When the digestion of nutrients is complete, absorption takes place. The tiny, thin-walled villi contain networks of capillaries, by which nutrients enter the bloodstream. Electrolytes, vitamins, and water are also absorbed.33
Finally, the large intestine completes the digestive system’s functions, producing some B vitamins and vitamin K, which are then absorbed, and forming and expelling feces. The large intestine is about 1.5 meters in length and is divided into four different regions, the ascending, transverse, descending, and sigmoid colons. Chyme, now essentially stripped of its nutrients, is prepared for expulsion by bacteria living in the colon. These bacteria also generate gases, the origin of flatulence. Peristaltic action pushes feces from the sigmoid colon to the rectum, where it is expelled through a reflex action.34
Thus we are nourished, and our metabolic processes sustained.
THE REPRODUCTIVE SYSTEM
This, of course, can be thought of as a system with two very different but complementary variations. It is the primary sex characteristics of the reproductive system that determine our male or female status. It is the job of the reproductive system to facilitate the bringing together of gametes—sex cells that contain genetic material—so that a new human might be conceived, nurtured, and birthed. The reproductive system works closely with the endocrine system in both males and females. (See below.)
From an evolutionary standpoint, the male reproductive system exists to generate sperm cells and deposit these cells in the female reproductive system. Sperm are generated in the testes (a part of the endocrine system) through what is called spermatogenesis. The testes are filled with structures called seminiferous tubules. Within them are spermatagonia, stem cells out of which sperm are made. These split to create either more spermatagonia or spermatocytes. Spermatocytes produce spermatids, which contain exactly half of the man’s chromosomes. (These chromosomes, of course, can come in many combinations.) It is from spermatids that sperm cells are made. Sperm cells are very small, not much more than cell nuclei and flagella. In the areas between the seminiferous tubules are Leydig cells. These are the cells that produce the hormone testosterone. Testosterone, in addition to causing the appearance of secondary sex characteristics in a male, is vital for sperm production.35 A series of tubes connects the testes to the urethra, the most significant of which is the vas deferens. Sperm cells travel through these tubes, and in the ejaculatory duct are intermixed with seminal fluid. Seminal fluid is generated mainly by secretions from the seminal vesicles, the prostate gland, and Cowper’s gland.
The male organ for accomplishing internal fertilization is, of course, the penis. The flaccid penis has a sheath of skin, known as the foreskin, that covers most or all of the glans. The foreskin retracts when the penis becomes erect. (The foreskin is often removed, of course, through circumcision.) When the penis is flaccid, there is relatively little blood flow in it, and the amount of oxygen gas in it (called the partial pressure of oxygen) is low as well. Men become sexually aroused because of visual stimulation, manual stimulation, a combination of both, or simply the imagining of erotic stimuli. Signals are sent from those centers of the autonomic nervous system in the brain that control the penis’s function. These signals reach the erectile tissue of the penis, structures containing smooth muscle cells. [Smooth muscle fibers are much narrower than skeletal muscle fibers, and are not striated.] This erectile tissue is known as the Corpora cavernosa. When a man is sexually aroused, neurotransmitters in the Corpora cavernosa are released, causing these smooth muscles to relax and the arteries and arterioles of the penis to dilate. Structures in the penis called sinusoids expand by trapping incoming blood. The covering of the erectile tissue, the tunica, stretches and helps reduce the outflow of blood through the penis’s veins. The partial pressure of oxygen in the penis increases drastically. As a consequence of all this, the penis is then erect, ready (potentially) to enter a woman’s vagina and, after tactile stimulation and ejaculation, deposit semen in it. Detumescence, or the return of the penis to a flaccid state, is characterized by the reversal of these processes.36
Human penises, although they come in a variety of shapes and sizes, and when erect stand out at various angles, are the largest in the entire primate order. From an evolutionary standpoint the long and semi-cylindrical shape of the penis is ideal for pushing out the walls of the vagina and releasing sperm in the deepest part of it. The ridge on the glans may even have evolved for the purpose of displacing rival sperm from the vagina.37
From an evolutionary standpoint, the job of the female reproductive system is to receive sperm from a male so that an ovum (see below) might be fertilized, and the resulting zygote nurtured and allowed to develop into a fully-formed infant. The birthing process completes this task. Consequently, the female reproductive system is more complex than that of the male. The outer genitalia include the mons veneris (or mons pubis), a fleshy protuberance above the vaginal opening, the labia majora and labia minora, lip-like structures at the opening of the vagina, and the clitoris. The clitoris is the female analog of the penis, and is highly sensitive to stimulation. It is located above the labia and is covered by a fold of skin called the prepuce, a fold which may be thought of as analogous to the foreskin of the penis. The mons, labia, clitoris, and opening of the vagina collectively constitute the vulva. The female urethral opening lies between the top of the labia minora and the clitoris.
The vagina ranges from about 7 to 10 cm in length. It is variable in shape and lined with muscle tissue. Sections of the vagina are physically supported by either the rectum or the ligaments of the cervix. The vagina is served by a complex network of arteries and veins, its major arteries being a branch of the pudendal artery, which serves the anterior [front] section, and a branch of the internal iliac artery that serves the posterior. The vagina does not have a large number of nerve endings, and is less sensitive than many other structures of the skin.38
When sperm are deposited in the vagina, they encounter a highly acidic environment, which limits their ability to swim. However, the alkalinity of seminal fluid balances the pH of the vagina considerably, which increases the ability of sperm cells to move up that passageway. Sperm cells move by rapidly moving their tails. Upon reaching the top of the vagina the sperm encounter the cervix and the cervical canal, the lower regions of the uterus. The cervical canal is the passageway between the vagina and the main part of the uterus. It is lined with mucus and cervical fibers. Estrogen released before ovulation (see below) liquefies this mucus somewhat. The cervical fibers, normally densely packed, are also affected by estrogen, which causes them to separate somewhat and create gaps. Sperm travel through these gaps almost single file at a speed of between 1.2 and 3.0 millimeters per minute. The vast majority of the sperm that enter the cervical canal are destroyed. The canal has many recesses in which sperm cells are trapped. One estimate is that about 182 million sperm cells (on average) start the journey to the cervix, and fewer than 1 million survive the cervical canal.39
The next structure on the journey is the main body of the uterus, the top of which is known as the fundus. Basically, the uterus is a small, hollow organ that is lined with smooth muscle. It normally weighs about two ounces. During pregnancy, it can grow to two pounds (almost half a kilogram). When a woman is not pregnant, the uterus is about three inches long and two inches wide in its main section. Its thickest part is about 1 inch in diameter, and the uterine wall itself is about one-half inch in diameter. The internal cavity of the uterus is lined with epithelial cells, a lining called the endometrium. (For a more detailed account of the endometrium and the evolution of the placenta, see pages 287-288.) Every month from the onset of the first menstrual period to early menopause [when period frequency becomes irregular] the uterus prepares to receive a fertilized egg. If none is received, the lining of the uterus is sloughed off during the menstrual period. [By the way, it is the pronounced contractions of the uterine muscles during the menstrual period that cause the miseries associated with menstrual cramps.] Normally positioned between the rectum and the bladder, the uterus can be pushed forward or backward depending on the distension (enlargement) of either. The position of the uterus can also vary depending on how many children a woman has had.40
When sperm cells enter the main part of the uterus, they move up the interior wall of it, some of them having been given momentum by the cervical fibers, others propelled by contractions of the uterine muscles. During sexual intercourse, the stimulation of the cervix causes the woman’s posterior pituitary gland to release the hormone oxytocin. The circulatory system delivers this to the uterus rapidly, and the oxytocin stimulates greater muscle contractions. But the sperm face more perils. White blood cells enter the uterine walls in great numbers, responding to the presence of sperm. They destroy large numbers of the sperm cells making their way up the walls. Only a few thousand survive to reach the next step in the reproductive process, the uterotubal junction, a narrow area separating the uterus from the oviduct.41
The oviducts, or Fallopian tubes, connect the uterus with the ovaries and the structures surrounding them. Since the ovaries are organs of the endocrine system, they also synthesize and release hormones. [The ovaries are therefore analogous to the testes in the male reproductive system.] An adult ovary is 2.5-5.0 cm in length and 1.5-3.0 cm in width. Ovaries are innervated by nerves of the ANS and they receive a particularly plentiful blood supply.
In the ovaries are immature female gametes (sex cells) called oocytes. Oocytes are enclosed in sacs of tissue called ovarian follicles. These sacs are tiny, measuring only about 50 micrometers in diameter. At this stage they are called primordial follicles. A baby girl has all the oocytes she will ever have at birth, about 2 million. By puberty, only 10% will remain. By menopause, the supply will be almost gone. A woman will ovulate between 400 and 500 mature oocytes over the course of a lifetime. The rest will die through a process known as atresia, a kind of programmed cell death.
Some of the primordial follicles will eventually grow into primary follicles, about 100 micrometers in diameter. The follicles are covered by layers of cells which get increasingly thick and supplied with blood. It then takes about four months for primary follicles to grow into secondary follicles. Only some of the secondary follicles will grow into tertiary follicles, a process taking 2-3 more months. The tertiary follicles are surrounded by thicker cell layers. These layers of cell contain hormones, enzymes, anticoagulants, and electrolytes.
Follicles secrete steroid hormones—estrogens, progestogens, and androgens. The pituitary gland secretes two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH) that control this process. The estrogens and progestogens play a role in transporting an egg down an oviduct, in addition to their other functions.
Tertiary follicles, which start out at 1-9 mm in diameter, grow into the more mature stages of follicle development. It takes about three weeks for them to grow into their most mature stage. In a menstrual cycle, about 20 tertiary follicles form in each ovary. But only one of these larger follicles will become fully mature per menstrual cycle. It is known as the preovulatory follicle. The rest die. The final, surviving follicle contains an ovum [Plural: ova], or egg, that has 23 of the female’s 46 chromosomes, chromosomes which can, of course, come in a wide range of combinations. The mature ovum escapes from the follicle. It is the breaking of the follicle’s wall that is the actual act of ovulation. The breaking of the wall seems to be facilitated by prostaglandins, a family of molecules found throughout the body. When an ovum escapes, it moves toward the oviduct that serves its side of the body. The oviduct is about 10 cm long. The ovum is captured by a portion of the oviduct containing tiny, finger-like structures. Cilia inside the oviduct move it in the direction of the uterus.42
Those sperm that have survived the vagina, cervical canal, and uterus reach the uterotubal junctions of the two oviducts. There are only a few thousand left out of perhaps 200 million that began the journey. About half of them are at the wrong place and enter an oviduct with no ovum in it. Of the half that are at the right place, only a few hundred make it past the opening. They reach a section of the oviduct called the isthmus. By now, there only about 20 of them in proximity to the ovum. As the ovum nears them, several make the attempt to fertilize it. Only one will succeed. The successful competitor will release an enzyme that helps it break down a section of the zona pellucida, the membrane that covers the ovum. A special protein in the sperm will bind with a sperm receptor on the ovum’s surface. The sperm’s nucleus enters the egg’s cytoplasm, and it moves toward the ovum’s nucleus. The ovum undergoes a chemical change that destroys all the receptor sites on its surface so no other sperm can attach itself to it. Finally, to oversimplify the story somewhat, the nuclear membranes of both sperm and egg break down and their haploid chromosome sets merge into the diploid chromosome set from which a new human will be built. This is syngamy—the actual moment of conception. Soon after, the zygote divides and makes two duplicates, the blastomeres. The development of a human being has started.43 The whole fertilization process itself takes about 24 hours from the time the sperm penetrates the ovum to the formation of the zygote. The fertilized egg is about the size of the period at the end of this sentence.44
After a few days, the fertilized egg enters the uterus. As we have previously seen, the fertilized egg attaches itself to a layer of cells in the endometrium of the uterus called the decidua basalis. From this point, the body’s only temporary organ, the placenta will begin to develop. After a total gestation period of 266-280 days, if all goes well—and all too frequently it does not—a baby will be born.
When can the reproductive sequence occur? In the menstrual cycle, there are three distinct phases: the follicular phase, when the tertiary follicles, as we have seen, are developing; the ovulatory period, when the ovum is released, and the luteal phase, in which the discarded follicle of the ovulated egg secretes hormones to help prepare the uterus for the implantation of a fertilized egg. For most women, the menstrual cycle is 25-30 days in length, with a cycle of 28-30 days being most common. Although women generally call the day they first start menstruating Day One of their period, it actually represents (as they well know) the end of the cycle, the shedding of the endometrium, indicating that no fertilized egg was implanted. In fact, the beginning of the follicular phase can even start one or two days before menstruation begins. The follicular phase lasts about 14 days, although this can vary somewhat, [see above] and the luteal phase is consistently 12-15 days in length. No fewer than four hormones regulate menstrual function, including FSH and LH.45
Opinions vary in regard to how long sperm can live in the body, with most researchers saying anywhere from two to five days (although some say as long as seven). And how long is an ovum fertilizable? Again, opinion varies, with estimates running from 12 to 48 hours. So it would appear that the middle 4 to 6 days of a woman’s menstrual cycle are the optimum time for a her to become pregnant.
What is the broader window of reproductive opportunity? The average age of the first menstrual period (menarche) is somewhere between 12 and 13, although there are many exceptions.46 At the other end of the scale, a major longitudinal study conducted in the United States established that the median age for the onset of menopause was 51.3, with most women falling into a range between 47 and 55 years of age.47 And while men of surprisingly advanced ages are capable of fathering children, the decline of testosterone that accompanies the aging process and increasingly frequent erectile difficulties tend to reduce the abilities of older men to reproduce.
Thus the human species is renewed. The journey of the sperm to the egg, by the way, takes place over a length of about six inches, or 15 centimeters.48
THE ENDOCRINE SYSTEM
We have already seen the huge impact the endocrine system has on our reproductive process. As we will see, it has a wide variety of other functions that are absolutely critical to the other systems of the body. So what is this system, and what do we mean by the term endocrine?
There are two kinds of glands in the body. One variety is called exocrine glands. Exocrine glands have ducts that release their products in a specific area (such as the way sweat glands operate on the skin). Endocrine glands are ductless, and release their products—hormones—directly into the cardiovascular system.
What is a hormone? Broadly speaking, a hormone is any chemical substance that performs a regulatory function inside of a human body. In the body, hormones are synthesized, go through an active phase, and then deteriorate. Different researchers categorize them in different ways. Some put each of the four varieties in their own category, others lump all the hydrophilic (“water-loving”) hormones in one category, and the hydrophobic (“water fearing”) hormones in another. The hydrophilic hormones are those based on proteins, peptides, and modified amino acids. The hydrophobic hormones are the steroids. All hormones bind themselves to receptors located on the outside of the cells they “target”. Agonists cause a particular effect to occur; antagonists block the agonist’s efforts to bind with the target cell.
Hormones are regulated by several mechanisms that maintain homeostasis (in this case, meaning chemical equilibrium) through various kinds of feedback loops. (To recall homeostasis and the function of feedback loops, see pages 134-136.) Sometimes excess production of a hormone stimulates the production of a second hormone, which suppresses the first. Hormones can act as mutual antagonists to each other as well. In some cases, the level of some particular substance can govern hormone production. If the level is too low, production is stimulated. If the level is too high, the production of the hormone is suppressed.49
Hormones are so significant that some scientists like to refer to the nervous and endocrine systems as a single over-arching entity. There is good reason for this. The brain, the center of the CNS, is the site of three endocrine glands: the pineal, the hypothalamus, and the pituitary. The pineal gland is in the lower-middle part of the brain, shaped like a pine cone (hence the name pineal) and is about the size of a garden pea, weighing about 100 to 150 mg. In its evolutionary history, it was a photoreceptor (light-sensor), and it retains a link to that past. The pineal gland secretes the hormone melatonin in response to the light and dark cycles a human lives in. The melatonin helps organize the body’s waking and sleeping rhythms in response to those cycles 50
The hypothalamus, to which we have already referred in connection with the ANS, is one of the most significant of all the endocrine glands. The hypothalamus is the primary interface between the nervous and endocrine systems. It is located just below the thalamus, which is to say just above the brainstem, in a region of the brain known as the diencephalon. It is very small in size compared to many other structures, and yet its functions are critical. The hypothalamus receives information from the cerebral cortex, the brainstem, the basal ganglia [which are involved in the regulation of body movement], the thalamus, and the olfactory regions [those associated with the sense of smell]. It has reciprocal relationships with several areas, and sends information to many other regions of the body. It has, strictly speaking, both endocrine and non-endocrine functions. It is involved with the secretion of oxytocin and vasopressin. [Vasopressin helps regulate blood pressure, increases peristalsis in the digestive tract, influences the uterus in combination with oxytocin, and helps regulate the kidneys.] The hypothalamus is involved with the regulation of the body’s temperature, the regulation of appetite (including the monitoring of blood glucose levels), the expression of emotion (especially sexual or aggressive emotions, and feelings of fear, horror, rage, or euphoria), sex drive (or libido), and the regulation of the heart.51
The pituitary gland, a small gland at the base of the skull, between the optic nerves, is also enormously important. It is also involved in the regulation of the body’s temperature. It affects the production of testosterone and estrogen, is involved with growth during childhood (through the secretion of growth hormone), is involved in the production of urine, and helps regulate all the rest of the endocrine glands as well. It is involved in the production of thyroid hormone (see below), and prolactin in women [which influences breast development and milk production].52
The thyroid gland, in the neck area, chiefly regulates the body’s metabolism through the secretion of thyroid hormone. The parathyroid glands, also in the neck, regulate bone formation and the elimination of calcium and phosphorus from the body. The adrenal glands, atop the kidneys, have a variety of functions. The aldosterone they secrete helps regulate the body’s salt/water balance. The dehydroepiandrosterone (DHEA) they secrete affects the immune system and a person’s mood, among other functions. The cortisol they secrete is strongly anti-inflammatory, affects blood pressure, blood glucose, and muscle strength, and also helps regulate the salt/water balance. And the epinephrine and norepinephrine they secrete stimulate the nervous system, the heart, the lungs, and the blood vessels. [Epinephrine is also known as adrenaline. It is the body’s chemical for dealing with extreme stress, emergencies, and dangerous situations in general. Adrenaline lies at the heart of the body’s “fight or flight” response to danger.] The pancreas, which we encountered in examining the digestive system, regulates blood sugar through two hormones, glucagon, which raises it, and insulin, which lowers it. Insulin generally affects how sugar is metabolized in the body. [Small structures in the pancreas called the Islets of Langerhans produce these hormones.]53 Finally, we have already examined the functions of the testes and ovaries.
The body comes under biological attack in many ways. So to see how it responds to those attacks, we now turn to…
THE IMMUNE-LYMPHATIC SYSTEM
The immune system evolved to attack various kinds of pathogens attempting to penetrate the skin or internal tissues. Invaders are first attacked by the innate immune system. Specialized white blood cells living in the body’s tissues called macrophages chemically detect bacteria (for example), devour them, and destroy them. Macrophages direct blood to points of infection (which is why these areas redden) and send out specialized proteins called cytokines to alert other kinds of immune cells to the infection’s presence. Among these cells are NK (Natural Killer) cells, dendritic cells, and neutrophils [very short-lived], that also devour invaders. This team of cells can fight not only bacteria, but cells infected by viruses (although they aren’t very effective against the viruses themselves), parasites, and even some kinds of cancer cells.54
Humans and other vertebrates also possess an adaptive immune system, one that adjusts to very specific kinds of invaders. We mentioned antibodies, which are chemically proteins, in connection with blood types. Some 75% of the antibodies in blood are of one kind: immunoglobulin G (IgG). There are four others: IgA, IgD, IgE, and IgM. They can chemically bind with antigens of all kinds to destroy them. Adaptive responses to invaders can provide immunity for life. Once the immune system has “figured out” what kind of pathogen attacked it, it can often defend the body from other attacks by this variety of pathogen because it chemically “recognizes it.” This is why, for example, a person only gets the measles once. [It’s also why vaccinations work, and are a very sound medical procedure.] Unfortunately, sometimes the immune system attacks harmless invaders, such as pollen, and the reactions they cause in many individuals are called allergies.55
The chief weapons of the adaptive immune system are known as lymphocytes, another form of white blood cell produced in the bone marrow. The class of lymphocytes called B cells secrete immunoglobulins, which circulate through the body and bind themselves to the invading pathogens. They can block viruses from attaching to cells, and can also block the toxins made by microbes. They can even “alert” phagocytes (bacteria-eaters) to come and destroy bacterial pathogens. The kind of lymphocyte called a T cell attacks antigens that are on the surface of a targeted cell. A T cell can even kill a cell that has been infected by a virus before it can reproduce. T cells can also send signals to macrophages to go on the attack.56
The name “lymphocyte” is indicative of the role the lymphatic system plays. The bone marrow that manufactures the various immune cells is in fact an organ of the lymphatic system. The spleen, the system of lymph nodes spread throughout sections of the body, the thymus gland [where T-cells mature], the tonsils, and appendix are all organs of the lymphatic system.57 The lymph nodes scattered throughout the body are of particular significance. Lymphocytes are constantly roaming between them, “looking” for antigens to attack. Cells with antigens are brought to the lymph nodes, where the antigens can be destroyed.58 [That’s why when your body has a serious infection, the lymph nodes swell.]
The lymphatic system captures fluids that have been lost through capillary walls into the interstitial [between the cells] spaces of tissue, and returns the fluids back to the cardiovascular system. This return of fluid is absolutely essential. There is an extensive system of vessels in the lymphatic system that work in very close conjunction with the cardiovascular system’s vessels. The lymphatic system also carries its own fluid, a liquid known simply as lymph, out of and back to the cardiovascular system.59
THE URINARY SYSTEM
The two most important structures of the urinary system are the kidneys, which are located toward the rear center of the body’s interior. The kidneys, traditionally described as “bean-shaped”, are each 10-12 cm in length, 5-7 cm wide, and 2-5 cm in thickness. In their interior structure, each kidney contains approximately 1.25 million units called nephrons. It is these structures that act as filters for the blood. The entire blood supply of a human passes through the kidneys many times a day. Twenty per cent of all blood pumped by the heart each minute goes to the kidneys. The left and right renal arteries carry blood to them at the rate of 1200 ml per minute. These arteries enter the kidneys and diverge into a system of smaller arteries, then arterioles, and finally capillaries. It is through the capillaries in the nephrons that the filtering process is done. A complex set of venules and veins, ending with the left and right renal vein, then transports blood from the kidneys.60
The kidneys have an impressive array of jobs. They regulate the blood levels of sodium, calcium, and potassium ions, among others. They regulate the blood’s pH. By retaining or eliminating water in the urine, they regulate blood volume. They help regulate blood pressure. They maintain something known as the blood’s osmolarity—the number of dissolved particles per liter in it. The kidneys produce hormones that help regulate calcium in the body and stimulate red blood cell production. They assist in the process of regulating the blood’s glucose level. And of course, their most familiar function is the excretion of wastes, which include ammonia and urea (products of the metabolism of protein), a number of by-products of the breakdown of substances used in the body, uric acid, and wastes introduced from outside the body, such as drugs.61
In each nephron, blood is filtered through a tuft of capillaries known as the glomerulus. Blood pressure in these capillaries drives the filtration process. These structures filter part of the blood flowing through them, removing contaminants from the plasma and sending most fluid back toward the venules and veins that will then send it back out of the kidneys. The waste products and water filtered out will form the urine.62 A complex set of structures transports the urine from the kidneys to the ureters, long tubes connecting the kidneys to the bladder, the holding area for urine. The bladder is innervated with nerves that signal when it needs to be voided. The bladder has a sphincter muscle controlling its output. It is connected to the urethra. In women, this is fairly straightforward. In men, the urethra passes through the prostate gland, which often causes issues. A healthy adult produces between one and three liters of urine per day, depending, of course, on the volume of liquid consumed, and the liquid’s diuretic qualities.
THE INTEGUMENTARY SYSTEM
Surrounding the tremendously complex and ceaseless activities of the other systems is the integument, the skin. It is the largest organ of them all, the waterproof covering of a human, the first line of defense against pathogens, and the sensory interface between a human and the outside world. It acts as a storage area for fats, glucose, salts and water, assists in the production of Vitamin D, and helps regulate the body’s temperature as well. On the average adult it covers 22 square feet (about 2 square meters). It weighs about 11 pounds (five kilograms) and varies in thickness from 4.0mm on the bottom of your heels to 0.5mm in the eyelids.63
The skin is also structurally more complex than we might imagine. One particular anatomy text points out that in one square centimeter of skin there are 3,000,000 cells, 12 heat-sensing apparatuses, two cold-sensing apparatuses, 200 nerve endings for registering pain, 25 pressure apparatuses to perceive tactile stimuli, four yards of nerves, 3,000 sensory cells at the end of nerve fibers, 700 sweat glands, one yard of blood vessels, 15 sebaceous (oil) glands, and 10 hairs.64 Of course, it should be understood that these are average distributions, and that not all areas and layers of the skin are equally sensitive to the same stimuli. As one research article put it, when discussing sensitivity to temperature,
The ability to perceive changes in skin temperature depends on a number of variables including the location on the body stimulated, the amplitude and rate of temperature change, and the baseline temperature of the skin. There is a 100-fold variation in sensitivity to changes in skin temperature across the body, with the cheeks and the lips being the most sensitive area, and the feet being the least sensitive region. A common finding in many studies of thermal thresholds is that despite the variability in thresholds across the body, all regions are more sensitive to cold than to warmth.65
The outer layer of the skin is the epidermis. Its cell types are, from top to bottom, keratinocytes, which produce keratin and are basically dead at the surface, melanocytes, the cells that produce the skin’s pigment, melanin, Merkel cells, which are touch sensory receptors, and cells that function as macrophages, known as Langerhan’s cells. The main layer of the skin is the dermis. This is where the sebaceous glands that secrete the oil that helps keep skin water-tight are found. The roots of the hair are here, accompanied by the arrector pili muscles that contract and make the hair stand up when a person is cold. The sweat glands that release the moisture that helps cool the skin in hot weather or after strenuous exertion are embedded in the dermis as well. The dermis is layered with sensory receptors and blood vessels. Most of the dermis is made out of collagen and elastin fibers that give the skin flexibility and strength, and help keep it hydrated. Finally, we should note that the toenails and fingernails are simply modifications of the epidermis.66
So there we have the body’s systems, ceaselessly working synergistically with each other. From cells to tissues to organs to systems to interacting and interdependent systems, increasingly higher levels of organization emerge. At the end of the process, the human being as a living, functioning entity emerges. The body’s systems are in many ways extraordinary. I am struck in particular by the enormous role the capillaries play at the interfaces between systems. Yet, the functioning of the systems is not always elegant, and their ways of accomplishing tasks not always optimal. But they meet the standard of evolution: they work. They are good enough. They have allowed Homo sapiens sapiens to survive and function in a dangerous world—and, increasingly, gain control of it.
Now, we will turn to other features of the human animal, looking at aspects of its physical nature that will allow us to gain a broader and more comprehensive understanding of it.
Appearance and General Structure
Humans are bilaterally symmetrical, although not precisely so. Their limbs are approximate mirror images of each other, the sides of their faces less so. (There seems to be a pronounced difference between the left and right sides of the face.) The lungs, kidneys, testes and ovaries come in pairs in physically normal humans, and the heart’s four chambers form much less symmetrical sides. Compared to chimpanzees, gorillas, and orangutans, humans have markedly less body hair; the hair on their heads probably exists because it was selectively useful in protecting the head, but the presence of hair in other areas is less well understood. Males, by far, have more facial hair than females, although females are certainly not devoid of it. The vast majority of humans have hair color ranging from brown to dark brown, almost black. Most humans also have brown eyes. (By the way, it is the section of the eye called the iris that holds its pigmentation.) The various shades of blonde and reddish-orange hair are found on a distinct minority of the human race, as are blue, green, or gray eyes.
Humans come in a variety of skin colors, ranging from very dark brown, almost black, to very pale, almost white, with various bronze-like, light brown, and reddish brown shades in between. In addition to skin color, many populations within the larger human species have hair and facial characteristics that set them apart from other sets of humans. These variations are the product, as we will see, of adaptations our ancestors made to life at different latitudes and in different climates. None of these variations is now of any significance in and of itself, but humans have generally used such features to categorize people by “race”. (See below.) Humans display the sexual dimorphism (differences in size) typical of many primates. Human heights and weights are quite variable but the “average” adult human’s height today falls in the range of approximately five to six feet (1.524 to 1.8288 meters), and the “average” adult human’s weight generally falls between 100 and 200 pounds (45.36 to 90.718 kilograms). There are many, many adult humans that fall outside of those parameters. Height and weight both seem to be a function of genetic inheritance and nutrition, although height is perhaps less influenced by nutrition than weight. In the typical human the arms are 70% the length of the legs.
Although no humans with the exception of monozygotic twins (or triplets, quadruplets, etc.) are physically identical in facial appearance, there are only so many possible combinations of jawline, nose type, cheek bone structure, lip type, skin tone, eye type, eye orbit structure, forehead structure, and general skull shape. Consequently, there are unrelated humans that resemble each other, sometimes strongly. (Of course, I mean unrelated in the general sense of the word. No human, as we have seen, is genuinely unrelated to any other.) Most humans, even in their best years of life, do not have faces that are considered to be “ideally” beautiful or handsome in their particular culture. Variations in attractiveness do have consequences in social interaction, as we will see in a subsequent volume.
Nutritional, Fluid, and Caloric Requirements
The World Health Organization has conducted extensive studies on the nutritional requirements of humans. They have concluded that the following are required on a daily basis for optimal physical function:
Protein, energy, vitamin A and carotene, vitamin D, vitamin E, vitamin K, thiamine, riboflavin, niacin, vitamin B6, pantothenic acid, biotin, vitamin B12, folate, vitamin C, antioxidants, calcium, iron, zinc, selenium, magnesium and iodine.67
Energy, of course, is derived from various proteins, fats, and carbohydrates, taken in as part of the human’s daily consumption of calories. Humans, unlike carnivores and many other animals, cannot synthesize vitamin C, and therefore must ingest it.
The United States Food and Drug Administration calculates that a human being weighing 150 pounds (68.18 kilograms) and 5 feet six inches (167.64cm) in height requires the following fluid and caloric intake:
Estimated Maintenance Fluid Requirement,
Child or Active Adult: 2451 ml/day
Estimated Caloric Requirements,
Child or Active Adult: 2871 kcal/day
Estimated Maintenance Fluid Requirement,
Sedentary person: 2386 ml/day
Estimated Caloric Requirements,
Sedentary person: 1632 kcal/day68
Obviously, these are simply broad guidelines. There are many degrees of difference in the terms “active” and “sedentary”. Very active adults working in very cold climates will need far more calories on average. Many humans in the economically advanced nations are consuming far more calories than necessary every day, given their fairly sedentary life styles. For many people in the underdeveloped world, this is not an issue.
Human Internal Temperature, Adaptations to Climate and Altitude
Humans, as are all mammals, are endothermic, meaning they can maintain a constant body temperature (in normal circumstances). Humans generally have been told that an internal temperature of 98.6° Fahrenheit (37.0° Centigrade) is normal. However, recent research indicates that 98.2°F (36.78° C) is the true norm. Further, in another study, older people, defined as those with an average age of 81, had body temperatures that never reached 98.6° F. Each individual of whatever age appears to have a “baseline” normal temperature that can differ from that of others.69
Hypothermia (dangerously low internal temperature) begins at 95° F (35° C). Lower temperatures than this are considered severe hypothermia. An internal temperature of 82.4° F (28° C) produces unconsciousness; being literally frozen stiff is 78.8° F (26° C) or lower.70 Hyperthermia (dangerously high internal temperature) is, according to almost every source, fatal at 107° F (41.7° C).
The ability of humans to withstand extreme temperatures is therefore limited. There are vast areas of the Earth considered uninhabitable (except by the hardiest and most determined humans) because of climatic extremes. Those populations of humans who do live in such regions have acclimatized themselves to them. Generally speaking, acclimatization is the adaptation of an animal to extremes of temperature. Yet acclimatization to such extremes by humans doesn’t so much involve changes in human anatomy as it does changes in human social and cultural behaviors. The non-human animal becomes acclimatized through natural selection. The human animal learns how to dress for the weather, and increase or decrease caloric intake based on external conditions. Contrary to the belief of some, people in cold regions are not usually fat. Through cultural experiences, they have learned how to keep themselves warm.
Humans can also adapt to high altitudes and low oxygen levels, such as the Quechua people in the Andes and the Tibetans and Nepalese in the Himalayas have done. It is not the level of atmospheric oxygen that presents a challenge at higher altitudes, it is the fall of atmospheric pressure and hence the pressure that allows the lungs to exchange gases. At about 18,000 feet (5500 meters), there is only about 50% of the atmospheric pressure there is at sea level.71 The populations of the Andes and Himalayas, living as high as about 13,000 feet (4000 meters) seem to have made somewhat separate but still effective adaptations, the Tibetan population in its lung functioning, the Andean population in their blood’s hemoglobin concentration.72
Variations in Human Physical Abilities
Humans demonstrate a wide range of differences in such areas as physical strength, with men generally stronger than women (but not universally so, by any means). Humans also display a great range of aerobic fitness, as measured by such tests as the ability to run long distances or repeated short bursts. Certain aspects of physical fitness appear to be genetic in origin. In both men and women, the age at which physical strength reaches its peak ranges from the late 20s to the early 30s, undergoing a gradual decline throughout the rest of life.73
According to research by specialists, infants from birth to two months of age require 12-18 hours of sleep per day. Infants three to eleven months in age require 14-15 hours. Children one to three years of age require 12-14 hours, children three to five years old need 11-13 hours, children five to ten years old require 10 to 11 hours, pre-teens and adolescents, ranging in age from 10 through 17 should get 8.5-9.25 hours per day, and adults generally need 7 to 9 hours.74 Many factors can hinder sleep, and older humans generally have less satisfactory rest.
So what is the foundation of the human’s various anatomical features and physiological processes? That lies within the genetic inheritance of Homo sapiens sapiens—the AMH genome.
The Human Genome’s General Features
The entire set of instructions for the building of an organism is called its genome. An organism’s genome is located in its DNA. As we have seen, every cell in the human body except gametes and mature red blood cells keeps a complete store of DNA in the 23 pairs of chromosomes found in its nucleus. When a cell holds both a maternal and paternal set of chromosomes, 46 chromosomes in total, it is known as a diploid cell. Since the gametes contain only a single set of chromosomes, those cells are known as haploid cells. There are, in each set of 23 chromosomes, 22 autosomes (non-sex chromosomes) and one sex chromosome, either an X or a Y. When two sets of chromosomes are combined at conception, if both sex cells are X’s, (XX) a female will eventually emerge. If there is one X and one Y, (XY) a male will eventually emerge. Why do I say eventually emerge? A science writer puts it this way:
By default, mammal embryos develop as females. A structure called the gonadal ridge eventually gives rise to the ovaries. It’s the presence of a gene called SRY that diverts the embryo down a male route. SRY sits on the Y chromosome and sets off a chain of activated genes that transform the gonadal ridge into testes instead. With SRY, you get a male; without it, a female.75
Interestingly, he adds that research indicates that genes for maleness or femaleness hold each other at bay throughout life, and as long as they are equally strong, there can be no change in a person’s sex. However, primary sex traits in laboratory animals have been altered by genetic manipulation, showing that such a change is possible.
Why is there no YY combination possible? Because all ova carry an X chromosome. Sperm cells carry either an X or a Y. Therefore, it is the male’s gametes that “determine” the sex of a child.
Each set of 23 chromosomes has about 3,000,000,000 base pairs (A-T, C-G combinations, with their accompanying sugar and phosphate groups), meaning that there are around 6,000,000,000 base pairs in a full complement of nuclear DNA. Each chromosome is a single DNA molecule with a particular number of base pairs, and the molecules of DNA vary in length. The longest chromosome is Chromosome 1, with an estimated 247 million base pairs. The shortest chromosome is Chromosome 21, with about 47 million base pairs. An X chromosome is much longer than a Y chromosome—154.9 million pairs compared to 57.8 million. The complete structure of each chromosome has yet to be ascertained, or as biologists say, sequenced.76
The number of genes arrayed along these chromosomes is still a matter of some discussion. Researchers from the Human Genome Project, which undertook to map the entire genetic code of AMH, estimate the number of genes that actually encode proteins is only about 25,000-30,000, many fewer than were expected. (Some estimates of the expected number were over 100,000.) 77 The final number has not yet been determined. Some researchers think there might turn out to be as few as 21,000 genes in the human genome. There are, by the way, only a few dozen genes in mitochondrial DNA.
Tremendous progress has been made in identifying what genes control specific biological processes and functions. The U.S. National Library of Medicine keeps a database of such information. Among the genes that have been identified:
- The CACNA1A gene, one of a group of genes that facilitate the making of calcium channels. Calcium channels are important in allowing a cell to generate and transmit electrical signals.
- The FOXC2 gene, a control gene that helps regulate the activities of many others. The FOXC2 gene gives instructions for making a protein that is crucial in the forming of a host of organs and tissues in human fetuses.
- The ADA gene, which provides instructions for producing an enzyme used by lymphocytes.
- The INS gene, which provides instructions for producing the hormone insulin.
- The BRCA1 gene, which produces a protein that helps suppress the growth of tumors, in this case by controlling cells that if allowed to grow in an uncontrolled fashion can lead to breast cancer.78
However, the function of about half of all genes is as yet unknown. Less than 2% of our genome is actually involved in making proteins, and there appear to be large areas of the genome that have no direct impact, but may still have an indirect impact by rearranging the physical structure of chromosomes. Most striking in some ways is the fact that 99.9% of all nucleotide bases appear to be identical in every human on Earth.79
It is common knowledge that our hair color, hair type, facial structure, eye color, skin color, and general body size are genetically determined. Yet these are only the most superficial signs of our genome’s significance. The human genome carries within it the heritage of millions of years of primate evolution. Its regulatory tasks are myriad. And its ability to effectively reproduce itself is phenomenal—but not flawless, as we have seen.
When DNA is reproducing, the process is carried out by an enzyme known as DNA polymerase. DNA polymerase plays an extraordinarily important role in life. Its task in human life is daunting—accurately reproduce all 6,000,000,000 base pairs in a cell’s DNA. It only makes a mistake in doing so about once in every 10,000,000,000 times, which appears to be an excellent record. But as one pair of researchers has pointed out, in a typical human lifetime cell division takes place 1,000,000,000,000,000 (one quadrillion) times.80 That amounts to about 100,000 errors by DNA polymerase. Even if many of these errors are repaired, there will still be many that slip through. Earlier in this volume, we discussed the various mutations which can alter the genome. I should emphasize again that the majority of these mutations are probably neutral in their effect. Yet, some of these errors can manifest themselves phylogenetically, and sometimes with very harmful effects.
A great deal of research has been done on genetically inheritable diseases. One very specialized text discusses no fewer than 88 major heritable disorders or syndromes, perhaps the most well-known of which is Down Syndrome. A single mutated gene can be responsible for an inherited disorder. Neurofibromatosis is an example of the effect of such a mutation, although this disease can also be inherited from a parent. Certain single gene disorders are sex-linked as well. Alternatively, disorders can arise from translocation, which, as we saw earlier, involves chromosomes becoming entangled with each other and exchanging genetic information. Finally, an inherited condition can come from a genetic predisposition, a genetic inheritance that can lie dormant until it is triggered by a particular environmental influence, or one which simply manifests itself after a long period of time.81
There are genetic predispositions to heart disease, hypertension, and cancer, among many others. In regard to cancer, an example of a predisposition is a mutation in the BRCA1 gene, a gene to which I referred above. The mutation is apparently inherited, and while it does not mean that the woman possessing it will definitely develop breast cancer, it puts her at much greater risk for it, somewhere between 56% and 86%. There are many kinds of cancer, of course, and they vary in the degree to which they are caused by genetic predisposition. Further, population bottlenecks can cause rare alleles to become common, simply by random chance.82
The difficulty in analyzing the causes of disease is that a genetic predisposition can be more likely to manifest itself because of individual human behaviors, or exposures to environmental toxins. When humans with a predisposition to heart disease eat recklessly, they are putting themselves at risk. When humans with a predisposition to lung cancer smoke tobacco, they are worsening their own odds. Naturally, there are no guarantees: a person with a healthy lifestyle can still succumb to an inherited disorder. But humans are often their own worst enemies in this regard. Similarly pollutants, not all of which are visible, can trigger predispositions as well.
Complicating the picture is the issue of gene expression, which we have encountered before. The activation or suppression of certain genes can be caused by natural selection, of course. But it can also be affected by drugs, as in the tragic episode in the 1950s and 1960s in which thousands of birth defects were triggered by Thalidomide. Gene expression can also be caused by the presence of certain chemicals, the saturation of sunlight in an area, and great variations of temperature. Of course, human behavior can greatly mitigate the impact of such factors as light or temperature, or exposure to drugs and noxious chemicals.83
When we discuss gene expression, we naturally encounter the issue of dominant versus recessive genes. Humans are subdivided into populations, as are all animals. All genes in a population come in two variations, or alleles, as we have seen, reflecting the contribution of both mother and father. When a person in this population has a gene with two different alleles, he or she is said to be heterozygous for that gene. When a person has two identical alleles of the same gene, they are said to be homozygous for it. In the anatomically modern human, certain physical traits are the product of dominant alleles, certain others are the product of recessive ones. If an individual is heterozygous for a trait, the dominant allele will always override the recessive one, and the phenotypic trait (such as eye color) will reflect that. The allele for brown eyes is dominant. Naturally, if a person’s alleles for the gene controlling eye color are both dominant, then they will definitely have brown eyes. If the person is heterozygous in the gene for eye color, even if the other allele of the gene is for blue eyes, the person will have brown eyes. But if the individual’s gene for eye color is homozygous for the recessive trait—in other words, if both alleles are for blue eyes, for example—then the person will be blue-eyed. This can happen when both parents are heterozygous in their genes for eye color. If each parent donates a recessive allele, their child will have blue (or green or grey) eyes. This is the straightforward phenomenon that Gregor Mendel, the founder of the science of genetics, noted in his experiments in pea plant breeding.
But, as always, things aren’t that simple. There is the phenomenon of incomplete dominance, when the heterozygous phenotype is actually a combination of or intermediate between two homozygous forms. There is also what is known as codominance, when both alleles in a heterozygous gene are fully expressed.84 Codominance can be found in blood types. For example, AB blood is the product of one parent’s A and the other parent’s B alleles.85 Incomplete dominance can be seen in sickle-cell anemia. A homozygous dominant person, one with no allele for the disease, will be disease-free, heterozygous individuals that have one allele for the disease will have a low-level of it, and homozygous recessive individuals, with two alleles for the disease will have the full-blown version of it.86
Examples of dominant traits are: brown eyes, dark hair, non-red hair, curly hair, normal vision, Roman (aquiline) nose, broad lips, extra digits, pigmented skin, type A blood, and type B blood. Examples of recessive traits are: Gray, green, hazel, or blue eyes, blonde hair, red hair, straight hair, nearsightedness, straight nose, thin lips, five digits, albinism, and type O blood.87
Are there genetic predispositions to mental illnesses? The proposition that there are is the subject of debate. One researcher, after having noted the decades-long failure [in his opinion] of psychiatric geneticists to find any genes underpinning mental illness, even with new data coming in from the Human Genome Project, has come a startling conclusion:
the only positive contribution that the ﬁeld of psychiatric genetics has ever made to the human condition is its apparent ﬁnding that genes for the major psychiatric disorders do not exist.88
To say that this opinion is not universally shared would be to say the least. A recent major study involving 50,000 individuals was released that flatly contradicts the contention that no genes for psychiatric disorders have been detected. In a study focusing on schizophrenia, an international team of researchers found an overlap between the origins of schizophrenia and bipolar disorders, and identified no fewer than seven locations on the genome that are implicated in causing schizophrenia, five of them newly discovered.89 I would add my own note: since the function of half of all genes has yet to be ascertained, how can anyone maintain that genetic predispositions to mental illness flat-out do not exist? In my view, this issue has not been settled.
Is intelligence heritable? There have long been debates about this matter, but solid evidence has been hard to come by. In 2011, the authors of a major study done on more than 3,500 subjects in the UK came to the conclusion that a minimum of 40% of the variation in crystallized intelligence [a person’s knowledge base] and a minimum of 51% of the variation in fluid intelligence [the ability to learn and assimilate new knowledge and skills] can be attributed to genetic factors, and that intelligence is polygenic, meaning many genes are involved in it.90 Such research is not the last word, of course, but it gives other researchers important findings which can assist with their own inquiries.
In a subsequent volume, we will examine the relationship between genetics and behavior in more detail, but suffice it to say that it appears that genetic predisposition and (partially) heritable traits such as intelligence, are in a continuous, interactive, reciprocating relationship with cultural inheritances and individual life experiences. Genetic predispositions and heritable traits affect life experiences and the interpretation of cultural inheritances, while life events and cultural inheritances help govern the degree and manner in which genetic predispositions manifest themselves and the relevance in a person’s life of heritable traits.
So with this very basic understanding of our genome, let us turn to an area that is significant primarily because so many people all through history have wrongly believed it to be—the origin of “race”.
The Evolution of “Racial” Characteristics
There is no topic that causes more intellectual confusion than the use of the term “race”. Race is a term widely used in ordinary life, but in scientific circles the concept of race has largely been abandoned. There are, of course, variations within and among human populations, distinctions that are genetically heritable. Still, the fact remains: the members of Homo sapiens sapiens are overwhelmingly similar in their genetic inheritance. (See above.) There is, as we have noted, some Neanderthal genetic material in non-Africans, but its significance is minor. So what accounts for the differences in skin color and other traits we find in modern humans?
What color were the first members of the genus Homo? Two scientists who have researched skin color’s origins very extensively argue that they were probably similar in color to chimpanzees, with very light skin covered with dark hair. Why are there different colors of skin in humans now? They believe, and there is extensive evidence to support their views, that skin color is strongly correlated with ultraviolet ray intensity at various latitudes on the surface of the Earth. Melanin, located in the melanocytes of the epidermis (see above) can form a barrier against UV rays. Darker melanin protects the lower layers of the skin from UV damage. Darker skin also protects the physical integrity of the sweat glands, without which effective cooling of the body in hot conditions cannot take place. Further, darker skin protects the vitamin D synthesized in the skin from being destroyed by excessive UV radiation. Perhaps even more important, darker skin protects the body’s folate from degeneration by UV rays. Folate [in its artificial form, folic acid] is crucial for keeping developing embryos/fetuses healthy. Darker skin, the product of heavier concentrations of melanin, was therefore selected for at those latitudes that received the most direct sunlight. As humans evolved from the earliest species to the more advanced ones, body hair was shed and skin became more and more exposed. Darker-skinned individuals survived in the regions of intense sunlight better than lighter-skinned ones, and thus darker-skinned humans became prevalent in equatorial regions.91
So what about the existence of lighter-skinned individuals? In the northern latitudes, with less direct sunlight, there is less opportunity for vitamin D synthesis. Lighter-skinned individuals had a lower melanin barrier to sunlight, and thus were able to absorb it and synthesize the critically important D vitamin more than darker-skinned individuals. Darker-skinned people suffer more from vitamin D deficiency than any other group. Research has shown that dark skin needs six times as much UV radiation as light skin to synthesize vitamin D. Although there is still no scientific consensus on this point, the vitamin D hypothesis seems the most plausible explanation of selection for lighter skin. The full emergence of light-skinned individuals may have been as recent as 11,000 ybp.92
Differences in nose type and eye type are more than likely adaptations to climate. There are hypotheses that longer and narrower noses heat incoming air more efficiently, making them useful for those in northern climates, while broader noses are more efficient at ventilating heat, giving them a selective advantage in warmer climates. These ideas are purely speculative. Epicanthic folds above the eye, characteristic of many Asian populations (and also the San people of southwest Africa) are thought by some researchers to have been adaptations to severe climates where blowing sand, heavy wind, or very cold conditions were common, but the genetic basis of the fold has yet to be determined. The genes that cause tightly curled hair, characteristic of many African populations, are being identified, but the adaptive advantage of such hair is unclear. Northern European populations appear to be the greatest sources of recessive genes for non-brown hair and non-brown eyes, but the reasons for this are, again, not clear. In truth, many scientists are not really very interested in these areas. There has been so much gene flow between human groups that there is no such thing as a “pure race” anywhere on this planet, and differences in physical features in certain populations are interesting from a social sciences standpoint, but they are not terribly significant from the standpoint of the natural sciences. The old “racial” categories are now considered archaic. There is only one “race”—Homo sapiens sapiens, the huge extended family that stretches across the world.
The “Just Good Enough” Body
The human body is extraordinary in many ways, but it is still an example of the rough-and-ready processes of evolution, the “goal” of which is reproductive advantage. Structures and functions don’t have to be perfect—they just need to work. There are many features of the body that from a purely engineering standpoint are sub-optimal. The male urethra passes through the prostate gland, and as a consequence, urinary problems in older men are common. The female pelvis is just barely big enough to accommodate the large-brained infants of our species, and because of this childbirth can be a harrowingly painful ordeal. In reproduction, only about one-ten millionth of one per cent of all ejaculated sperm get anywhere near an ovum. Capillaries leak so much liquid that a parallel circulatory system is needed to get it back into the cardiovascular highway. The common passageway for air and food can cause a choking hazard. The proximity or even overlap of excretory and reproductive structures can lead to infection. Our own immune system can turn on us, causing untold suffering. The protection afforded by the skull and the vertebrae can be tragically incomplete. Knees and hips wear out simply from prolonged use. Perhaps 40% of all conceptions end in miscarriage. And we are vulnerable to a disturbingly long list of diseases—more than 100 types of cancer93, dozens of major heart and vascular conditions94, more than 40 major respiratory illnesses95, more than 100 autoimmune disorders96, and on and on. We learn a hard truth when we are young, and understand that truth more deeply every year we live—we are not built to last. No matter how lucky we are in avoiding injury, no matter how diligently we look after our health, we will deteriorate if we live long enough. The body—which is everything we are—will finally give out. So in the last chapter of this volume, we will consider the processes of aging and the nature of death, and seek to understand why our lives will indeed have that last moment.