Human Nature, Physical Aspects

A consideration of the physical aspects of human nature leads to viewing human nature as embodied. Embodiment as a concept is fluid, taking its forms from pathways of inquiry that inevitably remake it, however provisionally, according to the task at hand. But surely this is not true of the body. The body as a physical object, a thing, is solid. One points to it, sees its movement, hears the sounds it makes, feels its heart beating, smells its fragrance, and tastes its sharp salinity. Having a body is an undeniable fact of life, a solid place of unity between one human and another, even between human beings and the more than human world. But having a body may do no more to unify than would having a car, wearing clothes, having a mother, speaking English, and dying. Establishing links between the concepts of body and concepts such as human unity requires much more than the simple facts associated with being bodied. Apart from pathways of inquiry, then, the fact of body—its sensory undeniability—seems indeed solid, unmovable, a mountain of inertia.

So the challenge is to give a technical review that transforms some of this inertia into movements along paths of inquiry linking science and religion. Sadly, this requires that much that is wonderful about the body will be left out. Further, some scientific results summarized below (e.g., in relation to physical beauty, human emotion, etc.) may be susceptible to cultural context; most of the studies summarized in this entry relied on Western approaches to science and worked exclusively with subjects within Western cultures.

The major dynamical systems of the body

Human biology partitions the functions of the human body into eleven major dynamical systems: cardiovascular, endocrine, gastrointestinal, hematologic, integumentary, lymphatic, musculoskeletal, nervous, reproductive, respiratory, and urinary (Seeley, Stephens, and Tate, 1995).

The cardiovascular (or circulatory) system includes the structures of the heart, blood vessels, and blood. Its functions include the transport of oxygen and waste gases (e.g., carbon dioxide), nutrients, waste products, and hormones; the regulation of body temperature; the regulation of blood pressure; and a contribution to the immune response.

The endocrine system includes the structures of the pituitary, thyroid, parathyroid, thymus, and adrenal glands, as well as the pancreas, ovaries, and testis. Its major functions are the regulation of the following: metabolism and growth, the absorption of nutrients, fluid balance and ion (i.e., chemicals in the body with a positive or negative charge) concentration, the stress response, and sexual characteristics, reproduction, birth, and lactation.

The gastrointestinal system includes the oral cavity, salivary glands, esophagus, stomach, liver, gallbladder, small intestine, large intestine, and rectum. Its functions include the breakdown of food, the absorption of nutrients, and the elimination of wastes from the body.

The hematologic system includes blood plasma (91.5 percent water by volume), blood cells, red bone marrow, spleen, liver, and kidneys. Blood cells include erythrocytes (i.e., red blood cells) for the transport of oxygen and waste gases; neutrophils for consuming microorganisms and other substances in the blood (i.e., phagocytosis); basophils for the release of histamine in inflammatory responses and heparin to prevent blood clots; eosinophils for the reduction of inflammation and the attack of some worm parasites; lymphocytes for the production of antibodies and other substances to destroy microorganisms and other substances foreign to the body (e.g., transplanted organs); monocytes for the phagocytosis of bacteria, dead cells of the body, cell fragments, and other tissue debris; and platelets for clotting blood. Red bone marrow is the only source of blood formation in adults and occurs mainly in bones along the body's central axis and in the joints of limbs (i.e., epiphyses) that are closest to the center of the body. The spleen holds a reservoir of blood, which is released in emergencies. The kidneys release a chemical, erythropoietin, to stimulate erythrocyte production. Enlarged monocytes in the liver, called macrophages, consume old or defective erythrocytes. The liver also produces most of the body's clotting factors.

The integumentary system includes the structures of the skin, hair, nails, and sweat glands. It functions mainly to protect other areas of the body against abrasions and ultraviolet light, to prevent the entry of microorganisms and other harmful substances, to reduce water loss, to regulate body temperature, to produce precursors to vitamin D (increases calcium and phosphate uptake in the intestine), and to provide sensory information about the body and the body's environment.

The lymphatic, or immune, system includes lymph (a clear fluid that is returned to the blood via the lymphatic vessels; three liters per day), lymph vessels, lymph nodes, lymph ducts, the tonsils, spleen, thymus gland, and red bone marrow. The functions of the lymphatic system include removing foreign substances from the blood and lymph, defending the body against elements of disease, maintaining fluid balance in the tissues, and absorbing fat. The two major cell types in the lymphatic system are B cells, which mature to secrete antibodies, and T cells, which recognize foreign molecular patterns on the surface of the body's own cells. Once T cells identify something that is foreign to the body, they either kill the cell or they activate other immune responsive cells in the body (e.g., B cells, macrophages).

The musculoskeletal system includes the bones of the skeleton and all the muscles attached to the skeleton. Its main functions are to provide movement of the body, to maintain body posture, and to produce body heat. This system does not include the muscle of the heart or the smooth muscles that are not typically under voluntary control.

The nervous system includes the brain, the spinal cord, the nerves, and sensory receptors (e.g., photoreceptors in the eye). Its main functions are to provide sensory input for bodily action, to control bodily action (the somatic nervous system), to control physiological processes typically beyond voluntary control (the autonomic nervous system), and to allow for human experience.

The reproductive system in women includes the vagina, uterus, uterine tube, and ovary, and in men the penis, prostate, seminal vesicle, ductus (or vas) deferens, the testis, the epididymis, and scrotum. Its main functions are to assist in the control and performance of sexual behavior.

The respiratory system includes the nose, nasal cavity, pharynx, oral cavity, larynx trachea, bronchi, and lungs. Its major functions are to transport oxygen to the lungs, to exchange waste carbon dioxide for oxygen, and to regulate the acidity of the blood (i.e., blood pH).

The urinary system includes the kidneys, ureter, urinary bladder, and urethra. Its major functions are to remove wastes from the cardiovascular system; to regulate blood pH, ion balance and fluid balance; and to assist in regulating blood pressure.

Paleoanthropology, archaeology, and the body

Humanity's origin narratives within Western science depend largely on the bodily remains of humanity's ancestors. Where were the remains found? What is their three-dimensional character? How old are they? What damage have they sustained? According to Ann Gibbons in "In Search of the First Hominids" (2002), recent unearthings of ancient primate bones have generated controversies in human evolution on questions ranging from whether bipedalism evolved on the savannah to what makes a primate a hominid. Nicknames given to some of these recently uncovered remains, such as Flat-Faced Man and Little Foot, are consistent with the importance of the body in paleoanthropology.

Since the discovery of Lucy, then the earliest known hominid, in Ethiopia in 1973, early hominids have been defined by their bodily resemblance to Lucy. Lucy was small, about the size of a female chimpanzee, had long arms, a relatively small volume inside her skull (i.e., intracranial volume), thick tooth enamel, large molars, smaller canines than earlier paleoanthropological fossils, foot bones that suggested bipedalism, and curved fingers. However, there have also been attempts by scientists to classify hominids by one or a few bodily characteristics: Ardipithecus ramidus (Aramis, Ethiopia; 4.4 million years ago) because its canines are more like human canines than those of chimpanzees (the converse is true for its molars); Ardipithecus ramidus kadabba (Aramis, Ethiopia; 5.2 to 5.8 million years ago) because the bones of its feet suggest bipedalism; Orrorin tugenensis (Tugen Hills, Kenya; 5.7 to 5.9 million years ago) because its thighbone (i.e., femur) looks more like human femurs than do those of Lucy and other Australopithecines, it has even thicker tooth enamel than Ardipithecus, and it has molars more like human molars than those of chimpanzees (the converse is true for its canines).

Controversy is also present in identifying the number and nature of the evolutionary step(s) to Homo sapiens from its ancestor, due to the differences in scientific opinions as to which measurements of the body are the deciding ones. Daniel Lieberman has proposed replacing the typically long list of measurements used to classify hominid skulls with two: the roundness of the skull and the degree to which the face and eyes are tucked under the frontal bone (Balter, 2002). Reducing the number of measurements would, in Lieberman's view, reduce the complexity involving theories regarding the evolutionary appearance of Homo sapiens. Typically, however, measurements of human skulls (i.e., human craniometry) in paleoanthropology and archaeology involve over sixty different measures (Howells, 1989; White, 2000).

Beauty and the body

Bodily symmetry is generally the most consistent factor to correlate with assessments of physical beauty (Geary, 1998). Women prefer men with high bodily symmetry, a strong chin and cheekbones, and an emotionally expressive mouth. These preferences may have adaptive value in that illnesses during puberty are known to reduce the secretion of male hormones (i.e., androgens) which in turn decreases bone size and density (Thornhill and Gangstad, 1993). Additionally, lower facial symmetry in men correlates with higher baseline metabolism (Manning, Koukourakis, and Brodie, 1997) and higher incidents of depression, anxiety, and minor illnesses (Schakelford and Larsen, 1997). Note, however, that this correlation does not hold for individuals who are assessed as either very attractive or as unattractive (Kalick et al, 1998). Men's assessments of physical beauty in women also correlate with bodily symmetry but rely more on facial features showing youthfulness relative to a man's own age (Kenrick and Keefe, 1992), except during male adolescence (Kenrick et al, 1996). Finally, men think women with a waist-to-hip ratio of around 0.7 are more attractive than women with other ratios, and men find women of average weight with this ratio to be more attractive than heavier or thinner women who have this ratio (Geary, 1998). There is evidence suggesting that women with ratios larger than 0.85 become more ill and have a harder time conceiving children than women with ratios around 0.7 (Singh, 1995).

Smell also plays a role in assessing physical beauty. Evidence associates women's ratings of the bodily fragrances of men with differences between their major histocompatibility complex (MHC). Men who differ more in MHC from women raters are assessed as having more pleasant fragrances than men more similar to the women's MHC (Apanius et al, 1997). Having a more variable MHC is associated with greater flexibility in one's immune response, and thus this fragrance preference could reflect the effects of natural selection.

Self determination according to the immune system

The immune system provides for both innate immunity and adaptive immunity. Innate immunity applies to parts of the immune system that do not adapt within an individual organism to a particular immune challenge. Adaptive immunity includes those systems that adapt within an organism to respond in ways specific to each challenge event.

Innate immunity as an organismal function is evolutionarily old since its components are found in both plants and animals. Even single-celled organisms have the capacity to recognize "microbial nonself" (Medzhitov and Janeway, 2002). Genetic changes in the molecular structure of components of innate immunity (i.e., pattern recognition receptors, or PRRs) happen slowly via evolution ( Janeway and Medzhitov, 2002). This in turn forces innate immunity to act only against those molecular patterns on nonself bodies (i.e., pathogen-associated molecular patterns, or PAMPs) that do not change rapidly across generations (i.e., antigens that are evolutionarily conserved). PRRs available in the blood or tissue fluid bind to PAMPs, providing a signal for pathogen destruction by cells such as macrophages or neutrophils or by complement. Complement is a group of proteins in blood plasma that undergo transition from inactive to active forms via action by PRRs and participate in the destruction of pathogens, largely by making a hole in the pathogenic cell (i.e., cell lysis). PRRs that are bound to cells are called Toll-like receptors (TLRs, because of similarities to immune-related proteins of the Drosophila Toll family). PRRs cannot differentiate between microorganisms that are pathogenic to the body and microorganisms that are beneficial to the body (e.g., those in the gustatory system) but are prevented from acting on beneficial microorganisms by physical barriers preventing their access.

Innate immunity also is responsible for what is called the recognition of missing self (Medzhitov and Janeway, 2002). The term missing self (instead of nonself) was chosen to highlight the observation that some components of innate immunity, instead of responding to molecular patterns of pathogens, respond to lower levels of molecular patterns specifically expressed by a body's own cells. This concept was introduced to account for observations that natural killer (NK) cells mainly kill tumor cells that lack MHC class I proteins. MHC class I proteins are adaptive immunity structures that can combine with parts of the body's own cells and are displayed on the surface of those cells to indicate the presence of a self cell. When cells in the body become cancerous, they display fewer or no MHC class I proteins bound with their own fragments. NK cells have proteins on their surfaces, called receptors, that recognize MHC class I proteins bound to self fragments and stop NK cells from killing (Medzhitov and Janeway, 2002). Other examples of innate immunity acting by recognizing a missing self include the activation mechanism of C3, a complement protein; the inhibition of macrophages and neutrophils through receptors on those cells that recognize sialic acid, which is expressed on vertebrate cells but generally not on microorganisms; and the inhibition of macrophages by the protein CD47, largely responsible for distinguishing between functioning and nonfunctioning erythrocytes. Note that these missing self strategies can be fooled if pathogens acquire the DNA that makes the self-specific molecules directly from the body's cells. Then they start looking like self according to the innate immune system. This is known to happen and is called horizontal gene transfer.

Adaptive immunity relies strongly on signals from the innate immune system. It is only present in jawed vertebrates, and its molecular components change in a challenge-specific manner. All jawed fish exhibit adaptive immunity, which is lacking in vertebrates without jaws, such as lampreys. Charles A. Janeway names this sudden appearance in the evolutionary record the "immunological 'Big Bang'" ( Janeway et al, 2001, p. 602). In a series of experiments culminating in 1998, it was discovered that the genes mediating the genetic recombination underlying adaptive immunity could also mediate the insertion of one DNA fragment into others, a process known as transposition (Hiom, Melek, and Gellert, 1998; Agrawal, Eastman, and Schatz, 1998). Scientists infer from this result that adaptive immunity was acquired from a transposable element that inserted itself into the DNA of an ancestor of jawed vertebrates. Significantly, adaptive immunity, unlike innate immunity, is not hereditary. Genetic modifications that occur in adaptive immunity occur in somatic cells, not in the germline cells (sperm or eggs). This leads immunologists to say that the "memory" of adaptive immunity is limited to the lifespan of the individual, and immunizations must be repeated for each generation. Adaptive immunity is thought to contribute to greater lifespan, though it is the cause of rejection in organ transplantation.

Antibodies, or B-cell receptors, are a key component of molecular pattern recognition in the adaptive immune system. There are on the order of one hundred billion different antibody specificities in the human body ( Janeway et al, 2001). The structure of an antibody molecule is modeled as a Y-shape. The stem of the Y is called the constant region, and the arms of the Y are called variable regions. There are five different classes of antibody: IgA, IgD, IgE, IgG, and IgM. IgG is the most abundant antibody class in humans. Each arm of an antibody's Y structure is composed of a heavy (H) chain and a light (L) chain. Moreover, each H and L chain in each arm of the Y is composed of a constant (C) and a variable (V) region, connected by a hinge.

Antibody diversity is produced in four major ways. The first two are controlled genetic recombination of gene segments forming the gene for the V-regions. Light chain V-region genes include the V gene segment (because it codes for most of the final V protein, 95 to 101 amino acids long) and the J gene segment (because it joins the V-region to the C-region, coding for up to thirteen amino acids). Heavy chain V-region genes include the V, J, and D (or diversity) gene segments. In addition to genetic recombination, diversity arises in different combinations of V and H chains at the protein level through different combinations of protein subunits. Finally, specialized mutations within B cells, occurring only at rearranged V-region DNA, add to the diverse antibody repertoire.

T-cell receptors are diversified much in the same way as B-cell receptors and are structurally similar to antibodies. T cells work in conjunction with the MHC, a gene complex whose proteins combine with small protein fragments inside a cell and take these fragments to the cell surface where they can be accessed by T cells. There are two different classes of MHC: I and II. T cells with CD8 proteins on their surface bind to MHC class I molecules, and those with CD4 proteins bind to MHC class II molecules. Both MHC class I and II molecules bind to protein fragments of the body's own cells if those cells are uninfected or otherwise harmed, though class II MHC molecules are largely responsible for binding to protein fragments from pathogenic microorganisms. CD4 T cells then recognize infection and activate other cells in the immune response. Human immunodeficiency virus (HIV) is particularly toxic to CD4 T cells, resulting in a lower level of these cells, which leads to acquired immunodeficiency syndrome (AIDS).

Emotions and bodies

Emotions are patterns of bodily activity that are often thought to have evolved because they allow an organism to respond to its environment in ways that enable survival and successful reproduction (Rosenberg and Ekman, 1997). In The Emotional Brain (1996) Joseph LeDoux says that "Emotions evolved not as conscious feelings, linguistically differentiated or otherwise, but as brain states and bodily responses. The brain states and bodily responses are the fundamental facts of an emotion, and the conscious feelings are the frills that have added icing to the emotional cake" (p. 302). The bodily responses LeDoux refers to include changes in position, posture and movement, facial expression, vocal expression, skin tone, heart rate, blood pressure, breathing rate, and hormone production.

Social affiliation and aversion are correlated with the amount of distance between two bodies, the orientation of one body to another, how much one body leans forward toward another, and the degree of welcome contact between two bodies (Collier, 1985). Two people who disagree but who like each other can show welcome physical contact during arguments (Scheflen and Scheflen, 1972; Collier, 1985). Bodily movement also indicates when someone is startled or suddenly afraid. In these cases, the eye blinks and the bodily movement freezes for a time (e.g., "My spine was frozen in fear."). Observers can infer happiness, sadness, anger, and occasionally pride simply from watching people move (Planalp, 1999).

Perhaps the main route of emotional communication in everyday human interaction is the face. Facial expression includes both the arrangement of the facial anatomy and the direction of eye gaze. There are sixteen muscles used to control facial expression, excluding those involving gaze direction. Surprise is expressed via the occipitofrontalis on the forehead; frowning is accomplished by the corrugator supercilii and the procerus, both of which work on the eyebrows, and by the depressor anguli oris, the depressor labii inferioris, the risorius, and the mentalis; and smiling (or sneering) is mediated by the levator labii superioris alaeque nasi, the levator labii superioris, the zygomaticus major and minor, and the levator anguli oris. Eyelids, the degree to which the eyes are closed, and the openness of the tear duct (i.e., the lacrimal gland) are controlled by the orbicularis oculi. Nasal dilation is controlled by the nasalis, levator labii superioris alaeque nasi, and depressor septi. The lips are controlled by the orbicularis oris. Gaze direction is mediated by the extraocular muscles, which are comprised of four rectus muscles (superior, medial, inferior, lateral) and two oblique muscles (superior, inferior).

Although there have been numerous studies of human facial expression before and since the time of Duchenne's The Mechanism of Human Facial Expression (1862) and Charles Darwin's The Expression of the Emotions in Man and Animals (1872), it did not receive great attention in modern psychology until behaviorism waned (Rosenberg, 1997). Facial expressions are assessed using either the maximally discriminative facial movement coding system (MAX); the Facial Action Coding System (FACS), or electromyography (EMG) of facial muscles. Both MAX and FACS rely on visual information about faces, while EMG depends only on electrical outputs of facial expression muscles, measured either at or under the skin. While MAX is framed in terms of what are generally considered universally recognizable features of emotional facial expression, FACS attempts to characterize all "visibly discernible facial movement" (Rosenberg, p. 12). However, FACS does not include gaze direction as a parameter.

Using these methods in combination with emotionally evocative stimuli and subject reports, there is evidence that (1) facial expressions and reports of some emotions cohere (Rosenberg and Ekman, 1997; Ruch, 1997); (2) verbal instruction can lead to the involuntary or voluntary suppression and enhancement of facial expressions relating to lower back pain (Craig, Hyde, and Patrick, 1997); (3) lowering the brows, tightening the areas around the eyes, and raising the lips are consistent signs that a person is in pain (Prkachin, 1997); (4) liars control their facial expressions more successfully than other bodily movements while lying; (5) it is possible to detect smiles while lying if one allows for different types of smile (Ekman, Friesen, and O'Sullivan, 1997); and (6) untrained adults have a difficult time distinguishing between what a baby is tasting (e.g., bitter versus sweet) simply by facial expression (Rosenstein and Oster, 1997).

See also Human Nature, Religious and Philosophical Aspects

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