ANTH 101 – Human Diversity Discussion - DUE TOMORROW

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14.  Human Variation: An Adaptive Signi�cance Approach

Leslie E. Fitzpatrick, Ph.D., Mercyhurst University

Learning Objectives

Describe how speci�c patterns of human adaptation are correlated to

natural selection processes.

Summarize the role of solar radiation in variations of human skin tone.

In your explanation, include information as to why reduced pigmenta- Previous: Race and Human Variation

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tion leading to lighter skin colors is advantageous for populations in-

digenous to northern latitudes.

Compare and contrast the various genetic mutations present in Ti-

betan, Andean, and Ethiopian populations that allow them to survive at

high altitudes.

De�ne the relationship between speci�c genetic mutations in some

human populations and certain infectious diseases, such as the sickle-

cell trait mutation and malarial infection.

In the previous chapters of this text, we explored the role of evolutionary forces in

human evolution as well as the basics of genetic variation. Within this framework,

we now shift our focus toward examining the numerous challenges our species has

faced throughout its evolutionary odyssey as well as how we have met those trials.

Genetic variability within and between modern populations of humans has been in-

�uenced by years of evolutionary forces, most notably natural selection and genetic

drift. As early humans left Africa and spread across the globe, they faced numerous

challenges related to their new environments. Beyond genetically in�uenced

changes in physiology as a result of evolution, humans have developed lifestyle

strategies to cope with and even thrive in a wide range of habitats. The ways popu-

lations of humans met such challenges, coupled with their geographic separation

throughout the majority of the last two centamillenia, have led to the many forms of

adaptation in our species. This chapter focuses on the complexities of modern hu-

man variation through the lens of human evolutionary history.

STRESS AND HOMEOSTASIS

All organisms, including humans, must maintain a baseline of normal functions

within their cells, tissues, and organs to survive. This constancy of internal functions

is referred to as homeostasis; however, homeostatic regulation may be challenged

by a variety of both external and internal factors. These stimuli are referred to as

stressors, exposure to which leads to a period in which there is a potential for the

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disruption of homeostasis. Within limits, all organisms have evolved certain physio-

logical mechanisms to respond to stressors in an effort to maintain homeostasis. For

example, some organisms, such as dogs, will develop a thicker coat of fur during

cooler periods and they will shed this additional fur during warmer periods. This

cyclical fur development linked to seasonal weather changes is but one way that

these organisms maintain their homeostasis. The range of changes in the physiology

(function), morphology (form), and/or behavior of organisms in response to their

environments and the potential stressors of those environments is regulated by its

phenotypic plasticity. A special case of phenotypic plasticity, polyphenism, is

broadly de�ned as the ability of a single genotype within the organism to produce

multiple phenotypes when exposed to different environmental conditions or stres-

sors. Changes to the organism’s physiology, morphology, and/or behavior that are

linked to its underlying phenotypic plasticity may result in changes that are merely

temporary or those that are permanent. With respect to human phenotypic plastic-

ity as well as evolutionary history, there are several primary mechanisms that have

led to variations among individuals and between populations. These mechanisms,

which are referred to as adjustments (behavioral, acclimatory, and developmental)

and adaptations, are explained in detail in the following sections.

AD�USTMENTS AND ADAPTATIONS

Adjustments

The term “adjustment” refers to an organism’s non-genetic way of coping with the

stressors of its environment. Although adjustments themselves are non-genetic in

nature, the ability of an organism to experience or develop an adjustment is based in

its phenotypic plasticity, which is linked to its evolutionarily guided genetic poten-

tial. Adjustments occur exclusively on the individual level. As such, different individ-

uals within a population may experience a wide range of possible adjustments in re-

sponse to a similar stressor. In general, the three main forms of adjustment are: be-

havioral, acclimatory, and developmental. Previous: Race and Human Variation

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Figure 14.1 Notice the lack of full spectrum color in this photo of a deep-water diver and the diver’s use of specialized equipment, such as a breathing apparatus to deliver gases for respiration, bodysuit to ensure thermal regulation, and �ashlight to increase visibility in the low-light setting.

Behavioral Adjustments

When you are cold, do you reach for a

blanket? When you are warm, do you seek

out shelter cooled by an air-conditioning

system? If so, you have likely been in�u-

enced to do so by the culture in which

you were raised. As noted earlier in the

text, the term “culture” refers to a collec-

tion of shared, learned behaviors among

individuals within a discrete population.

Behavioral adjustments are regarded as

cultural responses to environmental

stressors. These adjustments are tempo-

rary in nature and, since they are non-

genetic, must be constantly altered to

meet novel situations posed by the envi-

ronment. For example, through the use of

a specialized mixture of gases for breathing, an apparatus for the delivery of the

gases, protective clothing, and gear to increase visibility, divers are able to reach ex-

traordinary depths (in excess of 300 meters below the surface) within the water. The

deeper a diver descends, the more atmospheric pressure the diver experiences re-

sulting in increased levels of potentially toxic byproducts of respiration within the

body. In addition, with increased depth there is a decrease in the ambient tempera-

ture of the water as well as a decrease in the availability of light within the visible

spectrum. Deep-water divers are well-versed in the environmental stressors of open

waters and employ a variety of strategies based on behavioral adjustments to meet

such demands. From wearing protective clothing to assist with maintaining the

body’s core temperature to waiting at a speci�c depth for a prescribed period of

time to facilitate the expulsion from the body of nitrogen gas that may have accu-

mulated within the bloodstream, divers employ numerous behavioral adjustments to

ensure their safety dive after dive (Figure 14.1). Without these culturally mediated

behavioral adjustments, a deep-water diver’s �rst dive would be their last.

In many developing countries, the use of refrigeration for the storage of perishable

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food products is uncommon; therefore, individuals within these cultures have devel-

oped a variety of behavioral adjustment strategies related to food preparation to ad-

dress the possibility of food spoilage. Through a cross-cultural analysis of spice use

in recipes, Sherman and Billing (1999) determined that cultures closest to the equa-

tor, where temperatures are hotter, tend to use both a greater number and a wider

variety of plant-based spices with bacteria-inhibiting phytochemical properties (e.g.,

garlic and onion) in their recipes than cultures located further from the equator. The

antimicrobial properties of the spices permits the consumption of foods, particularly

animal-based protein sources, for a period of time beyond that which would be con-

sidered safe. There are some acclimatory adjustment bene�ts to the use of some

pungent spices as well, which are explored in the following section.

Acclimatory Adjustments: �ermal Stressors

Acclimatory adjustments are temporary, reversible changes in an organism’s physi-

ology in response to environmental stressors. Although they are regarded as non-

genetic in nature, the range of acclimatory adjustments an organism is capable of

producing to accommodate a given stressor is linked to its underlying phenotypic

plasticity and duration and severity of the stressor.

Before we discuss how varying ambient temperatures affect the human body, we

must detail the thermodynamic mechanisms through which heat may be gained or

lost. There are four pathways for the loss of heat within the human body: conduc-

tion, convection, evaporation, and radiation (Figure 14.2).

Through conduction processes, heat will move from a warmer body to a cooler one

through direct contact. An example of this is when you accidentally touch a hot

cooktop with your hand and the heat is transferred from the cooktop to your skin.

With convection, when a warm body is surrounded by a cooler �uid (e.g. air or wa-

ter) heat will be transferred from the warmer body to the cooler �uid. This is why

we will often employ the behavioral adjustment of wearing multiple layers of cloth-

ing during the winter in an effort to prevent heat loss to the cooler atmosphere.

Conversely, if your body temperature is cooler than that of the air surrounding you,

your body will absorb heat.

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Figure 14.2 Various thermodynamic mechanisms related to heat gain and loss in the human body.

Depending on your physical condition,

most people will begin to sweat around

37.2℃ to 37.7℃ (98.9℉ – 99.9℉). Sweating is an example of evaporation, which oc-

curs when a liquid, such as the water

within our bodies, is converted to a gas.

Phase conversions, such as those under-

lying the evaporative processes of trans-

forming liquids to gases, require energy.

In evaporation, this energy is in the form

of heat, and the effect is to cool the

body.

The �nal mechanism for heat loss within

the human body is radiation, through

which energy in the form of electromag-

netic waves is produced at a wavelength

that typically lies below that which is

visible to the human eye. Although humans gain and lose heat from their bodies

through radiation, this form of heat transfer is not visible. Humans are capable of

losing and gaining heat through conduction, convection, and radiation; however,

heat may not be gained through evaporation.

As the ambient temperature decreases, it becomes increasingly dif�cult for the hu-

man body to regulate its core temperature, which is central to the maintenance of

homeostasis. The hypothalamus is a small portion of the human brain located near

its base. It is responsible for numerous functions, including the regulation of body

temperature. As measured orally, normal human body temperature averages 37 ℃ (98.6°F). When an individual’s body temperature falls below 34.4℃ (93.9°F), the hy- pothalamus becomes impaired leading to issues with body temperature control. A

total loss of the ability to regulate body temperature occurs around 29.4℃ (84.9°F), which may result in death. When the ambient temperature (e.g. air temperature)

falls below the critical temperature of 31℃ (87.8°F), a nude human body that is at rest will respond with a series of physiological changes to preserve homeostasis (Figure

14.3). Previous: Race and Human Variation

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Figure 14.3 Example of overall body heat maintenance in cold and warm ambient environments.

In response to colder temperatures, the human body experiences two main types of

physiological responses: those that increase the production of heat within the body

and those that seek to retain the body’s heat. The production of heat within the

body is accomplished through short-term increases in the body’s basal metabolic

rate. This rate is a measure of the energy required to maintain necessary body pro-

cesses while the body is in a resting state. As the body’s basal metabolic rate in-

creases, an individual must consume greater quantities of energy-providing nutri-

ents to maintain the increase. Of course, such increases may not continue forever as

they are energetically expensive. As with all acclimatory adjustments, an increase in

the basal metabolic rate is merely temporary.

Another form of a temporary heat-generating acclimatory adjustment to cold stress

is the physiological response of shivering. Shivering results when the hypothalamus

stimulates increased muscular activity that leads to an elevation of the muscular

metabolism. Much like the increased muscular metabolism that occurs during peri-

ods of strenuous exercise, the elevation of muscular metabolism rates during shiver-

ing leads to higher rates of body heat generation.

Other physiological mechanisms the body uses to assist with the maintenance of

temperature related to homeostasis involve the preservation of heat already con-

tained within the body. Of these mechanisms, the most notable is the constriction of

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peripheral capillaries in the skin through a process called vasoconstriction. The de-

creased surface area of the capillaries through vasoconstriction results in less heat

reaching the surface of the skin where it would be dissipated into the atmosphere.

In addition, vasoconstriction leads to the maintenance of heat near the core of the

body where the vital organs are located. This is one of the reasons that an individual

may experience cold-related injuries, such as frost-bite leading to tissue necrosis

(tissue death) in regions of the body that are most distant from the core (e.g. �ngers,

toes, nose, ears, cheeks, chin, etc.).

Just as cold stress presents challenges to maintaining homeostasis within the body

with respect to temperature, heat does as well. In hot climates, the body will begin

to absorb extra heat from its surroundings (through conduction, convection, and ra-

diation) resulting in potential heat-related disorders, such as heat exhaustion. When

the human body is exposed to ambient temperatures above 35℃ (95°F), excess body heat will be lost primarily through evaporative processes, speci�cally through

sweating. All humans, regardless of their environment, have approximately the same

number of sweat glands within their bodies. Over time, individuals living in hot, arid

environments will develop more sensitive forms of sweat glands resulting in the

production of greater quantities of sweat. In an effort to prevent dehydration due to

this form of acclimatory adjustment, there will be an additional reduction in the vol-

ume of urine produced by the individual.

As noted in the previous section, some cultural groups, particularly those in equato-

rial regions, add pungent spices to their foods to inhibit the colonization of bacteria

(Sherman and Billing 1999). Although the addition of spices to foods to decrease

spoilage rates is a behavioral adjustment, the application of some forms of peppers

triggers an acclimatory adjustment process as well. Compounds referred to as cap-

saicinoids are the secondary byproducts of chili pepper plants’ metabolism. Capsai-

cinoids are produced by the plants to deter their consumption by some forms of

fungi and mammals. When mammals, such as humans, consume the capsaicinoids

from chili peppers, a burning sensation may occur within their mouths and along

their digestive tracts. This burning sensation is the result of the activation of cap-

saicin receptors along the body’s nerve pathways. Although the peppers themselves

may be at ambient temperature so their consumption is technically not causing any

form of body temperature increase, the human body perceives the pepper as elevat-

ing its core temperature due to the activation of the capsaicin receptors. Even Previous: Race and Human Variation

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though the body’s temperature has not actually been elevated due to the consump-

tion of capsaicin, the hypothalamus will react as if it has, which leads to the initia-

tion of sweating processes in an attempt to lower body temperature and maintain

homeostasis. The increased piquancy (application of pungent spices to food) as a

means of inhibiting food-borne bacterial colonization in warm climates, as well as

spices’ ability to trigger sweating processes as a method for cooling the body, is an

example of the intersection between behavioral and acclimatory adjustments that

may be utilized by individuals within certain populations.

In addition to increased sweat production in the body as a means of regulating in-

ternal body temperature to maintain homeostasis, vasodilation may occur (Figure

14.4). Vasodilation occurs when there is an expansion of the capillaries within the

skin leading to a more effective transfer of heat from within the body to the exterior

to allow conductive, convective, radiative, and evaporative (sweating) processes to

occur.

Figure 14.4 This image features an illustration of the vasoconstriction processes that occur within the peripheral vascular system when an individual is exposed to cold ambient temperatures and the vasodilation that occurs in warmer environments.

Physiologically-based acclimatory adjustments to hot, dry climates may be comple- Previous: Race and Human Variation

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mented by behavioral adjustments as well. For example, individuals in such climates

may limit their physical activity during the times of day when the temperature is

typically the hottest. Additionally, these individuals may wear loose �tting clothing

that covers much of their skin. The looseness of the clothing allows for air to �ow

between the clothing and the skin to permit the effective evaporation of sweat. Al-

though it may seem counterintuitive to cover one’s body completely in a hot climate,

the covering of the skin keeps the sun’s rays from directly penetrating the skin and

elevating the body’s core temperature.

Acclimatory Adjustments: Altitudinal Stressors

The challenges posed by thermal conditions are but one form of environmental

stressor humans must face. High-altitude environments, which are de�ned as alti-

tudes in excess of 2400 meters above sea level (m.a.s.l.) or 7874 feet above sea level

(f.a.s.l.), pose additional challenges to the maintenance of homeostasis in humans.

Some of the main stressors encountered by those living within high-altitude envi-

ronments include: decreased oxygen availability, cold temperatures, low humidity,

high wind speed, a reduced nutritional base, and increased solar radiation levels. Of

these challenges, the most signi�cant is the decreased availability of oxygen.

To visualize how altitude affects the availability of oxygen, imagine two balloons that

are each �lled with the same quantity of oxygen molecules. One of these balloons is

positioned at sea-level and the other is placed high upon a mountain peak. For the

balloon at sea level, there is more atmospheric pressure pressing down on the mole-

cules within this balloon. This leads to the oxygen molecules within the sea level

balloon being forced into a more compact organization. In contrast, the mountain

peak balloon has less atmospheric pressure pressing down on it. This leads to the

oxygen molecules within that balloon spreading out from each other since they are

not being forced together quite as strongly. This example highlights the availability

of oxygen molecules in each breath than we take in low- versus high-altitude envi-

ronments. At 5,500 m.a.s.l. (approximately 18,000 f.a.s.l.), the atmospheric pressure

is approximately 50% of its value at sea level (Peacock 1998). At the peak of Mount

Everest (8,900 m.a.s.l. or approximately 29,200 f.a.s.l.), the atmospheric pressure is

equivalent to only about 30% of their sea level amounts (Peacock 1998) (Figure 14.5).

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Figure 14.5 As altitude increases, atmospheric pressure decreases, which allows for more space between air molecules.

Due to the decreased avail-

ability of oxygen at higher

altitudes, certain acclima-

tory adjustments are re-

quired to ensure the main-

tenance of homeostasis for

individuals other than those

who were gestated, born,

and raised at high altitude.

For these people, their rate

of breathing will increase to

permit greater quantities of

air containing oxygen into

the lungs when they ascend

into higher altitude envi-

ronments. An increased

speed and depth of breath-

ing, which is referred to as hyperpnea, is not sustainable inde�nitely; thus, the rate

of breathing begins to decrease as the person becomes acclimatized to the altitude.

During the initial phases of high-altitude-related hyperpnea, the heart will begin to

beat faster but the stroke volume (the amount of blood pushed through during each

beat) will decrease slightly. In addition, the body will divert energy from non-critical

bodily functions, such as digestive processes.

Once the atmospheric oxygen reaches the alveoli (small air sacs) in the lungs, it dif-

fuses (spreads) across the alveolar membrane and enters erythrocytes (red blood

cells). When the oxygen reaches the erythrocytes, it will loosely bind with hemoglo-

bin, which is an iron-rich protein. It is within the alveoli that the oxygen combines

with the hemoglobin. When the erythrocytes carrying the hemoglobin-binded oxy-

gen molecules reach the capillaries where the partial pressure of oxygen is relatively

low, the oxygen will be released by the hemoglobin so that it is free for diffusion into

body cells. High-altitude-related hyperpnea leads to an increase in the pH of the

blood, which makes the blood more alkaline. An increase in the alkalinity of blood is

directly related to the partial pressures of oxygen and carbon dioxide in the blood-

stream (Shah et al. 2006). Previous: Race and Human Variation

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Figure 14.6 Premature infant born at 30 weeks, 4 days gestation to a mother with altitudinal-induced preeclampsia. Blue light assists the infant’s liver with processing high levels of bilirubin.

Similar to the acclimatory adjustments related to thermal conditions (e.g., shivering

or sweating), those related to high altitude may not be sustained permanently due to

their energetically expensive nature. Over a period of days or weeks, the human

body will begin to compensate for the increases in respiration as well as pH through

the urinary excretion of bicarbonate (a metabolic byproduct). In addition, to assist

with the transportation of oxygen in the absence of decreased respiration, there will

be an increase in the following: hematocrit (percentage volume of erythrocytes in

the blood), myoglobin (oxygen- and iron-binding protein in muscle tissues), red

blood cell mass, pulmonary artery pressure, and quantity of capillaries in the skele-

tal muscular tissue.

Although the long-term acclimatory adjustments

that an individual from low altitude experiences

in a high-altitude environment may permit them

to reside there successfully, reproduction within

such settings is frequently complicated. With in-

creased altitude comes an increased risk of mis-

carriage, lower birth weights, and higher infant

mortality rates. As the mother’s body seeks to

preserve its own homeostasis when faced with

challenges related to high-altitude living, there is

often a decreased rate and volume of blood �ow

to the uterus as compared to a pregnant woman

of similar physiological condition at a lower alti-

tude (Moore et al. 1998). Of course, a decreased rate and volume of uterine blood

�ow results in a decrease in the amount of oxygen that will be passed through the

uterus and placenta to the developing fetus. In addition, women who experience

pregnancy at higher altitudes are more prone to developing preeclampsia (severe

elevation of blood pressure), which is linked to increased rates of both fetal and ma-

ternal death (Moore et al. 1998) (Figure 14.6).

Developmen�al Adjustments

Although there are often signi�cant challenges for a mother gestating a fetus at high

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Figure 14.7 Quechua woman from high-altitude region of the Peruvian Andes.

altitude, individuals who are native to such environments experience a form of ad-

justment referred to as a developmental adjustment. Developmental adjustments oc-

cur only in individuals who spent their developmental period (i.e. childhood and

adolescence) within a high-altitude environment and they do not apply to those who

moved into these environments in the post-developmental (i.e., adult) phase. Fur-

thermore, the degree of developmental adjustment within an individual is directly

related to their underlying phenotypic plasticity (polyphenism) as well as the

amount of time during the crucial growth and development period the individual re-

sides within the challenging environment. Although humans have the remarkable

capacity to develop and survive within environments that are not overly conducive

to the successful maintenance of homeostasis, there are de�nitely physiological

costs associated with this ability.

In general, high-altitude natives tend to grow more slowly

and physically mature later than their low-altitude coun-

terparts (Figure 14.7). Decreased growth and maturity

rates are linked not only to the increased physiological

demands placed on the body due to the decreased partial

pressure of oxygen but to decreases in the quality of the

nutritional base at higher altitudes. Increased terrain

complexity, elevated solar radiation levels, and higher

wind speeds coupled with decreased temperatures and

lower humidity levels at higher altitudes leads to dif�cul-

ties with growing and maintaining crops and raising live-

stock. Overall, there is a decreased quality in the available

nutritional base as altitude increases, which is correlated

to a lack of the nutrients necessary to ensure proper

physiological growth and development in humans. Thus, even though individuals

may be able to develop and grow within high-altitude environments, they may not

reach their full genetically mediated growth potential as they would in a lower-alti-

tude environment.

The heart and lung capacity of individuals who are lifelong residents of high-altitude

environments is larger than for individuals from lower-altitude regions. In addition,

the high-altitude individuals are more ef�cient than those from low altitudes at dif-

fusing oxygen from the bloodstream to the body’s various tissues. Of course, as Previous: Race and Human Variation

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noted previously, the developmental adjustments that an individual may experience

are de�ned by their underlying genetic composition (phenotypic plasticity) and re-

searchers have begun to discover some of the genetic factors that appear to be

unique to certain populations of humans who have resided in high-altitude environ-

ments for signi�cant periods of time. The time frame necessary for natural selection

processes to act upon the underlying genetic composition of a population is on the

order of many millennia; thus, the physiological changes to indigenous high-altitude

populations on a whole are best described in the following section on adaptations.

Not all developmental adjustments are linked to environmental pressures such as

climate or altitude; rather, some of these adjustments are correlated to sociocultural

or behavioral practices. As noted earlier, some behavioral adjustments permit indi-

viduals to maintain homeostasis in challenging environments (e.g., wearing heavy

clothing layers to maintain body temperature in cold weather). Other behavioral ad-

justments, many of which are based on sociocultural principles, may affect the phys-

iological appearance of an individual when they are practiced consistently during

the development and growth phases.

Sudden infant death syndrome (SIDS) has no de�nitive cause; however, the Ameri-

can Academy of Pediatrics published a report in 1992 linking SIDS to infants (under

the age of one) sleeping on their stomachs. The “Back to Sleep” campaign champi-

oned by the American Academy of Pediatrics helped educate members of the medi-

cal community as well as the public that the best sleep position for infants is on

their backs. Per the American Academy of Pediatrics Task Force on Infant Sleep Po-

sition and Sudden Infant Death Syndrome (2000� 1,245), between 1992 and 2000 the

frequency of infants being placed on their stomachs to sleep decreased from more

than 70% to less than 20% and the SIDS rate decreased by more than 40%.

Placing infants on their backs to sleep has led to decreased infant mortality (death)

rates due to SIDS; however, it has led to an unintended consequence: infant cranial

deformation. The cranial deformations experienced by infants who sleep solely on

their back tend to manifest in one of two forms: brachycephaly and plagiocephaly

(Roby et al. 2012). With positional brachycephaly, the back of the infant’s head ap-

pears rather uniformly �attened due to repetitive contact with a �at surface, such as

a crib mattress or car seat back. In cases of positional plagiocephaly, the back of the

infant’s head appears asymmetrically �attened (Figure 14.8). This asymmetry is typi-

cally due to an uneven distribution of mechanical forces resulting from the manner

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Figure 14.8 Infant with positional plagiocephaly. Notice the irregular shape of the posterior (back) of the skull.

in which the infant’s head is in contact with a �at

surface. The forms of cranial deformation resulting

from sleep positioning do not affect the infant’s

brain development. For many individuals, the ap-

pearance of the deformation is minimized during

later development. Still, some individuals will

maintain the pattern of cranial deformation ac-

quired during their infancy throughout their lives.

The unintentional cranial deformation resulting

from placing infants on their backs to sleep as a

means of preventing SIDS-related deaths is a

physiological indicator of a behavioral adjustment.

Adaptations

As we have just explored, survival and reproduction at high altitudes present numer-

ous physiological challenges for most humans, but what if there were some humans

who were specially adapted to life at high altitudes? The behavioral, acclimatory, and

developmental adjustments discussed above are all related to the phenotypic plas-

ticity of the individual; however, most adjustments are temporary in nature and they

affect a single individual rather than all individuals within a population. But, what if

the physiological changes were permanent? What if they affected all members of a

population rather than just a single individual? The long-term, micro-evolutionary

(i.e., genetic) changes that occur within a population in response to an environmen-

tal stressor are referred to as an adaptation. From an evolutionary standpoint, the

term “adaptation” refers to a phenotypic trait (i.e., physiological/morphological fea-

ture or behavior) that has been acted upon by natural selection processes to in-

crease a species’ ability to survive and reproduce within a speci�c environment.

Within the �eld of physiology, the term “adaptation” refers to traits that serve to re-

store homeostasis. The physiology-based interpretation of adaptations presumes

that all traits serve a purpose and that all adaptations are bene�cial in nature; how-

ever, this may be a fallacy, since some traits may be present without clear evidence

as to their purpose. As such, during the following discussion of various forms of Previous: Race and Human Variation

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adaptations in human populations, we will focus our attention to phenotypic traits

with an evidence-based purpose.

Adap�ation: Altitudinal Adap�ation

As mentioned in the previous section, there is genomic research supporting the

evolutionary selection of certain phenotypes and their corresponding genotypes

within indigenous high-altitude populations across the globe. The following discus-

sion focuses on three high-altitude indigenous populations from Tibet, the Andes,

and Ethiopia (Figure 14.9). Although these populations share many common genetic

traits based on relatively similar evolutionary histories in�uenced by similar envi-

ronmental stressors, there is support for local genetically based adaptation as well,

based on different genes being acted upon by environmental stressors that may be

unique to Tibet, the Andes, and Ethiopia (Bigham 2016).

Figure 14.9 Highlighted regions feature ( from left to right) the Andean, Simian (Ethiopian), and Tibetan Plateau high-altitude regions.

Tibetan populations have resided in the Tibetan Plateau and Himalayan Mountain

regions at elevations exceeding 4,000 m.a.s.l. (13,100 f.a.s.l.) for at least the past 7,400

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years (Meyer et al. 2017). A gene referred to as EPAS1 is involved in the regulation of

red blood cell (and hemoglobin) production as well as catecholamine homeostasis.

Catecholamines are hormones (e.g., epinephrine) secreted as part of the sympa-

thetic nervous system’s acute stress (“�ght-or-�ight”) response. An acute stress re-

sponse typically includes an increase in: heart rate, blood pressure, and blood glu-

cose levels. Long-term elevation of catecholamines in the body may lead to hyper-

tension (elevated blood pressure), increased blood pH levels, the development of a

form of cardiovascular disease leading to narrowing of the arteries (atherosclerosis),

and blood clots. In the short term, an acclimatory adjustment leading to the increase

of catecholamines and hemoglobin production by the hypoxia (low-oxygen level)-

induced activation of the EPAS1 gene may assist individuals from lower altitudes as

they ascend to signi�cantly higher altitudes; however, such increases may not be

maintained for long before they cause damage to the body. For indigenous high-alti-

tude populations of Tibet, a mutation in the EPAS1 gene inhibits increased red blood

cell production and assists with catecholamine regulation. The red blood cell count

of high-altitude Tibetans with the EPAS1 point mutation is about the same as for in-

dividuals residing at sea level.

Interestingly, individuals in populations from the high-altitude Andean Altiplano of

Peru and Bolivia, such as the Quechua and Aymara, lack the EPAS1 point mutation, so

their red blood cell counts are relatively elevated compared with the Tibetan popu-

lations. In addition, populations from the Andean Altiplano also have an increased

arterial oxygen saturation and low, hypoxia-induced breathing pattern (minor hy-

perventilation) as compared to their Tibetan counterparts (Bigham 2016). Popula-

tions indigenous to the Semien Plateau of Ethiopia, such as the Oromo and Amhara,

share a similar but not identical EPAS1 point mutation with the Tibetan population

(Bigham 2016). This suggests that the EPAS1 mutations occurred independently from

each other; however, their effects are still similar in that they permit the Tibetan and

Ethiopian populations to survive at high altitudes. Not all adaptations are related to

life in high-altitude environments, however. In the following sections, we will ad-

dress two more general examples of adaptation in human populations: variations in

skin color and differences in body build.

Adap�ation: Skin Tone Basics Previous: Race and Human Variation

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When you think about your own skin tone and compare it to members of your fam-

ily, do you all possess exactly the same shade? Are some members of your family

darker than others? What about your friends? Your classmates? Skin tone occurs

along a continuum, which is a re�ection of the complex evolutionary history of our

species. The expression of skin tone is regulated primarily by both melanin and he-

moglobin. Melanin is a dark brown-black pigment that is produced by the oxidation

of certain amino acids (e.g., tyrosine, cysteine, phenylalanine) in the melanocytes.

Melanocytes are specialized cells located in the base layer (stratum basale) of the

skin’s epidermis as well as several other areas within the body (Figure 14.10). Within

the melanocytes, melanin is produced in the special organelle called a melanosome.

Melanosomes serve as sites for the synthesis, storage, and transportation of

melanin. Melanosomes transport the melanin particles through cellular projections

to epidermal skin cells (keratinocytes) as well as to the base of the growing hair root

(root sheath portion). In the eye, however, melanin particles produced by the

melanosomes remain present within the iris (iridial melanocytes) and are not trans-

ported beyond their origin location. The two main forms of melanin related to skin,

hair, and eye color are eumelanin and pheomelanin. All humans contain both eume-

lanin and pheomelanin within their bodies; however, the relative expression of these

two forms of melanin determines an individual’s overall coloring. Eumelanin is a

brown-to-black colored melanin particle and pheomelanin is more pink-to-red. In-

dividuals with darker skin or hair color have a greater expression of eumelanin than

those with lighter-colored skin and blonde or red hair.

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Figure 14.10 Diagram featuring the relative numbers of melanocytes and melanosomes in light and dark shades of skin tone.

SPECIAL TOPIC: SKIN TONE GENETIC REGULATION

The melanocortin 1 receptor (MC1R) gene acts to control which types of

melanin (eumelanin or pheomelanin) are produced by melanocytes. The

MC1R receptor is located on the surface of the melanocyte cells (Quillen

et al. 2018). Activation of the MC1R receptors may occur through expo-

sure to speci�c environmental stimuli or due to underlying genetic pro-

cesses. Inactive or blocked MC1R receptors results in melanocytes pro-

ducing pheomelanin. If the MC1R gene receptors are activated, then the

melanocytes will produce eumelanin. Thus, individuals with activated

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MC1R receptors tend to have darker pigmented skin and hair than indi-

viduals with inactive or blocked receptors.

The alleles of another gene, the major facilitator, superfamily domain-

containing protein 12 (MFSD12) gene, affect the expression of

melanocytes in a different way than the MC1R gene. Instead of affecting

the activation of melanocyte receptors, the MFSD12 alleles indirectly af-

fect the membranes of melanocyte lysosomes (Quillen et al. 2018). The

melanocyte’s lysosomes are organelles containing digestive enzymes,

which ultimately correlate to varying degrees of pigmentation in humans.

Variations in the membranes of the melanocyte lysosomes ultimately

correlate to differing degrees of pigmentation in humans.

Ancestral MFSD12 allele variants are present in European and East Asian

populations and are associated with lighter pigmentation of the skin

(Crawford et al. 2017; Quillen et al. 2018). In addition, this ancestral variant

is also associated with Tanzanian, San, and Ethiopian populations of Afro-

Asiatic ancestry (Crawford et al. 2017; Quillen et al. 2018). In contrast, the

more-derived (i.e. more recent) allele variants that are linked to darker

skin tones are more commonly present in East African populations, par-

ticularly those of Nilo-Saharan descent (Crawford et al. 2017; Quillen et al.

2018). The notion that ancestral alleles of MFSD12 are associated with

lighter skin pigmentation is in opposition to the commonly accepted idea

that our pigmentation was likely darker throughout early human evolu-

tion (Crawford et al. 2017; Quillen et al. 2018). Due to the complexity of

the human genome, MFSD12 and MC1R are but two examples of alleles

affecting human skin tone. Furthermore, there is genetic evidence sug-

gesting that certain genomic variants associated with both darker and

lighter skin color have been subject to directional selection processes for

as long as 600,000 years, which far exceeds the evolutionary span of

Homo sapiens sapiens (Crawford et al. 2017; Quillen et al. 2018). So, evolu-

tionary processes may lead to skin becoming more darkly pigmented as

well as more lightly pigmented.

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Figure 14.11 Penetration of skin layers by UVA and UVB rays.

Adap�ation: Melanogenesis

Although all humans have approximately the

same number of melanocytes within the epider-

mis, the production of melanin by these

melanocytes varies. There are two forms of

melanogenesis (the process through which

melanocytes generate melanin): basal and acti-

vated. As discussed previously, the expression of

eumelanin and pheomelanin by the melanocytes

is genetically regulated through the expression

of speci�c receptors (e.g., MC1R) or other

melanocyte components (e.g., MFSD12).

Basal melanogenesis is dependent upon an

individual’s inherent genetic composition and is

not in�uenced by external factors. Activated melanogenesis occurs in response to

ultraviolet radiation (UV) exposure, speci�cally UV-B (short UV wave) exposure. In-

creased melanogenesis in response to UV-B exposure serves to provide protection

to the skin’s innermost layer called the hypodermis, which lies below the epidermis

and dermis (Figure 14.11). Melanin in the skin, speci�cally eumelanin, effectively ab-

sorbs UV-B radiation from light meaning that it will not reach the hypodermal layer.

This effect is often more apparent during periods of the year when individuals tend

to be outside more and the weather is warmer, which leads to those individuals don-

ning fewer protective garments. The exposure of skin to sunlight is, of course, cul-

turally mediated with some cultures encouraging the covering of skin at all times.

As previously noted, hemoglobin is an iron-rich protein that binds with oxygen in

the bloodstream. For individuals with lighter-colored skin, blood vessels near the

surface of the skin and the hemoglobin contained within those vessels is more ap-

parent than in individuals with darker skin. The visible presence of hemoglobin cou-

pled with the pink-to-red tone of the pheomelanin leads to lighter-skinned individ-

uals having a pale pink skin tone. Individuals with lighter skin more readily absorb

UV radiation as their basal melanin expression is directed more toward the produc-

tion of pheomelanin than eumelanin. But, why are there so many variations in skin Previous: Race and Human Variation

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Figure 14.12 Evolutionary basis for human skin color variation.

tone in humans? To answer this question, we now turn toward an exploration of an

evolutionary-based adaptation of skin tone as a function of the environment.

Adap�ation: Evolutionary Basis for Skin Tone Variation

Skin cancer is a signi�cant concern for many in-

dividuals with light skin tone as the cumulative

exposure of the epidermis and underlying skin

tissues to UV radiation may lead to the develop-

ment of abnormal cells within those tissues lead-

ing to malignancies. Although darker-skinned in-

dividuals are at risk for skin cancer as well, they

are less likely to develop it due to increased lev-

els of melanin, speci�cally eumelanin, in their

skin. Even though skin cancer is a serious health

concern for some individuals, most skin cancers

occur in the post-reproductive years; therefore,

it is improbable that evolutionary forces favoring

varying melanin expression levels are related to a

selective pressure to avoid such cancers. Fur-

thermore, if avoiding skin cancer were the pri-

mary factor driving the evolution of various skin

tones, then it reasons that everyone would have

the most signi�cant expression of eumelanin

possible. So, why do we have different skin tones

(Figure 14.12)? The term cline refers to the con-

tinuum or spectrum of gradations (i.e., levels or

degrees) from one extreme to another. With re-

spect to skin tone, the various tonal shades oc-

cur clinally

with darker skin being more prevalent near the

equator and gradually decreasing in tone (i.e.,

decreased melanin production) in more distant

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latitudes. For individuals who are indigenous to equatorial regions, the increased

levels of melanin within their skin provides them with a measure of protection

against both sunburn and sunstroke as the melanin is more re�ective of UV radia-

tion than hemoglobin. In cases of severe sunburn, eccrine glands are affected, re-

sulting in an individual’s ability to sweat being compromised. As sweat is the body’s

most effective means of reducing its core temperature to maintain homeostasis,

damage to the eccrine glands may lead to numerous physiological issues related to

heat that may ultimately result in death.

Even though avoiding severe sunburn and sunstroke is of great importance to indi-

viduals within equatorial regions, this is likely not the primary factor driving the

evolutionary selection of darker skin within these regions. It has been proposed that

the destruction of folic acid, which is a form of B-complex vitamin, by UV radiation

may have led to the selection of darker skin in equatorial regions. For pregnant

women, low levels of folic acid within the body during gestation may lead to defects

in the formation of the brain and spinal cord of the fetus. This condition, which is

referred to as spina bi�da, often signi�cantly reduces the infant’s chances of survival

without medical intervention. In men, low levels of folic acid within the body lead to

an inhibition in the production of sperm. Thus, in geographic regions with high UV

radiation levels (i.e., equatorial regions), there appears to be an evolutionarily driven

correlation between darker skin and the maintenance of fertility.

If darker skin tone is potentially correlated to more successful reproduction, then

why do lighter shades of skin exist? One hypothesis is that there is a relationship be-

tween lighter skin tone and vitamin D synthesis within the body. When skin is ex-

posed to the UV-B radiation waves in sunlight, a series of chemical reactions occur

within the epidermis leading to the production of vitamin D3. Before the body can

use vitamin D3, it must travel to the liver and then to the kidneys where it is con-

verted into different forms of bioactive molecules. Ultimately, it is converted into

the bioactive molecule calcitriol (Vukić et al. 2015). Within the human body there are

numerous cell types with binding receptors for calcitriol, so it is capable of adhering

to the DNA of those cells (Snoddy et al. 2016). Calcitriol serves as a regulator in cel-

lular-replication processes within the body, including those for pancreatic, breast,

colon, and kidney cells (Snoddy et al. 2016). Insuf�cient calcitriol is associated with

an increased risk of: some forms of cancer (colon, prostate, etc.), autoimmune dis-

eases (multiple sclerosis, lupus, type I diabetes, etc.), cardiovascular diseases, and Previous: Race and Human Variation

Next: Bioarchaeology and Forensic Anthropology

Figure 14.13 Children with rickets in various developmental stages.

infections (e.g., tuberculosis, in�uenza) (Snoddy el al. 2016; Chaplin and Jablonski

2009). De�ciencies in calcitriol production and absorption within the human body

may be linked to underlying genetic factors, such as a mutation in the vitamin D re-

ceptors present in some of the body’s cells (Chaplin and Jablonski 2009). Alterna-

tively, it may be linked to inadequate exposure to the UV-B rays necessary to stimu-

late calcitriol production or to a nutritional de�ciency in vitamin D-rich foods. Re-

gardless of the cause of the de�ciency, individuals with a calcitriol (vitamin D3) de�-

ciency may also be at risk for the development of certain skeletal abnormalities in

addition to the previously mentioned health issues.

Vitamin D is required for the absorption of certain

nutrients, such as calcium and phosphorus, in the

small intestine. These nutrients are among those

that are critical for the proper growth and mainte-

nance of bone tissue within the body. In the ab-

sence of adequate minerals, particularly calcium,

bone structure and strength will be compromised

leading to the development of rickets during the

growth phase. Rickets is a disease affecting chil-

dren during their growth phase and is character-

ized by inadequately calci�ed bones that are softer

and more �exible than normal. Individuals with

rickets will develop a true bowing of their femora,

which may affect their mobility (Figure 14.13). In ad-

dition, deformation of pelvic bones in women may occur as a result of rickets lead-

ing to complications with reproduction. In adults, a de�ciency in vitamin D will of-

ten result in osteomalacia, which is a general softening of the bones due to inade-

quate mineralization. This softening is the result of impaired bone metabolism that

is primarily linked to insuf�cient levels of bioavailable vitamin D, calcium, and phos-

phate. In addition, it may be linked to inadequate absorption of calcium in the

bloodstream. As noted, a variety of maladies may occur due to the inadequate pro-

duction or absorption of vitamin D, as well as the destruction of folate within the

human body; so, from an evolutionary perspective, natural selection should favor a

skin tone that is best suited to a given environment.

In general, the trend related to lighter skin pigmentation further from the equator Previous: Race and Human Variation

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follows a principle called Gloger’s Rule. This rule states that within the same species

of mammals the more heavily pigmented individuals tend to originate near the

equator while lighter-pigmented members of the species will be found in regions

further from the equator. Gloger’s Rule applies latitudinally; however, it does not ap-

pear to hold for certain human populations near the poles. Speci�cally, the Inuit

people (Figure 14.14), who are indigenous to regions near the North Pole and cur-

rently reside in portions of Canada, Greenland, Alaska, and Denmark. The Inuit have

a darker skin tone that would not be anticipated under the provisions of Gloger’s

Rule. The high re�ectivity of light off of snow and ice, which is common in polar re-

gions, necessitates the darker skin tone of these individuals to prevent folic acid

degradation just as it does for individuals within equatorial regions. The consump-

tion of vitamin D–rich foods, such as raw �sh, permits the Inuit to reside at high lat-

itudes with darker skin tone while preventing rickets.

Figure 14.14 Inuit family, 1917.

Genome studies have identi�ed a number of genes (TYR, OCA2/HERC2, TYRP1,

SLC45A2, HPS6i, etc.) related to the expression of melanin and pigmentation presen-

tation in humans. Compared to the exceptionally large number of genes within the

human genome, those regulating the expression of melanin are relatively few and

appear on distinct loci. The genes at these loci are generally pleiotropic in nature, so

there is a relatively predictable patterning in skin, hair, and eye color combinations

(Sturm and Duffy 2012). For example, some populations that are indigenous to

higher latitude regions tend to have lighter skin, hair, and eye color than their coun-

terparts from equatorial regions. Still, since the genes affecting skin, hair, and eye

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color are actually independent, it is possible that variations may produce many phe-

notypic combinations. Turning again to our example of individuals indigenous to

higher latitudes, it is theoretically possible to encounter an individual with dark hair,

light-toned skin, and blue eyes within this region due to the variability of pheno-

typic combinations.

Adap�ation: Shape and Size Variations

In addition to natural selection playing a role in the determination of melanin ex-

pression related to skin tone, which is correlated to the environment, it plays a sig-

ni�cant role in the determination of the shape and size of the human body. As previ-

ously discussed, the most signi�cant thermodynamic mechanism of heat loss from

the body is radiation. At temperatures below 20 ℃ (68 ℉ ), the human body loses around 65% of its heat to radiative processes; however, the ef�ciency of radiation as

a means of heat reduction is correlated to the overall body shape and size of the in-

dividual. There is a direct correlation between the ratio of an object’s surface area to

mass and the amount of heat that may be lost through radiation. For example, two

metal objects of identical composition and mass are heated to the same tempera-

ture. One object is a cube and the other is a sphere. Which object will cool the

fastest? Geometrically, a sphere has the smallest surface area per unit mass of any

three-dimensional object, so the sphere will cool more slowly than the cube. In

other words, the smaller the ratio of the surface area to mass an object has, the

more it will retain heat. With respect to the cube in our example, mass increases by

the cube, but surface area may increase only by the square, so size will affect the

mass to surface area ratio. This, in general, holds true for humans, as well.

In regions where temperatures are consistently cold, the body shape and size of the

individuals who are indigenous to the area tend to be more compact. These individ-

uals have a relatively higher body mass to surface area (i.e., skin) than their counter-

parts from equatorial regions where the average temperatures are considerably

warmer. Individuals from hot climates, such as the Fulani (Figure 14.15a) of West

Africa, have limbs that are considerably longer than those of individuals from cold

climates, such as the Inuit of Greenland (Figure 14.15b). Evolutionarily, the longer

limbs of individuals from equatorial regions (e.g., the Fulani) provide a greater sur-

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face area (i.e., lower body mass to surface area ratio) for the dissipation of heat

through radiative processes. In contrast, the relatively short limbs of Arctic-dwelling

people, such as the Inuit, allows for the retention of heat as there is a decreased sur-

face area through which heat may radiate away from the body.

Figure 14.15a The Fula people of Burkina Faso (pictured here in 1974) are from a tropical environment where the rapid dispersal of heat is necessary to maintain homeostasis.

Figure 14.15b These Inuit people from Greenland live in an arctic environment where the conservation of heat in the body’s core is of critical importance.

As described above, there are certain trends related to the general shape and size of Previous: Race and Human Variation

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Figure 14.16a These organisms are representative of Bergmann’s rule. The animal on the left depicts an ungulate from a cooler environment with increased body weight and decreased surface area, compared to the slender ungulate on the right.

human bodies in relation to the thermal

conditions. To better describe these

trends, we turn to a couple of general

principles that are applicable to a variety

of species beyond humans.

Bergmann’s Rule predicts that as average

environmental temperature decreases,

populations are expected to exhibit an

increase in weight and a decrease in sur-

face area (Figure 14.16a). Also, within the

same species of homeothermic animals,

the relative length of projecting body

parts (e.g., nose, ears, and limbs) in-

creases in relation to the average environmental temperature (Figure 14.16b). This

principle, referred to as Allen’s Rule, notes that longer, thinner limbs are advanta-

geous for the radiation of excess heat in hot environments and shorter, stockier

limbs assist with the preservation of body heat in cold climates. A measure of the

crural index (crural index=tibia length [divided by] femur length) of individuals from

various human populations provides support for Allen’s Rule since this value is lower

in individuals from colder climates than it is for those from hot climates. The crural

indices for human populations varies directly with temperature, so individuals with

higher crural index values are generally from regions with a warmer average envi-

ronmental temperature. Conversely, the crural indices are lower for individuals from

regions where there are colder average temperatures.

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Figure 14.16b These animals are representative of Allen’s rule. Note the shorter limbs and ears of the rabbit on the left that you might �nd in cold temperatures. Note the length of the ears on the rabbit on the right that you might �nd in a warm climate. Rabbits do not sweat like humans, heat is dissipated primarily through their ears.

Nasal shape and size (Figure 14.17) is another physiological feature that is affected by

way of an individual’s ancestors’ environments. The selective role of climate in de-

termining human nasal variation is typically approached by dividing climates into

four adaptive zones: hot-dry, hot-wet, cold-dry, and cold-wet (Maddux et al. 2016).

One of the principal roles of the nasal cavity is to condition (i.e., warm and humidify)

ambient air prior to its reaching the lungs. Given that function of the nasal cavity, it

is anticipated that different nasal shapes and sizes will be related to varying environ-

ments. In cold-dry climates, an individual’s nasal cavity must provide humidi�cation

and warmth to the dry air when breathing in through the nose (Noback et al. 2011).

Also, in that type of climate, the nasal cavity must conserve moisture and minimize

heat loss during when the individual exhales through the nose (Noback et al. 2011).

From a physiological stress perspective, this is a stressful event.

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Figure 14.17 Human nasal morphological variation as in�uenced by four major climate-based adaptive zones: hot-dry, hot-wet, cold-dry, and cold-wet. Note that images are presented left-to-right in relation to the climate-based adaptive zones, respectively.

Conversely, in hot-wet environments, there is no need for the nasal cavity to provide

additional moisture to the inhaled air nor is there a need to warm the air or to pre-

serve heat within the nasal cavity (Noback et al. 2011). So, in hot-wet climates, the

body is under less physiological stress related to the inhalation of ambient air than

in cold-dry climates. As with most human morphological elements, the shape and

size of the nasal cavity occurs along a cline. Due to the environmental stressors of

cold-dry environments requiring the humidi�cation and warming of air through the

nasal cavity, individuals indigenous to such environments tend to have taller (longer)

noses with a reduced nasal entrance (nostril opening) size (Noback et al. 2011). This

general shape is referred to as leptorrhine, and it allows for a larger surface area

within the nasal cavity itself for the air to be warmed and humidi�ed prior to enter-

ing the lungs (Maddux et al. 2016). In addition, the relatively small nasal entrance of

leptorrhine noses serves as a means of conserving moisture and heat (Noback et al.

2011). Individuals indigenous to hot-wet climates tend to have platyrrhine nasal

shapes, which are shorter with broader nasal entrances (Maddux et al. 2016). Since

individuals in hot-wet climates do not need to humidify and warm the air entering

the nose, their nasal tract is shorter and the nasal entrance wider to permit the ef-

fective cooling of the nasal cavity during respiratory processes.

Adap�ation: Infectious Disease Previous: Race and Human Variation Next: Bioarchaeology and Forensic Anthropology

Throughout our evolutionary journey, humans have been exposed to numerous in-

fectious diseases. In the following section, we will explore some of the evolutionary-

based adaptations that have occurred in certain populations in response to the

stressors presented by select infectious diseases. One of the primary examples of

natural selection processes acting on the human genome in response to the pres-

ence of an infectious disease is the case of the relationship between the sickle-cell

anemia trait and malaria.

Malaria is a zoonotic disease (type of infectious disease naturally transmitted be-

tween animals and humans; covered in more detail in Chapter 16� Human Biology

and Health) caused by the spread of the parasitic protozoa from the genus Plasmod-

ium (Figure 14.18). These unicellular, eukaryotic protozoa are transmitted through

the bite of a female Anopheles mosquito. During the bite process, the protozoan par-

asites that are present within an infected mosquito’s saliva will enter the blood-

stream of the individual where they will be transported to the liver. Within the liver,

the parasites multiply and will eventually be released into the bloodstream where

they will infect erythrocytes. Once inside the erythrocytes, the parasites will repro-

duce until they exceed the cell’s storage capacity, causing it to burst and release the

parasites into the bloodstream once again. This replication cycle will continue as

long as there are viable erythrocytes within the host to infect.

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Figure 14.18 Life cycle of the malaria parasite.

General complications from malaria infections include: enlargement of the spleen

(due to destruction of infected erythrocytes), lower number of thrombocytes (also

called platelets, required for coagulation/clotting of blood), high levels of bilirubin (a

byproduct of hemoglobin breakdown in the liver) in the blood, jaundice (yellowing of

the skin and eyes due to increased blood bilirubin levels), fever, vomiting, retinal

(eye) damage, and convulsions (seizures). According to the World Health Organiza-

tion, in 2016 there were 445,000 deaths from malaria globally with the highest per-

centage of those deaths occurring in Africa (91%) and Southeast Asia (6%) (World

Health Organization 2017). In sub-Saharan Africa, where incidents of malaria are the

highest in the world, 125 million pregnancies are affected by malaria, resulting in

200,000 infant deaths (Hartman et al. 2013). Pregnant women who become infected

during the gestational process are more likely to have low-birthweight infants due

to prematurity or growth restriction inside the uterus (Hartman et al. 2013). After

birth, infants born to malaria-infected mothers are more likely to develop infantile

anemia (low red blood cell counts), a malaria infection that is not related to the ma-

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ternal malarial infection, and they are more likely to die than infants born to non-

malaria-infected mothers (Hartman et al. 2013).

For children and adolescents whose brains are still developing, there is a risk of cog-

nitive (intellectual) impairment associated with some forms of malaria infections

(Fernando et al. 2010). Given the relatively high rates of morbidity (disease) and mor-

tality (number of deaths) associated with malaria, it leads to reason that this disease

may have served as a selective pressure during human evolution. Support for natural

selection related to malaria resistance is related to genetic mutations associated

with sickle cell, thalassemia, glucose-6-phosphate dehydrogenase (G6PD) de�-

ciency, and the absence of certain antigens (molecules capable of inducing an im-

mune response from the host) on erythrocytes. For the purposes of this text, we will

focus our discussion on the relationship between sickle cell disease and malaria.

Sickle cell disease is a group of genetically inherited blood disorders characterized

by an abnormality in the shape of the hemoglobin within erythrocytes. It is impor-

tant to note that there are multiple variants of hemoglobin, including, but not lim-

ited to: A, D, C, E, F, H, S, Barts, Portland, Hope, Pisa, and Hopkins. Each of these

variants of hemoglobin may result in various conditions within the body; however,

for the following explanation we will focus solely on variants A and S.

Individuals who inherit a mutated gene (hemoglobin with a sickled erythrocyte vari-

ety, HbS) on chromosome 11 from both parents will develop sickle cell anemia, which

is the most severe form of the sickle cell disease family (Figure 14.19). The genotype

of an individual with sickle cell anemia is HbSS; whereas, an individual without sickle

cell alleles has a genotype of HbAA representing two normal adult hemoglobin type

A variants. Manifestations of sickle cell anemia (HbSS) range from mild to severe

with some of the more common symptoms being: anemia, blood clots, organ failure,

chest pain, fever, and low blood oxygen levels. In high-income countries with ad-

vanced medical care, the median life expectancy of an HbSS individual is around 60

years; however, in low-income countries where advanced medical care is scarce, as

many as 90% of children with sickle cell disease perish before the age of �ve (Longo

et al. 2017).

Considering that advanced medical care was not available during much of human

evolutionary history, it stands to reason that the majority of individuals with the

HbSS genotype died before the age of reproduction. If that is the case though, why

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Figure 14.19 Normal and sickled erythrocytes.

do we still have the HbS variant present in mod-

ern populations? As covered earlier in this text-

book, the genotype of an individual is composed

of genes from both biological parents. In the case

of an individual with an HbSS genotype, the sickle

cell allele (HbS) was inherited from each of the

parents. For individuals with the heterozygous

genotype of HbSA, they have inherited both a

sickle cell allele (HbS) and a normal hemoglobin

allele (HbA). Heterozygous (HbSA) individuals who

reside in regions where malaria is endemic may

have a selective advantage. They will experience a

sickling of some, but not all, of their erythrocytes.

Unlike an individual with the HbSS genotype,

someone with HbSA may experience some of the

symptoms listed above; however, they are gener-

ally less severe.

As noted earlier, the mechanism through which

Plasmodium protozoan parasites replicate involves human erythrocyte cells. How-

ever, due to their sickled shape, as well as the presence of an abnormally shaped

protein within the cell, the parasites are unable to replicate effectively in the ery-

throcyte cells coded for by the HbS allele (Cyrklaff et al. 2011). An individual who has

an HbSA genotype and an active malaria infection will become ill with the disease to

a lesser extent than someone with an HbAA genotype. Although normal erythro-

cytes (regulated by the HbA allele) allow for the replication of the parasite, the para-

sites will not be able to replicate in HbS erythrocytes of the heterozygote. So, indi-

viduals with the HbSA genotype are more likely to survive a malaria infection than

an individual who is HbAA. Although individuals with the HbSA genotype may en-

dure some physiological complications related to the sickling of some of their ery-

throcytes, their morbidity and mortality rates are lower than they are for HbSS

members of the population. The majority of individuals who are heterozygous or ho-

mozygous for the HbS trait have ancestors who originated in sub-Saharan Africa, In-

dia, Saudi Arabia, regions in South and Central America, the Caribbean, and the

Mediterranean (Turkey, Greece, and Italy) (Centers for Disease Control and Preven-

tion 2017) (Figure 14.20).

Previous: Race and Human Variation

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Figure 14.20 Distribution of sickle cell and associated erythrocytic abnormalities for Africa and Asia.

With respect to the history of these regions, during the early phases of settlement

horticulture was the primary method of crop cultivation. Typically performed on a

small scale, horticulture is based on manual labor and relatively simple hand tools

rather than the use of draft animals or irrigation technologies. Common in horticul-

ture is swidden, or the cutting and burning of plants in woodland and grassland re-

gions. The swidden is the prepared �eld that results following a slash-and-burn

episode. This practice fundamentally alters the soil chemistry, removes plants that

provide shade, and increases the areas where water may pool. This anthropogeni-

cally altered landscape provides the perfect breeding ground for the Anopheles mos-

quito, as it prefers warm, stagnant pools of water (Figure 14.21).

Previous: Race and Human Variation

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Figure 14.21 The effects of human horticultural activities on the balancing selection of populations in relation to sickle cell disease genotype variants.

Although swidden agriculture was historically practiced across the globe, it became

most problematic in the regions where the Anopheles mosquito is endemic. These

areas have the highest incidence rates of malaria infection. Over time, the presence

of the Anopheles mosquito and the Plasmodium parasite that it transmitted acted as

a selective pressure, particularly in regions where swidden agricultural practices

were common, toward the selection of individuals with some modicum of resistance

against the infection. In these regions, HbSS and HbSA individuals would have been

more likely to survive and reproduce successfully. Although individuals and popula-

tions are far more mobile now than they have been throughout much of history,

there are still regions where we can see higher rates of malaria infection as well as

greater numbers of individuals with the HbS erythrocyte variant. The relationship

between malaria and the selective pressure for the HbS variant is one of the most

prominent examples of natural selection in the human species within recent evolu-

tionary history.

Previous: Race and Human Variation

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Figure 14.22 Interpolated map depicting the percentage of adults with the lactase persistence genotype in indigenous populations of the Old World. Circles denote sample locations.

Adap�ation: Lac�ase Persistence

With the case of sickled erythrocytes and their resistance to infection by malaria

parasites, there is strong support for a cause-and-effect-style relationship linked to

natural selection. Although somewhat less apparent, there is a correlation between

lactase persistence and environmental challenges. Lactase-phlorizin hydrolase

(LPH) is an enzyme that is primarily produced in the small intestine and permits the

proper digestion of lactose, a disaccharide (composed of two simple sugars: glucose

and galactose) found in the milk of mammals. Most humans will experience a de-

crease in the expression of LPH following weaning, leading to an inability to prop-

erly digest lactose. Generally, LPH production decreases between the ages of two

and �ve and is completely absent by the age of nine (Dzialanski et al. 2016). For these

individuals, the ingestion of lactose may lead to a wide variety of gastrointestinal ail-

ments including abdominal bloating, increased gas, and diarrhea. Although the

bloating and gas are unpleasant, the diarrhea caused by a failure to properly digest

lactose can be life-threatening if severe enough due to the dehydration it can cause.

Some humans, however, are able to produce LPH far beyond the weaning period.

Individuals who continue to

produce LPH have what is

referred to as the

lactase persistence trait.

The lactase persistence trait

is encoded for a gene called

LCT, which is located on hu-

man chromosome 2 (Ran-

ciaro et al. 2014; see also

Chapter 3). From an evolu-

tionary and historical per-

spective, this trait is most

commonly linked to cultures

that have practiced cattle

domestication (Figure 14.22).

For individuals in those cul-

tures, the continued expression of LPH may have provided a selective advantage.

Previous: Race and Human Variation

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During periods of environmental stress, such as a drought, if an individual is capable

of successfully digesting cow’s milk, they have a higher chance of survival than

someone who suffers from diarrhea-linked dehydration due to a lack of LPH. Per

Tishkoff et al. , the “frequency of lactase persistence is high in northern European

populations (more than 90% in Swedes and Danes), decreases in frequency across

southern Europe and the Middle East (less than 50% in Spanish, French, and pas-

toralist Arab populations), and is low in non-pastoralist Asian and African popula-

tions (less than 1% in Chinese, less than 5% to 20% in West African agriculturalists)”

(2007� 248). Although the frequency of the lactase persistence trait is relatively low

among African agriculturalists, it is high among pastoralist populations that are tra-

ditionally associated with cattle domestication, such as the Tutsi and Fulani, who

have frequencies of 90% and 50%, respectively (Tishkoff et al. 2007).

Cattle domestication began around 11,000 years ago in Europe (Beja-Pereira et al.

2006) and 7,500 to 9,000 years ago in the Middle East and North Africa (Tishkoff et

al. 2007). Based on human genomic studies, it is estimated that the mutation for the

lactase persistence trait occurred around 2,000 to 20,000 years ago for European

populations (Tishkoff et al. 2007). For African populations, the lactase persistence

trait emerged approximately 1,200 to 23,000 years ago (Gerbault et al. 2011). This

begs the question: Is this mutation the same for both populations? It appears that

the emergence of the lactase persistence mutation in non-European populations,

speci�cally those in East Africa (e.g., Tutsi and Fulani), is a case of

convergent evolution. With convergent evolution events, a similar mutation may

occur in species of different lineages through independent evolutionary processes.

Based on our current understanding of the genetic mutation pathways for the lac-

tase persistence trait in European and African populations, these mutations are not

representative of a shared lineage. In other words, just because a person of Euro-

pean origin and a person of African origin can each digest milk due to the presence

of the lactase-persistence trait in their genotypes, it does not mean that these two

individuals inherited it due to shared common ancestry.

Is it possible that the convergent evolution of similar lactase-persistence traits in

disparate populations is merely a product of genetic drift? Or is there evidence for

natural selection? Even though 23,000 years may seem like a long time, it is but a

blink of the proverbial evolutionary eye. From the perspective of human evolution-

ary pathways, mutations related to the LCT gene have occurred relatively recently. Previous: Race and Human Variation

Next: Bioarchaeology and Forensic Anthropology

Similar genetic changes in multiple populations through genetic drift processes,

which are relatively slow and directionless, fail to accumulate as rapidly as have lac-

tase-persistence traits (Gerbault et al. 2011). The widespread accumulation of these

traits in a relatively short period of time supports the notion that an underlying se-

lective pressure must be driving this form of human evolution. Although to date no

de�nitive factors have been �rmly identi�ed, it is thought that environmental pres-

sures are likely to credit for the rapid accumulation of the lactase-persistence trait

in multiple human populations through convergent evolutionary pathways.

Human Variation: Our Story Continues

From the time that the �rst of our species left Africa, we have had to adjust and

adapt to numerous environmental challenges. The remarkable ability of human be-

ings to maintain homeostasis through a combination of both nongenetic (adjust-

ments) and genetic (adaptations) means has allowed us to occupy a remarkable vari-

ety of environments from high-altitude mountainous regions to the tropics near the

equator. From adding piquant, pungent spices to our foods as a means of inhibiting

food-borne illnesses due to bacterial growth to donning garments specially suited to

local climates, behavioral adjustments have provided us with a nongenetic means of

coping with obstacles to our health and well-being. Acclimatory adjustments, such

as sweating when we are warm in an attempt to regulate our body temperature or

experiencing increased breathing rates as a means of increasing blood oxygen levels

in regions where the partial pressure of oxygen is low, have been instrumental in our

survival with respect to thermal and altitudinal environmental challenges. For some

individuals, developmental adjustments that were acquired during their develop-

ment and growth phases (e.g., increased heart and lung capacities for individuals

from high-altitude regions) provide them with a form of physiological advantage not

possible for someone who ventures to such an environmentally challenging region

as an adult. Genetically-mediated adaptations, such as variations in the pigmenta-

tion of our skin, have ensured our evolutionary �tness across all latitudes.

Will the human species continue to adjust and adapt to new environmental chal-

lenges in the future? If past performance is any measure of future expectations, then

the human story will continue as long as we do not alter our environment to the

Previous: Race and Human Variation

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point that the plasticity of our behavior, physiological, and morphological bound-

aries is exceeded. In the following chapters, you will explore additional information

about our saga as a species. From the concept of race as a sociocultural construct to

our epidemiological history, the nuances of evolutionary-based human variation are

always present and provide the basis for understanding our history and our future

as a species.

Review �uestions

Detail at least two examples of how natural selection has in�u-

enced human variation. Speci�cally, what was the selective pres-

sure that may have led to a preference for a speci�c trait and how

is that trait related to an increased level of �tness?

Why is reduced pigmentation of the skin advantageous for individ-

uals from northern latitudes? What role does darker skin pigmen-

tation serve for individuals near the equator? What is the relation-

ship between skin pigmentation and �tness?

What are some of the risks associated with pregnancy at high alti-

tude? Compare and contrast the various genetic mutations of the

indigenous Tibetan, Andean, and Ethiopian high-altitude popula-

tions. In your answer, speci�cally address the issue of pregnancy at

high altitudes.

What is the relationship between the sickle cell mutation and the

Plasmodium parasite? Would having the HbSA genotype still be ad-

vantageous in a region where such parasites are not common? Why

or why not?

Key Terms Previous: Race and Human Variation

Next: Bioarchaeology and Forensic Anthropology

Acclimatory adjustments: Processes by which an individual organism adjusts to

maintain homeostasis in response to environmental challenges.

Activated melanogenesis: Increase in melanin production in response to ultraviolet

radiation (UV) exposure.

Adaptation: alteration in population-level gene frequencies related to environmen-

tally induced selective pressures; leads to a greater level of �tness for a population

related to a speci�c environment.

Adjustment: Non-genetic-based ways in which organisms adjust to environmental

stressors.

Allen’s Rule: Due to thermal adaptation, homeothermic animals have body volume-

to-surface ratios that vary inversely with the average temperature of their environ-

ment. In cold climates, the anticipated ratio is high and it is low in warm climates.

Basal melanogenesis: Genetically-mediated, non-environmentally in�uenced base

melanin level.

Behavioral adjustments: An individual’s culturally mediated responses to an envi-

ronmental stressor in an effort to maintain homeostasis.

Bergmann’s Rule: For a broadly distributed monophyletic group, species and popu-

lations of smaller size tend to be found in environments with warmer climates and

those of larger size tend to be found in ones that are colder.

Cline: A continuum of gradations (i.e., degrees or levels) of a speci�c trait.

Conduction: Mechanism of heat transfer between objects through direct contact.

Convection: Movement of heat away from a warm object to the cooler surrounding

�uid (i.e., gas or liquid).

Convergent evolution: Evolutionary process where organisms that are not closely

related independently evolve similar traits as a product of adaptation to similar evo-

lutionary parameters.

Erythrocyte: Red blood cell; most common form of blood cell—principle means of

transporting oxygen throughout the circulatory system.

Previous: Race and Human Variation

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Evaporation: Mechanism of heat transfer where liquid is transformed into a gas uti-

lizing energy (e.g., heat).

Folic acid: Form of B complex vitamin necessary for proper fetal development.

Gloger’s Rule: For mammals of the same species, those with more darkly pigmented

forms tend to be found closer to the equator and those with lighter forms are found

in regions further from the equator.

Hematocrit: Volume percentage of red blood cells within the blood.

Homeostasis: Condition of optimal functioning for an organism.

Hyperpnea: Increased depth and rate of respiration.

Hypothalamus: Small portion of the human brain responsible for body temperature

regulation.

Lactase persistence: Genetic mutation permitting the continued production of lac-

tase-phlorizin hydrolase enzyme in the small intestine past the weaning period.

Melanin: Black-brown pigment produced by melanocytes; one of the primary pig-

ments in skin.

Melanocytes: Specialized cells that produce melanin.

Phenotypic plasticity: Ability of one genotype to produce more than one phenotype

dependent on environmental conditions.

Polyphenism: Multiple discrete phenotypes from an organism’s genotype in re-

sponse to the environment; a special form of phenotypic plasticity.

Radiation: Mechanism of heat transfer involving electromagnetic energy being

emitted from an object.

Sickle cell disease: A group of genetically inherited blood disorders characterized by

an abnormality in the shape of the hemoglobin within erythrocytes (red blood cells).

Stressor: Any stimulus resulting in an imbalance in an organism’s homeostatic

balance. Previous: Race and Human Variation

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Leslie Fitzpatrick

Vasoconstriction: Narrowing of the blood vessels due to contractions of the muscu-

lar vessel walls.

Vasodilation: Dilation of the blood vessels due to relaxation of the muscular vessel

walls.

About the Author

Leslie Fitzpatrick, Ph.D.

Mercyhurst University, L�[email protected]

Leslie Fitzpatrick is an Assistant Professor in the

Department of Biological Anthropology and Ap-

plied Forensic Sciences at Mercyhurst University

in Erie, Pennsylvania. She earned a Ph.D. in An-

thropology from the University of Wyoming

(2017), an M.A. in Anthropology from Georgia

State (2012), and a B.S. in Mechanical Engineer-

ing from Georgia Tech (2000). Her primary re-

search focus is the stable-isotope analysis of hu-

man remains as a means of interpreting past mobility and diet pro�les for both

modern and archaeological populations. In addition to her work in the classroom

and laboratory, she has worked as a bioarchaeologist at �eld sites in Germany, Spain,

Croatia, Mexico, Peru, and throughout the United States.

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Next: Bioarchaeology and Forensic Anthropology

Figure Attributions

Figure 14.1 Deep water diver by Leslie E. Fitzpatrick is under a CC BY-NC 4.0 License.

Figure 14.2 Mechanisms of heat transfer original to Explorations: An Open Invitation

to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 14.3 Body heat maintenance in cold and warm original to Explorations: An

Open Invitation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0

License.

Figure 14.4 Vasoconstriction and vasodilation original to Explorations: An Open Invi-

tation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 14.5 Atmospheric pressure original to Explorations: An Open Invitation to Bi-

ological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 14.6 Premature infant by Leslie E. Fitzpatrick is under a CC BY-NC 4.0

License.

Figure 14.7 Quechua Woman in Peru by Alexander Fiebrandt (Alecconnell at

de.wikipedia) is under a CC BY-SA 2.0 DE License.

Figure 14.8 Плагиоцефалия (Plagiocephaly) by Medical advises at http://larece.ru/?

p=27115 is under a CC BY-SA 3.0 License.

Figure 14.9 World Map of HVR adaptation in high altitude populations by Chkuu is

used under a CC BY-SA 4.0 License.

Figure 14.10 Skin Pigmentation (Anatomy and Physiology, Figure 5.8) by OpenStax is

used under a CC BY 4.0 License.

Figure 14.11 Penetration of skin layers by UVA and UVB rays a derivative work original

to Explorations: An Open Invitation to Biological Anthropology by Katie Nelson is

under a CC BY-NC 4.0 License. [Includes Skin Anatomy by NIH National Cancer In-

stitute, public domain].

Figure 14.12 Evolutionary basis for human skin color variation original to Explo- Previous: Race and Human Variation

Next: Bioarchaeology and Forensic Anthropology

rations: An Open Invitation to Biological Anthropology by Katie Nelson is under a CC

BY-NC 4.0 License.

Figure 14.13 Rachitis, stages of development for children (slide numbers 7181 and

7182; photo number: M0003399) by Wellcome Collection is under a CC BY 4.0

license.

Figure 14.14 Eskimo Family NGM-v31-p564 by George R. King [original from National

Geographic Magazine, Volume 31 (1917)] is in the public domain.

Figure 14.15a COLLECTIE TROPENMUSEUM Fulani vrouwen en kinderen putten wa-

ter uit de waterput van Santaba TMnr 20010199 (Fulani women and children draw

water from the Santaba water well) by Tropenmuseum, part of the National Museum

of World Cultures is used under a CC BY-SA 3.0 License.

Figure 14.15b Greenland 1999 (01) by vadeve has been designated to the public do-

main (CC0).

Figure 14.16a Bergmann’s Rule original to Explorations: An Open Invitation to Biolog-

ical Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 14.16b Allen’s Rule original to Explorations: An Open Invitation to Biological

Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 14.17 Human nasal morphological variation original to Explorations: An Open

Invitation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 Li-

cense.

Figure 14.18 Malaria parasite life cycle-NIAID by NIH National Institute of Allergy and

Infectious Diseases is in the public domain.

Figure 14.19 Sickle cell 01 by The National Heart, Lung, and Blood Institute (NHLBI) is

in the public domain.

Figure 14.20 Red Blood Cell abnormalities by Armando Moreno Vranich has been

designated to the public domain (CC0).

Figure 14.21 Sickle cell disease a derivative work original to Explorations: An Open

Invitation to Biological Anthropology by Katie Nelson is under a CC BY-NC 4.0 Li- Previous: Race and Human Variation

Next: Bioarchaeology and Forensic Anthropology

LICENSE

Explorations by Beth Shook, Katie

Nelson, Kelsie Aguilera, and Lara Braff,

Eds. is licensed under a Creative

Commons Attribution-NonCommercial

4.0 International License, except where

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Figure 14.22 Lactose tolerance in the Old World by Joe Roe is used under a CC BY 4.0

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