1. Discovering Physical Anthropology
Lecture Slides
FOURTH EDITION
Discovering Physical Anthropology
Clark Spencer Larsen

2. Chapter 5
Biology in the Present: Living People

3. BIOLOGY in the Present: Living People
Questions addressed in this chapter:
Is race a valid, biologically meaningful concept?
What do growth and development tell us about human variation?
What are the benefits of our life history pattern?
How do people adapt to their environmental extremes and other circumstances?
It is tempting to take what we have learned from the last chapter and conclude that different human populations have been isolated for a long period of time and have developed a unique set of adaptations to survive in their particular environments. It is also tempting perhaps to simplify matters by grouping these people into categories and calling those categories “races.” In fact, people have been classifying themselves, and others, into different races for a long time. As you’ll see in this chapter, “races” are in fact biologically meaningless categories that do little to explain the variation we see in humans today. We’ll examine this question of race. In addition, we’ll look at how humans adapt through their lifetime—a subject termed “human growth and development.” Additionally, we’ll take a trip around the world and look at how different humans have adapted biologically to their environments. Humans are remarkably adaptable, living in extreme temperatures and altitudes. In this chapter, we’ll examine how humans are able to do this biologically. First, let’s deconstruct the concept of race.

4. Debunking the Race Concept: Blumenbach and Boas
Divided humans into 5 racial categories.
Boas
Found that skull shape widely varied and was not fixed as Blumenbach had thought.
For a long time, it was thought that all humans could be categorized into different racial categories. When early European explorers, such as Christopher Columbus, discovered people of different lands, they emphasized the physical differences rather than the similarities. Even anthropologists engaged in this activity, seeking to find the biological and skeletal patterns that distinguish the different races of humans. At the right of this slide is one such image from the 18th-century German anatomist Johann Friedrich Blumenbach, who studied human skulls and categorized them into five races: Mongoloids, Malays, Ethiopians (Africans), American Indians, and Caucasoids. (Only four are illustrated here.) Critical to this way of thinking was the idea that these races were fixed. But these ideas were flat-out wrong.

5. Debunking the Race Concept: Geneticist R. C. Lewontin
Biological/genetic variation does not follow racial categorization.
There is more human variation within a population than between populations.
The great anthropologist Franz Boas tested the idea that races were static by studying the skull shapes of American immigrants whose parents were born in Europe. He found that skull shape could vary; it was not fixed as would be predicted by typological classification schemes predicted by Blumenbach. In the 1970s, the geneticist R. C. Lewontin moved beyond skull shapes to DNA. He surmised that if races were real groupings, with biological foundations, then blood types, serum proteins, and enzyme variants should all cluster according to racial categories. They do not. Given these data, how should we think about human diversity?

6. How Do We Account for Human Diversity?
Geographic Clines
Human diversity is biologically meaningful when in the context of gradual change or on a continuum when observing phenotypic characteristics from one population to the next.
Personal genomics
But all individuals possess biological variations of their own, making it impossible to place individuals into “races.”
How should we think about human diversity? One way that does seem to be biologically meaningful is the idea of a cline, or a gradual change in a phenotypic character from one population to the next. Blood types seem to follow this pattern, as does skin coloration. Interestingly enough, not all phenotypes follow the same patterns, meaning that racial categories can work only if just a few phenotypes are considered (like skin coloration, for instance). But by doing this, we ignore the other phenotypes that do not cluster in the same way. We’ll return to variation in skin coloration later in this lecture. Personal genomics is the branch of genomics focused on sequencing individual genomes. This is useful in understanding the complexities of an individual’s unique genetic signature.

7. How Do We Account for Human Diversity?
Microsatellite analysis
Can identify patterns of genetic variation within and between traditional ethnic groups.
However, these patterns represent the result of social histories and migration patterns, not biological categories.
How should we think about human diversity? Personal genomics is the branch of genomics focused on sequencing individual genomes. This is useful in understanding the complexities of an individual’s unique genetic signature. Microsatellites in DNA can be used to identify patterns of genetic variation within and between traditional ethnic groups. While these patterns do not represent biological categories, they do represent the result of human social histories and migration patterns. Understanding these geographical-associated microsatellite variations can assist the medical community in understanding genetic markers linked to specific diseases or other health risks.

8. Life History: Growth and Development
Fertilization
Prenatal stage
3 trimesters; ends 40 weeks after conception
Postnatal stage
Neonatal (month 1)
Infancy (month 2–end of lactation)
Childhood (3–7 years)
Juvenile (7–10 years girls; 7–12 years boys)
Puberty (days or weeks)
Adolescence (5–10 years post-puberty)
Adult stage
Reproductive period
Senescence
The last few chapters may lead us to think that a living organism is built quite mechanically from the DNA blueprint, but it does not work quite this way. Instead, DNA is like a recipe. And like a recipe, many external factors, like the oven temperature, for instance, or the erroneous addition of an ingredient not in the recipe, can alter the final outcome. So how does the human organism achieve its final outcome? Through stages of growth and development that, while underlain by genetic control, are highly influenced by the environment. This is called the “life history” of an individual. The life history of a human, which is essentially the timing of all the major developmental events in life, can be broken down into the stages listed on this slide. First, fertilization occurs. You started out as a single egg fertilized by a single sperm. Following this event is the prenatal stage, which consists of the 40 weeks of gestation, often divided up into 3 trimesters of pregnancy. After fertilization, the postnatal stage consists of all the stages that happen when you are a kid. There is the first month after birth, called the neonatal stage. Then comes infancy, which is month 2 up until the child stops breastfeeding. This happens in human hunter-gatherers at around the age of 2.5 or 3. Childhood follows and ends around the age of 7, when the juvenile period begins, and lasts until puberty. The post-puberty, but pre-adult, stage is called adolescence. These are the “teen” years. Adulthood consists of the period from about 20 years old until death. This can be divided into two stages: a reproductive period, and a period of senescence, which follows the childbearing years.

9. Prenatal Health
Stressors in the uterine environment such as smoking, alcohol, drugs, and inadequate nutrition can affect an individual’s health later in life.
Stressors in the uterine environment can affect adult health. The first trimester is a particularly sensitive time for the developing embryo—the fertilized egg goes from one cell to millions, and the different organ systems begin to develop. Stressors in the intrauterine environment—such as drugs or poor nutrition—can permanently and adversely affect development at this time. In fact, these stressors have been shown to increase the likelihood of a premature birth or a stillborn infant. The second and third trimesters are primarily about growth and organ maturation. However, malnourished babies who are born with a very low birth weight for their gestational age are at a greater risk for high blood pressure, high cholesterol levels, and coronary heart disease as adults. It is thought that the uterine environment can alter gene expression (called epigenetics) and can predispose an individual for diseases that happen much later in life. This is called the fetal origins hypothesis and was first proposed by Dr. David Barker, a British cardiovascular physician and professor.

10. Life History: Growth and Development
The human body grows at different rates. The brain grows the fastest, reaching full development around the age of six. As seen in the chart, the dentition closely follows in growth rate. The reproductive system does not begin to develop at a substantial rate until the onset of puberty, which can vary by age depending on the population, but usually reaches completion around the age of for girls.

11. Life History: Growth and DevelopmentPostnatal Stage
Growth velocity – rate of growth per year
Infancy
Period of the most rapid growth.
Deciduous dentition – eruption begins shortly after birth.
Weaning varies, but generally the infant has all 20 deciduous teeth by this time.
Motor skills – walking, running, etc. develop during the first 2 years.
Obviously, the prenatal stage of life matters quite a bit, but plenty of important events that shape the biology of an individual occur during the postnatal stage as well. Each of the postnatal periods (neonatal, infancy, childhood, juvenile, adolescent) show a different growth velocity. Not only does growth change from period to period, but other anatomical changes occur as well. For instance, during infancy, the deciduous (baby) teeth erupt. This happens at a predictable rate until weaning time, when the infant no longer relies on his or her mother’s breast milk, and all 20 deciduous teeth have erupted.

12. Life History: Growth and DevelopmentPostnatal Stage
Cognitive abilities; brain growth
Brain grows very rapidly.
Puts pressure on the mother to supply the energy necessary for this growth.
Even after weaning, there is a need for steady, energetically expensive growth of brain tissue into childhood.
Obviously, the prenatal stage of life matters quite a bit, but plenty of important events that shape the biology of an individual occur during the postnatal stage as well. Each of the postnatal periods (neonatal, infancy, childhood, juvenile, adolescent) shows a different growth velocity. During infancy, both motor skills and cognitive skills are acquired rapidly, so that by the time childhood begins, a three-year-old can walk and talk. Throughout infancy, the brain grows at a very fast pace. This growth puts a very large energetic burden on the mother, who supplies the energy for brain growth in her infant with milk through lactation. Even after weaning, this steady and energetically expensive growth of brain tissue continues into childhood. It is no accident that during childhood, as the brain is growing and maturing, important skills are learned. By the age of about six, the brain is fully grown in volume, which is why children have very large heads for their body size. Brain maturation continues well past adolescence and into early adulthood. Also starting around the age of six, permanent teeth begin to erupt and replace the deciduous teeth. Notice that in the early teen years, brain and tooth growth have ceased, but body and reproductive growth begin to pick up the pace. This is the onset of puberty, which causes breast development and menstruation (menarche) in girls and deepening of the voice, among other changes, in boys. There are also developmental changes to the reproductive organs and genitals. The growth spurt that occurs at this time is, among primates, unique to humans.

13. Life History: Growth and DevelopmentBone Growth
Growth occurs because of bone growth occurring at the growth plates.
Epiphysis – the end of bones
Diaphysis – main bone shaft
This happens at varying ages on many bones throughout the body.
Growth can occur because the bones themselves are growing. This happens at growth plates, the regions of cartilage separating the ends of the bones (epiphyses) and the bone shafts (diaphyses). In the image, look for the growth plate. It appears as a distinctive white band in the MRI to the left, and as a “crack” in the dry bones to the right. Once the cartilage in the growth plates stops dividing, the epiphysis fuses to the diaphysis, the growth plate vanishes, and growth stops. This happens typically around the start of what is called adulthood.

14. Life History: Secular Trend
How does growth change over time?
Genes largely control how and when long bones start to grow and continue to grow.
Environment also influences long-bone growth.
Poor nutrition can stunt growth by limiting the production of cartilage in the growth plate.
To a large extent, genes control how long growth continues and how tall an individual can become. However, the environment also has a strong influence on long-bone growth. In some populations, poor nutrition can stunt growth by limiting the growth of the cartilage and by prematurely fusing the epiphysis to the diaphysis. As environmental and nutritional conditions improved, the average height increased quite dramatically. This is what is called a secular trend.

15. Life History: Secular Trend
Examine the graph at the top of this slide as an example. These are data showing the average height of American-born males of European descent from 1710 to Notice that around 1830, heights began to plummet. Is this because shorter individuals survived better, or reproduced more often? Probably not. Instead, the urbanization of America, the crowded cities, the poor nutrition, and the spread of disease led to the attenuated growth seen in this graph. However, as environmental and nutritional conditions improved, the average height increased quite dramatically. Many factors have contributed to this particular secular trend—improved nutrition and the elimination of many diseases the most notable. But this secular trend has not happened everywhere. The bottom graph shows the growth curves of kids in the Amazon River basin in Brazil. Here, nutrition remains suboptimal, and it has resulted in a lower slope to the growth curve, and ultimately to individuals shorter than 95% of American-born children.

16. Life History: Aging and Senescence
Homeostatis – internal equilibrium.
Senescence – biological aging.
Body’s ability to regulate bone mass begins to break down causing bone loss.
Bone becomes more porous and susceptible to breaks (osteoporosis).
The final stage of life: adulthood. Growth and development have ceased for the most part, though the body’s tissues continue to be replaced through basic maintenance. But over time, the body begins to break down, and it is not as good at keeping itself in homeostasis—internal equilibrium. Biological aging—also called senescence—begins to occur. For example, the body’s ability to regulate bone mass begins to break down, and there can be a net loss of bone. Bone can become porous and more susceptible to breaks. This is known as osteoporosis. Notice in the graph at the bottom that women suffer from osteoporosis more than men, in part because of the important role that estrogen plays in bone growth. The decrease in estrogen later in a woman’s life can result in a loss of bone, shown as the gray areas in the top right X-ray of a female pelvis.

17. Life History: Aging and Senescence
The “Grandmother Effect”
If women stop reproducing in their 50s, then evolutionarily speaking, why do they live decades longer?
One theory is that grandmothers can assist in raising grandchildren and ultimately increase their own fitness in doing so.
Another aspect of female senescence related to the drop in estrogen levels is menopause, which occurs when a woman, usually in her 50s, stops producing eggs and stops menstruating. Menopause signals the end of a woman’s reproductive life. Men can continue to produce sperm into their later years, though the sperm lose motility as men age. There is an important evolutionary question to ask here. If women stop reproducing in their 50s, why do they continue to live another 20, 30, sometimes 40 years? The answer may be something called the “grandmother effect.” This postmenopausal period has been selected for in humans. Grandmothers can help out new parents and can assist with the raising of grandchildren, thus increasing their own fitness (indirectly). Certainly, their vast knowledge and a means to communicate it (through language) may have been the impetus for selection favoring longevity in ancestral humans.

18. Adaptation: Meeting the Challenges of Living
Four levels of adaptation:
Genetic
Developmental (Ontogenetic)
Acclimatization (Physiological)
Cultural (Behavioral)
Functional adaptations: adaptations that occur during an individual’s lifetime (acclimatization and developmental) to increase fitness in a given environment.
All living organisms can adjust to new conditions and challenges, but perhaps no living organism can do this better than humans. These responses within an environmental context are called adaptation and can happen at four different levels. The first is genetic, and is the basis of natural selection. Genetic adaptations are not reversible; they happen because natural selection has favored the phenotypic products of certain genetic variants. But adaptation is not all genetic. The adaptations that occur during the growth and development of a child are called developmental, or ontogenetic. For instance, children living at high altitudes develop a larger chest girth and larger lung capacity than if they had been raised at low altitude. The genetic adaptation would simply be the capacity to change chest size depending on the environment, but the actual adaptation (increased lung volume) happens developmentally. Acclimatization or physiological adaptation also occurs at the level of the individual but can happen at any time in an individual’s life. A tan, for instance, is a physiological adaptation to increased sun exposure. Finally, the use of material culture to adapt to our environment is particularly prominent in humans. For instance, our ability to make and wear clothing is a cultural adaptation that allows us to stay warm in climates not otherwise inhabitable by a primate. The middle two types of adaptations, developmental and acclimatization, which occur over an individual’s lifetime, have been termed functional. All of these adaptations can help an individual maintain the body’s normal functioning, thereby increasing survival and reproductive opportunities. In other words, these adaptations all increase an individual’s fitness.

19. Climate Adaptation: Heat Stress
How do we adapt to heat?
Vasodilation
Sweating and hairlessness
Body shape: Bergmann’s and Allen’s rules
Humans are primates, and primates live along a narrow strip of mostly forested land in the tropics of the Americas, Africa, and Asia. However, humans have expanded their territory and now live in extreme environments, from the Sahara Desert to the Arctic Circle. We’ve been able to do this because of all four of the adaptations discussed in the previous slide: genetic, developmental, acclimatization, and cultural innovation. Still, extreme heat and extreme cold can certainly kill a person. So how does the human body deal with heat stress? One physiological response is vasodilation, in which the blood vessels on the extremity of the body enlarge. This permits more blood, and therefore more heat, to move to the perimeter of the body, where it can dissipate into the surrounding air, thus decreasing the core body temperature of the person. This process is enhanced by sweating, in which heat is removed from the body through evaporation of water on the skin’s surface. But sweating does not reduce body temperature nearly as effectively in areas covered with hair. Thus, the hairless body—rather unique to humans—may also have evolved to help keep the body cool in hot environments. In addition to these physiological responses, there are genetic adaptations to heat. Human populations living in hot environments tend to have two adaptations related to body shape. First, heat-adapted individuals tend to be quite thin, whereas cold-adapted individuals tend to be wider, a body shape that helps retain body heat. This is known as Bergmann’s rule, and is graphically illustrated at the bottom of this slide. Additionally, Allen’s rule describes the observation that heat-adapted individuals tend to have longer limbs, which increases the surface area over which heat can be lost. Cold-adapted individuals, therefore, have shorter limbs and a stockier build. These body shapes appear to be genetic adaptations, helping populations adapt to their local climates by means of natural selection.

20. Climate Adaptation: Cold Stress
How do we adapt to cold?
Vasoconstriction
Shivering
Elevated BMR
Clothing and shelter
Extreme cold is as challenging to the human body as extreme heat. The body needs to maintain an internal body temperature of around 98.6 degrees Fahrenheit. In cold environments, we can easily lose body heat through our extremities, leading to hypothermia, or low body temperature. If the drop in body temperature is severe enough, a person can suffer permanent injury, or even die. Adaptations to cold include cultural innovations, in which traditional societies living in cold environments wear certain clothing, or build shelters to protect them from the cold. Physiologically, the body can adapt by narrowing the blood vessels, a process called vasoconstriction, which reduces the flow of blood away from the body’s core. Shivering is a physiological mechanism for increasing internal body temperature by generating heat through rapid muscle contraction. After just a few days in cold temperatures, the body acclimates, and does not shiver as much, even if the temperature remains low. Internal body temperature can also be elevated by raising the basal metabolic rate (BMR), or the rate at which fuel (in the form of food) is metabolized, or “burned.” Populations with a higher BMR tend to live in cold environments. Following Bergmann’s and Allen’s rules, humans living in arctic conditions tend to have large, wide bodies and short limbs to reduce the surface area over which heat loss can occur.

21. Climate Adaptation: Solar Radiation and Skin Coloration
Nina Jablonski and George Chaplin – skin reflectance
The darkest skin (low skin reflectance) is associated with higher levels of UV radiation.
The lightest skin (high skin reflectance) is associated with lower levels of UV radiation.
Humans display remarkable variation in skin coloration. Unfortunately, this human polymorphism has been used in racist thinking for centuries. Why do humans have different skin colors? And how does this occur? Certainly, skin coloration is a flexible characteristic that can change over the course of an individual’s life, or even from one season to the next. Your skin is quite complex; it is composed of a layer of cells called the epidermis and a deeper level called the dermis. At the epidermis/dermis junction are cells called melanocytes, which produce a pigment called melanin. Those with higher melanin production have darker skin. Exposure to the sun can lead the melanocytes to produce more granules of melanin. This is called a tan. It is clear, therefore, that the concentration of melanin and melanocytes, making the skin darker, is a protective agent against the sun. It is also clear that human skin coloration varies according to the intensity of ultraviolet radiation from the sun. Dark-pigmented individuals tend to live close to the equator, while the lightest-skinned individuals live in the higher latitudes, where the UV rays from the sun are considerably less intense. But why? The anthropologists Nina Jablonski and George Chaplin have developed an elegant explanation for the evolution of skin coloration.

22. Climate Adaptation: Solar Radiation and Skin Coloration
Dark-skinned individuals near the equator get enough rays to produce ample vitamin D, while protecting their folic acid with high concentrations of melanin. Light-skinned individuals far from the equator are not exposed to UV radiation intense enough to threaten their folic acid, and have light skin so that vitamin D can be produced. This creates a balance between skin color, solar radiation, vitamin D, and folic acid.

23. Climate Adaptation: Skin Coloration and UV Radiation
UV radiation helps synthesize vitamin D
Necessary for proper skeletal development.
Lack of active vitamin D can lead to rickets in children and osteomalacia in adults.
UV radiation depletes folic acid
Necessary for DNA synthesis and spinal development.
While UV radiation is necessary for vitamin D production, it can destroy folate (folic acid), which has been shown to be absolutely critical to proper synthesis and repair of DNA. Additionally, those with folate deficiencies are at a greater risk of having children with neural-tube defects, like spina bifida. Light-skinned individuals living in areas with intense UV radiation will make plenty of vitamin D, but they are more likely to have depleted folic acid stores and are at a greater risk of having children with severe birth defects. As you can see, there is a tug-of-war going on here between vitamin D, which needs sunlight, and folic acid, which needs protection from sunlight. The balance reached by natural selection is the gradient of skin coloration we see in humans across the planet. Dark-skinned individuals near the equator get enough rays to produce ample vitamin D, while protecting their folic acid with high concentrations of melanin. Light-skinned individuals far from the equator are not exposed to UV radiation intense enough to threaten their folic acid, and have light skin so that vitamin D can be produced. These are the extremes, and every possible shade between the lightest light and the darkest dark can be found on our planet, corresponding to the intensity of UV exposure from the sun. Of course, with global travel, we now have humans of all different skin colors living in all different places. Our foods are fortified with vitamin D, and pregnant women can take folic acid supplements. Our culture has allowed us to adapt to our new standing as a species that can migrate from one side of the planet to the other in hours instead of millennia.

24. Climate Adaptation: High Altitudes
Humans living in high altitudes face a lack of oxygen, which can lead to hypoxia if they are not adapted to the environment.
Example – Populations in Peruvian Andes have wider chests to accommodate an increase in lung volume.
Humans living at high altitudes have yet another challenge: the lack of oxygen. An insufficient amount of oxygen is call hypoxia, and humans can easily get “altitude sickness” if they are not acclimated to such conditions. High-altitude environments also tend to be quite cold and dry, with poor food quality. Some of these areas are shown on this map (in red). They include the Andes mountain chain along the west coast of South America, and the populations living in the Himalayas in Asia. People who travel to high altitudes can acclimate as their body adapts by producing more oxygen-carrying red blood cells. But long-term, developmental, and even genetic adaptations can be found in mountain populations. Children who grow and develop in mountainous regions grow larger lungs than they would have had they grown up in an area closer to sea level. Populations in the Peruvian Andes have wide chests because of their increased lung volume. They also tend to be quite small, in part because of the poor nutrition in some of these regions. Women in the Tibetan highlands, shown in the bottom image, have more surviving children if they carry an allele for a better oxygen-binding hemoglobin molecule. Research on this aspect of Tibetan biology demonstrates that high-altitude populations adapt not only developmentally but genetically through natural selection.

25. Nutritional Adaptation
Basal metabolic requirement – the minimum amount of energy an individual requires to survive, grow, and reproduce.
Total daily energy expenditure – the total amount of energy an individual requires, given his or her activity level.
Of course, humans are not subject only to the external pressures of heat, cold, altitude, and UV radiation. We are also, as the saying goes, what we eat. Humans are remarkably diverse in terms of the kinds and amounts of food we eat, and our bodies adapt to food via the same mechanisms already described for climate adaptations. In some places, scorpions are a delicacy. This may sound repulsive to some, but is it much different from a New Englander enjoying another multilegged, clawed creature—namely, a lobster? In fact, diet is one of the best ways in which to understand the complex relationship between our biology and our culture. Ultimately, food is fuel—the energy we require to run our bodies, to survive, and to reproduce. The minimum amount of energy we require to survive is called the basal metabolic requirement. But just surviving isn’t enough. We need additional energy to grow and reproduce, work and exercise. The total amount of energy we require, given our activity level, is the total daily energy expenditure.

26. Nutritional Adaptation
What if we do not ingest all that is recommended?
Diet requires macro- and micronutrients.
Macronutrients
Carbohydrates, proteins, fats
Micronutrients
Vitamins and minerals
Diet is a complex matter, and we require many macronutrients (carbohydrates, fats, and proteins) in addition to micronutrients (vitamins and minerals). All the different macro- and micronutrients that humans are recommended to ingest in a day can be found in your book, and you should have a look at this exhaustive list. But this raises an interesting question: What if we do not ingest all that is recommended?

27. Nutritional Adaptation: Malnutrition
Malnutrition – typically consuming fewer than 2,000 calories per day.
Immune systems are compromised by a lack of proper nutrition.
Poor environments lead to infectious diseases.
Growth, while strongly genetic, is also influenced by nutrition.
Sadly, many humans today are malnourished, consuming fewer than 2,000 calories per day. Efforts to combat global malnutrition have focused on calories, with efforts made to get grains, like rice and corn, to these populations. Although this has certainly helped the global starvation epidemic, a lack of calories is only one form of malnutrition. The lack of essential micronutrients is the other. The immune systems of undernourished populations are compromised by a lack of proper nutrition. In addition, many of these individuals live in poor environments that are rife with infectious diseases. As we might expect, malnourishment also takes its toll on growth. It could be argued that shortness is a genetic adaptation to poor nutrition, with smaller individuals surviving because they require less food. However, the data indicate that growth, while undoubtedly having a genetic component, is strongly influenced by nutrition.

28. Nutritional Adaptation: Overnutrition
Taking in more calories and exercising less – humans, especially in the United States, are facing an obesity epidemic.
In the United States:
50% of adults are considered overweight
20% of children are considered overweight
There is a flip side to the nutrition coin: overnutrition. By taking in more calories and exercising less, humans are now facing an obesity epidemic. Why this is happening is complex, but it appears to boil down to the availability of inexpensive, high-calorie, high-fat food, with fewer and fewer micronutrients. One by-product of overnutrition is hypercholesterolemia, or high cholesterol, which is a severe risk factor for heart disease. It is a mistake to think that our ancestors did not eat meat as we do today—there is ample evidence that they did. However, we now eat meats, in addition to vegetable fats and oils, that have a very high fat content compared to nondomesticated animals and plants hunted and gathered by our ancestors.

29. Nutritional Adaptation: Overnutrition
Consequences…
High blood pressure
High cholesterol
Type 2 diabetes
As much as 50% of adults in the United States are considered overweight (a BMI above 25 in the graph shown); 20% of children are obese (a BMI over 30). A major problem associated with overnutrition is the spike in type 2 diabetes cases—100 million worldwide. The high-fat, high-sugar diets we consume cause our pancreas to secrete an abnormal amount of insulin. Our tissues—like muscle and the liver—eventually become nonresponsive to this insulin, and these tissues cannot store and access the glucose they need to function properly. The excess glucose is stored as fat, or it remains in the blood, where it raises its viscosity (thickens it). This taxes the blood vessels, and organs like the kidneys. The high frequency of type 2 diabetes in some populations can also be attributed to nutritional stress in the womb, leading gestating fetuses to be “programmed” into thinking they are being born into a world of famine, when instead they are born into a world of plenty.

30. Nutritional Adaptation: Obesity Increase
Overnutrition resulting in obesity is not just a challenge faced by Americans. As this graph shows, obesity is on the rise in Brazil, China, India, Mauritius, Russia, and Nauru (an island nation in the South Pacific). Why is this happening? Researchers suspect that because humans were hunter-gatherers for 99% of our existence, we evolved physiological traits that are effective at storing energy during food shortages. When times were good, humans ate and stored as fat what they did not immediately need. During famines, these fat reserves were metabolized. In many cultures today, however, the risk of famine is low, activity levels are on the decline, and there is excessive food in the chain restaurants around every corner. Take, for example, the growth of McDonald’s around the world. With these and other factors—such as fat-promoting chemicals found in plastics and canned foods—becoming global trends, the obesity epidemic remains one of the major health challenges of the 21st century.

31. Skeletal Adaptation
The ability of our bones to change their size and shape to adapt to the forces exerted on them
Bones influenced largely by genetics, but also environmental factors
Osteoblasts – produce more bone to increase the strength of the bone
Osteoclasts – resorb bone that is not used
Let’s look at one more adaptation: the ability of our bones to change their size and shape to adapt to the forces we exert on them. This is called skeletal adaptation. Bones are critically important because they are the anchors for our muscles, which use the bones as leverage to move the body. But bones must be strong enough to withstand high forces. A broken bone in the wrong place, at the wrong time, could be fatal. Bones are under genetic control, but they are also responsive to external forces. Cells called osteoblasts can produce more bone to increase the strength of the bone. Cells called osteoclasts resorb (break down and assimilate the components of) bone that is not being used.

32. Skeletal Adaptation
Wolff’s Law – repetitive action can stimulate osteoblasts; likewise, lack of activity can stimulate osteoclasts that reduce bone density.
The principle that bone changes in response to external forces is called Wolff’s law. Repetitive action can stimulate these osteoblasts to make more and more bone. For instance, certain athletes, such as tennis players, have a thicker upper-arm bone (humerus) in the dominant compared to the nondominant arm. Lack of physical activity causes an imbalance in which the osteoclasts are more active than the osteoblasts, and bone density is reduced. Inactive children tend to have smaller, less developed bones than more physically active ones. Inactive children have bones that are less dense and are therefore susceptible to osteoporosis and fractures later in life.

33. The prime period of adulthood occurs
Concept Quiz
The prime period of adulthood occurs
A. at 7-10 years old.
B. at the completion of brain growth.
C. at 20 years old to the end of the reproductive years.
D. at the end of the reproductive years to death.
Answer: C
Feedback: Refer to the Concept Check for Life History Stages in Humans (now on page 142). The Prime period of Adulthood is described as 20 years to end of reproductive years; stability in physiology, behavior, and cognition; menopause in women commencing at about age 50.

34. A gradual change in phenotype over a geographical area is called a
Concept Quiz
A gradual change in phenotype over a geographical area is called a
A. race.
B. cline.
C. category.
D. group.
Answer: B
Feedback: A cline is a gradual change or continuum of phenotypic traits from one population to the next while race is a socially constructed category with no biological basis.

35. Thinking Anthropologically: Discussion
Read Anthropology Matters: Life in an Obesogenic World. Explain how understanding our evolutionary strategies for acquiring and consuming foods can help us better understand the obesity pandemic, according to anthropologist Leslie Sue Lieberman.

36. Thinking Anthropologically: Discussion
Describe some examples of human cultural adaptations to the environment.

37. Thinking Anthropologically: Discussion
Summarize why “race” is not a useful and meaningful biological concept.

38. Discovering Physical Anthropology
Lecture Slides
FOURTH EDITION
Discovering Physical Anthropology
Clark Spencer Larsen