Anthropology help

*

*

*

*

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 and group these people into categories, and call 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. We’ll examine human growth and development. Additionally, we’ll take a trip around the world and look at how different humans have biologically adapted to their environments. Humans are remarkable adaptable—living in extreme heat and extreme cold, and high altitudes. In this chapter, we’ll examine how humans are able to do this biologically. First, let’s deconstruct the concept of race.

*

For a long time, it was thought that all humans could be categorized into different racial categories. Even anthropologists engaged in this activity, seeking to find the biological and skeletal patterns that distinguish the different races. This slide shows one such image from the eighteenth century German anatomist Johann Friedrich Blumenbach, who studied human skulls and categorized them into five races: Mongoloids, Malays, Ethiopians (Africans), American Indians, and Caucasoids. Critical to this way of thinking was the idea that these races were fixed. But, these ideas were flat-out wrong. 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, and was not fixed as would be predicted by typological classification schemes. In the 1970s, the geneticist R. C. Lewontin moved beyond skull shapes and to DNA. He surmised that if races were real groupings, with biological meaning, then blood types, and serum proteins, and enzyme variants should all cluster according to racial categories. They do not. So, 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 another. Blood types seem to follow this pattern, as does skin coloration. Interestingly though, not all phenotypes follow the same patterns, meaning that racial categories can only work if just a few phenotypes are considered (like skin coloration, for instance). But, by doing this, one ignores the other phenotypes that do not cluster in the same way.

*

The last few chapters may lead one to think that a living organism is built quite mechanically from the DNA blueprint. But, it does not quite work this way. Instead, DNA is like a recipe and like a recipe, many external things, like the oven temperature and too much of one ingredient or another, could 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. The life history of a human, which is essentially the timing of all of the major developmental events in life, can be broken down into the stages listed on this slide. There is the prenatal stage, which consists of the 40 weeks of gestation, often divided up into the three trimesters of pregnancy. Shown here is a developing human. 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, like 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, before birth happens at approximately 40 weeks after conception. The postnatal stage consists of all of 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 two up until the child stops breastfeeding. This happens in human hunter-gatherers at around the age of 2.5 or three. Childhood follows and ends around the age of seven, 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 broken into two stages: a reproductive period, and a period of senescence, which follows the childbearing years.

*

In the last slide, we detailed the prenatal stage, but not the postnatal stage. Each of the postnatal periods (neonatal, infancy, childhood, juvenile, adolescent) show a different growth velocity. This is shown in the graph to the upper left of this slide. Notice that the growth velocity starts quite high at birth, and continues at a high, but decreasing, rate until about the age of two. At that point, growth levels off to between 4–6 cm/year until puberty hits. At puberty, the growth velocity accelerates again—the so-called ‘growth spurt.’ During adolescence, growth continues, albeit at a decreased pace, until adulthood is reach and growth velocity reaches zero. Not only does growth change from period to period, but other anatomical changes occur as well. For instance, during infancy, the deciduous, or baby teeth, erupt as is shown in the upper right image. This happens at a predictable rate until weaning time, when the infant no longer relies on his or her mother’s breastmilk, and all 20 deciduous teeth have erupted. During infancy, both motor skills and cognitive skills are acquired rapidly so that by the time childhood begins, the three-year-old can walk and talk.

*

Through infancy, the brain grows at a very fast pace as can be seen in the top graph. This puts a very large energetic burden on the mother, who sustains this brain growth with high-energy 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, which is why children have very large heads for their body size. But, brain maturation continues well past adolescence and into early adulthood. Also starting around the age of six, permanent teeth begin to erupt, as can be seen in the image on the bottom left, and replace the deciduous teeth. Look again at the upper graph. Notice that in the early teen years, brain and tooth growth has ceased, but body and reproductive growth begin to pick up the pace. This is the onset of puberty, which causes breast development and menstruation (menarch) in girls, and deepening of the voice in boys. Of course, there are also developments to the reproductive organs and genitals as well. Unique to humans, among primates, is the growth that happens at this time, the growth spurt.

*

Growth can occur because the bones themselves are growing. This happens at growth plates, which are regions of cartilage separating the ends of the bones, called epiphyses, and the bone shafts, called diaphyses. In the top 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 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.’

*

Of course, genes impact how long growth continues and how tall an individual can become. However, the environment also has a strong influence. In some populations, poor nutrition can stunt growth by limiting the growth of the cartilage and by prematurely fusing the epiphysis to the diaphysis. Examine the graph at the top of this slide. These are data showing the average height of American-born males of European descent from 1710 to 1970. 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 conditions improved, the average height increased quite dramatically. This is called a secular trend. Many factors have contributed to this, most notable are improved nutrition and the elimination of many diseases. But, this secular trend has not happened everywhere. The bottom graph shows the growth curves of kids in the Brazilian Amazon River basin. Here, nutrition is suboptimal, and it has resulted in a lower slope to the growth curve, and ultimately to shorter individuals.

*

Before we summarize, let’s look at the final stage of life: adulthood. Growth and development have ceased, though the body’s tissues continue to be replaced through basic maintenance. But, over time, the body begins to breakdown and is not as good at keeping itself in homeostasis, or in an internal equilibrium. Biological aging, or 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 top left that women suffer from osteoporosis more than men in part because of the important role that estrogen plays in bone growth. Estrogen decrease later in a woman’s life can result in a loss of bone, such as the gray areas seen on this X-ray of a pelvis. 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 in older men. 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 seems to be something called the “grandmother effect.” This postmenopausal period has been selected for in humans. Grandmothers help out new mothers 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.

*

This is a summary slide of the table on page 104 in your book. You should review it to make sure you understand all of the different stages of human life history and the important events that characterize each stage. But, few of these events will be completely foreign to you. You are, after all, a human and have experienced many of these stages. The trick now is to assign some labels to these stages and to understand how these stages help us understand our biology, our evolutionary history, and how we adapt to changes in our environment. That is the subject of the rest of the chapter—adaptation.

*

Living organisms can adjust to new conditions and challenges, and perhaps no living organism can do this better than humans. These responses within an environmental context, called adaptation, can happen at four different levels. The first is genetic, and is the basis of natural selection. Genetic adaptations are not reversible, and happen because natural selection has favored the phenotypic products of certain genetic variants. We generally call these adaptations. But, adaptation is not all genetic. Some adaptations happen during the growth and development of a child; these are called developmental or ontogenetic adaptations. 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 happens at the level of the individual, but can happen at anytime 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 otherwise not inhabitable by a primate. The middle two types of adaptations, developmental and acclimatization, which occur over an individual’s lifetime, have been termed functional adaptations. All of these adaptations help the organism maintain normal functioning of the body, thereby increasing survival and reproductive opportunities. In other words, these adaptations all increase an individual’s fitness.

*

Humans are primates, and primates live along a narrow strip of mostly forested land in the tropics of the Americas, Europe, and Asia. However, humans have expanded their territory and now live in extreme environments, from the Saharan 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 moves more blood, and more heat, 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 also genetic adaptations to heat as well. 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, stockier limbs. These body shapes appear to be genetic adaptations, helping populations adapt to their local climates by means of natural selection.

*

As challenging to the human body is extreme cold. The body needs to maintain an internal body temperature around 98 degrees and can easily lose body heat through the extremities, leading to hypothermia, or low body temperature. If the drop in body temperature is severe enough, the individual can be permanently impacted, or can even die. Adaptations include cultural ones, in which societies living in cold environments wear certain clothing, or build shelters to protect them from the cold, like the shelter shown here in an old picture from Siberia. Physiologically, the body can adapt by vasocontricting blood vessels, reducing the flow of blood away from the body’s core. Shivering also 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 a higher basal metabolic rate (BMR), or the rate at which fuel (in the form of food) is “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.

Image of cold-adapted family from Siberia courtesy of Wikicommons creative license.

*

Humans display remarkable variation in skin coloration, and unfortunately this human polymorphism has been used in racist thinking for centuries. But, why do humans have different skin colors? And how does this occur? Certainly, this is a flexible character. Exposure to the sun can lead cells in the skin ,called melanocytes, to produce larger granules of melanin, the pigment that darkens the skin. This is called a tan. It is clear, therefore, that 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 the 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? Anthropologists Nina Jablonski and George Chaplin have developed an elegant explanation for the evolution of skin coloration. Solar radiation is not entirely a bad thing. It is absolutely necessary for the proper development of vitamin D, which helps regulate calcium absorption and is critical for proper skeletal development. Without proper vitamin D synthesis, the bones do not develop normally, and a condition called rickets can occur. This is shown here in these two unfortunate individuals who did not have enough exposure to sunlight to synthesize enough vitamin D to mineralize their bones. So, in high-latitude areas, those individuals with dark skin would be blocking what little UV radiation there is, and would not produce enough vitamin D to grow a functional skeleton. These individuals would not survive as well as the lighter-skinned individuals. So, why doesn’t everyone have light skin? The answer lies in another chemical: folic acid.

*

While UV radiation is necessary for vitamin D production, it can destroy folate (folic acid), which has been shown to be absolutely critical for proper synthesis and repair of DNA. In fact, 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. Hopefully, you see the tug-of-war going on here between vitamin D, which needs sunlight, and folic acid, which needs protection from sunlight. The balance 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 intense enough UV radiation 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 vitamin D-fortified, 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.

*

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. This research shows that high-altitude populations do not just adapt developmentally, but genetically through natural selection.

*

It is useful now to review how humans adapt to heat, cold, varying levels of UV radiation, and high altitude. The second column categorizes the response as either a quick physiological response to the initial exposure, a longer-term functional adaptation either developmentally or physiologically, or a true genetic adaptation through natural selection. Read through this chart on your own, and make sure you understand how each of these examples helps us understand the adaptability of humans.

*

Of course, humans are not just subject 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 also adapt to food via the same mechanisms already described for climate adaptations. In some places, scorpions are a delicacy. But, is that much different from a New Englander enjoying another multi-legged, clawed creature, namely a lobster? In fact, diet is one of the best ways we can 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, we don’t just survive, 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. Diet is a complex matter, and there are many macronutrients, like carbohydrates, fats, and proteins, that we require, in addition to micronutrients, like vitamins and minerals. On page 114 in your book is a list of all of the different macronutrients, vitamins, and minerals that humans are recommended to ingest in a day. But, what if we do not ingest all that is recommended?

Images of scorpion and lobster courtesy of Wikicommons creative license.

*

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 lack of calories is only one form of malnutrition. The lack of essential micronutrients is the other. Undernourished populations also suffer the double-whammy of having their immune systems compromised by a lack of proper nutrition, PLUS the fact that many of these individuals live in poor living conditions, rife with infectious diseases. As one might expect, malnourishment also takes its toll in 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 shows that growth, while having a genetic component, is strongly influenced by nutrition. As shown in this figure, the height of kids living in Germany dropped during the years of World Wars I and II, when food was not as abundant, only to catch up after the wars ended.

*

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. 50% of adults in the U.S. are considered overweight; 20% of children are obese. 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. This 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 meat, in addition to vegetable fats and oils, that have a very high fat content compared to non-domesticated animals and plants hunted and gathered by our ancestors. One additional problem associated with overnutrition is the spike in type 2 diabetes cases: 100 million strong worldwide. The high fat, high sugar diets we consume cause our pancreas to secrete an abnormal amount of insulin. Our tissues, like muscle and liver, eventually become non-responsive to this insulin, and these tissues cannot store and access the glucose they need to properly function. The excess glucose is instead stored as fat, or it remains in the blood where it raises the viscosity of blood (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.

*

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 critical, because they are the anchors for our ligaments, tendons, and muscles, which use the bones as leverage to move the body. But, the bone 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 bone that is not being used. 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, athletes, like tennis players, have a thicker upper arm bone (humerus) in the dominant compared to the non-dominant 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 kids. These same kids are then susceptible to osteoporosis and fractures later in life.

*