Jill MChurch. 21st Century Anthropology: A Reference Handbook. Editor: H James Birx. Volume 1. Thousand Oaks, CA: Sage Reference, 2010.
An adaptation is a feature of an organism that was produced through natural selection to perform a specific function. The Oxford Dictionary of Biology defines adaptation as “any change in the structure or functioning of successive generations of a population that make it better suited to its environment” (Hine, 2008, p. 9). Adaptations allow organisms to cope with environmental pressure or stress. Adaptations can be biological (physiological) or behavioral (cultural) and occur in all living organisms; however, this chapter will focus solely on human adaptations.
Human adaptation happens on a variety of levels. In addition to genetic adaptation through the mechanism of natural selection and cultural adaptations (e.g., clothing, shelter, social systems, rules/taboos), humans are capable of three additional forms of physiologic adaptation. Acclimation refers to very short-term changes in response to a stress, such as shivering in the cold or sweating in the heat. Acclimatization occurs over days or months, such as adjusting to breathing thinner air at high altitudes. Developmental acclimatization is a change in body structure over an individual’s lifetime, such as the larger chest size and greater lung capacity of an individual raised at those high altitudes. The ability of humans to respond physiologically or developmentally to environmental stresses is called plasticity. Human responses to new environmental conditions often occur in a combination of physiologic and behavioral changes. Genetic changes would only be seen after the passing of many generations.
A cohesive theory to explain evolutionary principles evolved in Western Europe, building on knowledge and observations that date back to the 16th century. Many of these ideas were borrowed from Arab, Chinese, and Indian scientists and philosophers. These earlier scholars proposed the concept of evolution, but had no suggestions to explain the theory, process, or mechanisms driving this force.
Charles Darwin, a British naturalist, was the first person to suggest an explanation of the mechanics of the evolutionary process. As he published his theory of natural selection, a Scottish naturalist named Alfred Russel Wallace independently reached the identical conclusion. Since scientific knowledge builds on previous knowledge and theories, it is possible to see the development of ideas that caused these two gentlemen to simultaneously develop the theory of evolution by natural selection.
It was generally accepted throughout the Middle Ages that all forms of life on the planet were static. All species existed exactly as God created them. This belief that a life-form could not change after creation is called the fixity of species, upholding the idea that God created all animals and plants with the features they needed to perform their tasks. Irish archbishop James Ussher (1581-1656) took this information in the early 17th century and studied the “begat” chapter of Genesis in the Bible. He determined that the earth was created in 4004 BCE. For decades afterward, this date was printed at the beginning of every Bible. The concept of vast geologic time simply did not exist.
As Europeans learned more about the wider world though, doubts about the common mind-set arose. From encounters with Chinese civilization claimed to have occurred before the date given for the creation of the universe to questions about how the inhabitants of the Americas got there after dispersing from Noah’s ark, the literal interpretation of Genesis was being systematically undermined.
It was a scientist named Copernicus (1473-1543) who began the revolution of modern scientific thought. He was a Polish mathematician and astrologer who simplified the Ptolemaic model of the universe by placing the sun at the center of the universe and the earth as an orbiting planet. This radical change to intellectual thought enabled scientists to view the physical universe in new ways. By the early 1700s, the concept of “motion” was widely accepted in the physical universe, but biologists still insisted on the fixity of species. One of the leading naturalists of the time, Carolus Linnaeus (1707-1778), declared that there was a continuum of life from algae to humans but each species was fixed and unchangeable. He developed a well-received classification system called Systema Naturae to classify organisms. He isolated common traits and assigned two Latin names to each organism—a generic term (genus) for the group of organisms and a more specific term (species) for the specific plant or animal. The two words together would be the name for that specific life-form.
While Linnaeus’s classification system was widely accepted, he had contemporaries that were vocal in their opposition to his views. Georges Louis Leclerc, Count Buffon (1707-1788), was a leader among his detractors. Buffon stressed the importance of change in the universe. He described the variety and number of minute changes in nature as a system of laws, elements, and forces. He felt the aim of scientists was to discover and explain these forces that drive nature, not simply categorize their result. Both arguments were widely discussed in intellectual circles well into the 19th century.
Erasmus Darwin (1731-1802), grandfather to Charles, was an eccentric scientist, doctor, and poet. He explored a number of evolutionary ideas, but tended to express his ideas in verse, making little impact on general scientific thought. While he believed in the process of evolution, he could not explain how it happened. Another scientist, Jean-Baptiste Pierre Antoine de Monet de Lamarck (1744-1829), was finally able to go one step beyond Buffon and Erasmus Darwin by organizing his ideas into a comprehensive theory of adaptation. Lamarck stressed that organic forms interacted with their environment. Their stability was proportional to their living conditions, and as those conditions changed, life-forms were impacted. In other words, physical changes were caused by an environmental need. As an organism made a repeated effort to do something, “fluids and forces” would go to that point in the body and develop an organ to eventually fulfill that need. Lamarck also believed that new organs, or appendages, developed this way would be passed on to the next generation. This theory of acquired characteristics, or Lamarckism, is known to be untrue today, but many of his views are as valid today as they were almost 200 years ago.
Lamarck made the concept of evolution popular, but there was vehement opposition to the notion that existing species could develop into new species. Georges Cuvier (1769-1832), a contemporary of Lamarck and a very well-known scientist often called the “Pope of Bones,” was very vocal about his criticism of Lamarck’s views. By this time it was widely accepted, from examinations of the fossil record, that new species of plants and animals had come into existence. Cuvier insisted on the fixity of species, and developed a theory of catastrophism to explain how new species could appear. His theory proposed that a series of natural disasters or catastrophes (like the formation of a mountain chain) would destroy all life in that area, and be reflected in the geological record. Over time, the area would be repopulated by life from surrounding areas unaffected by the disaster. This would explain the appearance of new life-forms in the fossil record of a location without mentioning evolution.
The most influential opponent to Cuvier’s views was Charles Lyell (1797-1875). Lyell was a lawyer with a great interest in geology. He befriended Charles Darwin when he returned to England after his 5-year voyage on the HMS Beagle. Lyell’s greatest contribution to science was his three-volume Principles of Geology. In this pivotal work, he rejected catastrophism and reaffirmed the principle of uniformitarianism proposed by James Hutton in 1785, namely, that there are no forces working today that were not also active in the past. Lyell showed that the earth’s crust formed through very slow, gradual changes like weathering or erosion by water, wind, and ice. These forces, over the vastness of geologic time, could create the mountains, rivers, deserts, and coasts seen in the present. Lyell believed the earth was hundreds of millions of years old, which gave Charles Darwin a conception of time that made the gradual process of evolution possible.
Another inspiration to both Darwin and Wallace in developing their theories of natural selection was an essay written by Thomas Malthus (1766-1834), an economist. Malthus pointed out that unrestrained human population growth would cause it to double every 25 years, but the capacity for food production would increase far more slowly. Animals in the wild had to struggle for survival, which would restrict the population growth, but humans would have to apply artificial restraints given their limited food resources but infinite breeding capacity. This gave Charles Darwin the missing insight needed to explain selection occurring in nature. He realized that individuals with favorable characteristics would be more likely to survive, and individuals with unfavorable characteristics would not. Previous scientists looked at a species as a single entity and minor differences within a species as irrelevant. Darwin was the first to realize that the struggle of the individual to survive was the mechanism that made evolution work. This is the process of natural selection by which individuals that share favorable characteristics will increase in number from generation to generation, so greater numbers within the species will share those adaptations better suited to the environment. Over time, successful adaptations will produce enough variation that a new species is formed. Darwin, Wallace, and others finally understood the importance of variation and adaptation and how these drive the process of natural selection, but no one in the 19th century understood how traits are passed to offspring. A contemporary of Darwin, the Augustinian monk named Gregor Mendel, was actually working out the rules of heredity, but his work was not recognized until the early 20th century.
In the early 1900s, the foundations of modern evolutionary theory were in place. Darwin and Wallace had articulated the importance of the process of natural selection in driving evolution, and Mendel’s work was rediscovered, establishing the mechanisms for inheritance. One would think that a comprehensive theory of evolution was developed quickly from this knowledge, but for the next three decades rival groups would vehemently argue different viewpoints. Some biologists took the Darwinian view stressing the importance of natural selection in the production of variation, while others stressed random mutations as the source of variation. A combination of these views, called the modern synthesis, was finally developed in the mid-1930s. Biologists working with mathematical models came to realize that both mutation and selection were needed to explain evolutionary change. Mutation alone does not produce evolutionary change, but mutations are the source of variation, which produces different characteristics that natural selection chooses for or against.
A central component of modern synthesis is the relationship between populations and species. A species is a group of populations whose members can interbreed and produce fertile offspring. A species has a geographic range it inhabits, with populations of individuals clustering into smaller areas within it. A population in a remote area with little outside contact may eventually select characteristics specific to surviving in that region which result in measurable physiologic differences from the rest of the species. The accumulation of many small genetic changes over generations results in the differences seen in populations today. From a modern genetic perspective, evolution is defined simply as a change in allele frequency from one generation to the next.
Anthropologists today know that human variation is the result of a number of evolutionary factors, including mutation, genetic drift, gene flow, and natural selection. Mutations are random, spontaneous changes in a gene that can be caused by any number of environmental factors. They are the ultimate source of all genetic variation. Genetic drift is also a random factor related to population size. In a small population, some individuals may contribute a disproportionate share of genes to succeeding generations. Gene flow is the exchange of genes between populations. It occurs when people migrate to a new area (either temporarily or permanently) and interbreed with another population. Finally, there’s natural selection, the principle mechanism of evolutionary change. It is the process by which individuals with advantageous characteristics for reproduction in a specific environment leave more offspring in the next generation with the same trait, increasing the proportion of their genes in the gene pool over time.
Cultural adaptations have also played a significant role in human evolution. Cultural adaptation refers to nonbio-logical responses of individuals or groups to alleviate environmental stress. It is an important mechanism that allowed humans to survive and colonize relatively inhospitable areas until physiological adaptations could occur. All the evidence to date suggests that hominids evolved in the hot savannas of East Africa. Humans today cope better with heat than they do cold, illustrating the long-term adaptations to heat that developed in our ancestors. As humans migrated to colder environments, they invented fire, clothing, and shelter to survive.
Throughout the course of human evolution, people have settled in almost every climatic zone of the world. Using a variety of adaptations, they have adjusted remarkably well to living in extremely hot or cold temperatures, exposure to solar radiation, very dry or humid air, thin atmosphere, and broad seasonal fluctuations in climate. For populations to cope with the challenge of new habitats, they must undergo changes through a combination of natural selection and physiological plasticity. The interaction between both processes is so intertwined it is difficult to isolate either.
General build and skin color are the most obvious adaptations. Ancient Greeks took this knowledge one step further by associating physical characteristics with the environment in which people lived. People from the interior of Africa had the darkest skin and it was assumed that the tropical sun was the cause. Similar associations were made in other animals. The average size and shape of indigenous individuals had a relationship to the temperature, while nose size and shape correlated to humidity. A zoologist by the name of Constantin Wilhelm Lambert Gloger first commented on this phenomenon in 1833. Gloger’s rule states that within a species of endotherm (warm-blooded mammal), skin pigment tends to be darker in warmer climates at lower latitudes or lower altitudes, and lighter in color in colder climates at higher altitudes or higher latitudes.
Adding to Gloger’s rule, a 19th-century zoologist named Carl Bergmann studied the relationship between body size and temperature in a variety of mammal species, explaining his findings in terms of heat loss. Bergmann’s rule (developed in 1847) states that if two mammals have similar shapes but different sizes, the smaller one will lose heat more rapidly. This makes the smaller animal better adapted to living in warm climates. Larger animals lose heat more slowly and would be better adapted to colder climates. The reason for this relationship is that heat production is a function of the total volume of an animal while heat loss is a function of total surface area. A final aspect of Bergmann’s rule factors the shape of a mammal into the relationship between heat production and loss. He states that two differently shaped animals with the same volume will produce the same amount of heat, but a linear shape would have a greater surface area and dissipate heat more rapidly. Therefore, mammals living in hot climates will have linear body shapes and those in cold climates will have stockier body shapes. Another zoologist, Joel A. Allen, applied these principles to body limbs and other appendages. Allen’s rule (developed in 1877) predicts that mammals in hot climates will have longer and leaner limbs and those in cold climates will have shorter, bulkier limbs.
The Bergmann and Allen rules apply to adult humans, but evidence to date suggests that a combination of genetic and environmental factors influence the relationship between climate, growth, body size, and body shape. When children grow up in a climate that differs from that of their ancestors, they tend to grow as indigenous children do.
Measuring the size and shape of the human head has long been a focus of racial classification. In the 19th century, a Swedish anatomist named Anders Retzius developed a measure of cranial shape called the cephalic index. This index is derived from two measurements: the total length of the head and its maximum width. The width of the head is divided by the length, and the result is multiplied by 100. The cephalic index among human populations ranges from 70 to 90. These values only apply to the average for a population. There is a certain amount of variation within a population, and the numbers for different populations do sometimes overlap.
As this data was compiled and compared geographically, a pattern emerged. Populations in colder climates tend to have wider skulls relative to length than those in hot climates. This correlation fits with the Bergmann and Allen rules. Rounded heads, those with a high cephalic index, would lose heat more slowly and be advantageous in cold climates. Narrow heads lose heat faster and would have the advantage in hot climates.
Another variation with a strong relationship to climate is the nasal index. This index is determined by dividing the width of the nasal opening by the height of the nasal opening, and multiplying by 100. Typical values for humans range from 64 to 104. In the past, stereotypical racial views associated wide noses (large nasal indices) with African populations, but in actuality, there are some African peoples with wide noses and others with long, narrow noses. Instead, the nasal index has a direct relationship to the temperature and humidity of an area. Populations in cold climates tend to have narrow noses, because high, narrow noses can warm more air before it reaches the lungs, which is advantageous in the cold. High, narrow noses also have greater internal surface area to moisten air in dry climates, either hot or cold. Wider noses are found in areas of high humidity.
The study of human variation and adaptation is useful in a variety of careers. The broad field of biological anthropology, also called physical anthropology, studies the mechanisms of biological evolution, genetic inheritance, human adaptation and variation, and primatology. The objects of study range from fossils and bones to living populations. In addition to researching and teaching in the anthropology department of a college or university, there are many situations that require the study of human adaptation.
A key opportunity to study human adaptations appears in biomedical research. Biomedical scientists focus on issues related to public health, including growth and development, nutrition, aging, disease, genetics, epidemiology, physiology, and forensics. Anthropology’s theoretical bases of evolution, human adaptation, human variation, and their relationship to cultural influences are very relevant to biomedical practices. A growing number of biological anthropologists are therefore transferring their skills and interests to research careers in schools of medicine and in private biomedical research facilities.
Museums also have anthropologists on staff. Specialists in various subfields are needed to manage collections and prepare exhibits in addition to conducting research. Skills in educating visitors about the relationship between biology and culture and explaining the importance of the collection to the public are an important part of a museum’s mission. Anthropologists also write grants to secure funding for museums to support additional research.
Another critical application of the study of human variation and adaptation occurs in the military. Knowing how troops will react to extended exposure from a variety of environmental stressors is necessary for the training and preparation of soldiers. Working in full body armor and gear requires specific adaptations, whether in extreme heat, cold, or high altitude. Psychological stress over long periods of time will also result in physiological changes to the body. This stress can come from a variety of sources, such as exposure to combat or the isolation of working in very remote places like submarines or arctic research stations.
There has been a great deal of research in the last several decades about human adaptation to conditions in space. Physical and psychological adaptations are necessary to endure long periods of time in a weightless environment. The majority of the data to date comes from missions of relatively short duration, making it difficult for scientists to extrapolate the effects of long-term exposure. In addition, virtually all studies have been on physically fit, male cosmonauts. The effects of space on average individuals, children, and the elderly are completely unknown. Hence, there is a great deal of work to be done in this field as humans contemplate future space colonization, and researching the ability of humans to adapt to these extreme living conditions will play a vital role.
Environmental conditions vary greatly around the world. Over thousands of years, human beings have adapted to living with extreme heat, extreme cold, high altitude, dietary limitations, and more. While human populations have a variety of cultural or behavioral methods to combat exposure to environmental stressors for rapid acclimation, the actual physical changes in over hundreds of generations can be seen in those native to a region. While demonstrating direct effects of natural selection is difficult, humans do show physiological differences in response to their environment. The question is whether these changes were due to adaptation through natural selection, or would any population of humans have the same physiological ability (plasticity) to adjust to that environment, given enough time?
Skin color is the best understood relationship between physical characteristics and climate. As the ancient Greeks hypothesized, there is a correlation between skin color and solar radiation. A pigment called melanin in the dermal layer of the skin is responsible for its color. Levels of melanin in a population are a genetic characteristic, but exposure to ultraviolet light will increase the amount of melanin in the skin of all populations at the same rate, regardless of the initial pigment level.
Ultraviolet light is strongest at the equator due to the way sunlight reaches the earth and weakens toward the poles. It is also stronger in the Southern Hemisphere than in the Northern Hemisphere. The distribution of human skin color around the world illustrates past evolutionary adaptations. It is believed that as hominids evolved in Africa, they developed more sweat glands and less hair to adapt to the hot climate. Darker skin would have been beneficial as protection from the damaging effects of ultraviolet radiation. As some human groups migrated out of Africa, lighter skin was selected for areas away from the equator.
Dark skin in areas of high solar intensity provides a number of benefits. Melanin blocks ultraviolet radiation, so the darker an individual’s skin, the more protection against skin cancer. Some scientists reject this benefit as a selection factor however, because skin cancer generally affects individuals past reproductive age. If someone dies after the reproductive years have passed, it would not impact the process of natural selection.
Protection against sunburn has also been suggested as a beneficial adaptation. During the thousands of years of human civilization prior to the development of antibiotics, severe sunburn could lead to skin damage and exposure to dangerous infection. However, this benefit would have a minimal impact on an entire population.
The most likely advantage to darker skin in equatorial regions involves the damage ultraviolet radiation can cause on the levels of folate in the body. Ultraviolet light destroys folate, and deficiency of this mineral in an individual can lead to both birth defects and decreased reproductive capacity. As humans migrated farther from the equator, the dangers of ultraviolet exposure were reduced. This does not explain why light-colored skin evolved though, only that it could evolve.
The most widely accepted model for the adaptation of lightly pigmented skin focuses on the ability of the human body to synthesize vitamin D. Vitamin D deficiency can cause poor bone development and bone diseases like rickets. These disorders can affect fertility and mortality. Modern humans receive enough vitamin D through vitamins or food additives (like fortified milk), but in the past, people obtained the vast majority of their vitamin D from sunlight. As human populations migrated away from the equator, their darker skin blocked too much ultraviolet radiation. Lighter skin would then be a beneficial adaptation, resulting in healthier individuals.
Because humans evolved in tropic or subtropic zones, they are genetically well adapted to hot, dry climates. They are one of very few mammals that can remain moderately active during the hottest part of the day. This is due to having the most efficient process of heat reduction in mammals—the ability to sweat.
Thermal sweat is produced by eccrine glands, which release a watery solution with virtually no fat or protein content, and very little salt. Most other mammals capable of sweating depend on apocrine glands, which produce a solution full of fats, proteins, and salt. These substances evaporate very slowly, reducing the rate of heat loss. Humans only have these glands on the face and hands. The Inuit, natives of arctic regions, demonstrate a unique adaptation here. The Inuit sweat less on their trunks and extremities, but more on their faces. This is an advantageous feature in the arctic, where moisture accumulating on clothing would be a hazard.
Human skin is covered by more than 1.5 million sweat glands, which can produce copious amounts of sweat over the entire body. Combined with the relative lack of body hair, sweating provides a very efficient cooling system for humans.
Humans also have a number of behavioral adaptations to living in hot climates. Clothing is important to protect individuals from solar radiation and the hot, dry winds of the desert. Typical desert clothing is lightweight and loose. This allows air to circulate near the skin and rapidly evaporates sweat. The layer of air between the body and clothing also adds a layer of insulation. Desert shelter is usually compact to minimize surfaces exposed to the sun. Light colors on the outside reflect heat. Doors and windows are minimal and kept closed during the day to keep the interior cool.
Heat stress in humid, tropical environments requires a modified set of behaviors. Humidity retards the evaporation of sweat, so clothing tends to be minimal to increase the likelihood of evaporation. Shelter is open, often lacking walls entirely, to maximize the circulation of air. Lastly, behavior is modified. People will be most active very early and very late in the day, taking a long break during the most intense midday heat.
Modern humans have a very low tolerance for cold, lacking the insulation of fur and hair. Exposure of the skin to temperatures as warm as 75° F causes constriction in the blood vessels of the skin. Temperatures in the 60s increase heat production in the body, resulting in shivering. Subcutaneous fat gives a little protection. It has low heat conductivity and helps retain core body heat, protecting internal organs.
If an adult submerges a finger into freezing water, blood immediately stops flowing to the area. Continued exposure would cause the body to force blood to the area in a cyclical fashion. Expansion and constriction of the blood vessels may be adaptive because this would keep heat loss to a minimum. Once temperatures drop below the freezing point though, the appendage would freeze without the heat caused by circulation. Therefore, the most adaptive response would depend on the length and severity of exposure to the cold.
There are measurable differences among populations exposed to this type of cooling. Men of black African descent have a much lower average of finger temperature in ice water. European men have a better physiological response, and men from the arctic and high altitude populations have the most effective response. The different levels of tolerance are due to vasodilation—the body constricts and relaxes blood vessels automatically, cycling the blood flow to the affected appendage.
Cultural adaptations to cold stress involve clothing and shelter. The Inuit, again, have adapted effectively to life in a polar environment. It is not enough to wear a great deal of heavy clothing to stay warm. Working hard in those conditions would cause an individual to overheat, and wet clothing in the arctic is hazardous. The Inuit instead wear layers of clothing that capture pockets of insulating air between them, much as the desert dwellers do. Their clothing is also designed with flaps and openings that can be adjusted as needed to prevent sweat buildup.
Inuit shelters are also highly specialized. Homes are designed with an underground entry that is curved to block incoming wind. The main living area inside is constructed on a higher level than the fireplace to maximize access to the heat and minimize drafts. When the Inuit are out hunting or fishing for long periods of time, they build igloos. These temporary shelters have thick walls of snow and ice, which provide efficient insulation. The reflective surface of the walls also helps to retain heat.
The Quechua Indians living in the cold, dry highlands of Peru do not have such effective temporary shelters. The temperature inside their temporary structures is often much the same as outside. Their most effective protection against heat loss is in their heavy, warm bedding—woven from the fur of the llamas and alpacas they herd.
There are a number of stresses associated with living at high altitudes. Low oxygen levels, cold, strong ultraviolet radiation, and sometimes, poor nutrition combine to create an inhospitable environment.
Oxygen deprivation, or hypoxia, is common at high altitudes. While the amount of oxygen in the air remains fairly constant more than 60 miles above the earth’s surface, barometric pressure decreases rapidly with an increase in altitude. The air is less compressed at high altitudes, making oxygen less concentrated. With this, there is less oxygen available to the hemoglobin in the blood. Hypoxia can then result in increased respiration, hyperventilation, and loss of appetite or weight loss. Memory, sensory abilities, and hormone levels may also be affected.
The thinner air results in higher concentrations of ultraviolet radiation, loss of rapid surface heat, and low humidity. Hypoxia is not only a danger to human life; it reduces plant and animal life as well. Trees cannot grow at altitudes over 13,000 feet, and the limited availability of plants and animals can be a source of nutritional stress.
Scientists have studied high- and low-altitude populations of Peruvian Indians and found two main differences. Chest dimensions and lung capacity are greater in all ages of the high-altitude group, and they have a shorter average height. Studies in other high-altitude areas around the world show similar chest and lung growth patterns, but not all groups reflect relatively short stature. Nutrition in the developmental years has a great influence on adult stature, and a limited diet appears to play an important role in the growth and development of Peruvian populations.
There are no human populations living under water, but there are groups that, for thousands of years, have lived by the sea. Tribal groups found in Southeast Asia are referred to as “sea gypsies,” known for their exceptional diving and swimming ability. One such tribe, the Moken, live along the coasts of Burma and Thailand. Moken children are expert divers, gathering shells, clams, and sea cucumbers from the sea floor. While most humans have poor vision under water (due to human eyes losing the ability to focus, making everything blurry and small objects very difficult to see), the Moken children can see twice as well as European children under water. A study done by Anna Gislén and her colleagues (2003) found that the pupils normally dilate under water because it is darker, but the pupils of the Moken children constrict for improved focus. It is not clear if this is a genetic adaptation in this population, or if it is an example of human acclimatization. Given their traditional lifestyle, the ability to accommodate under water may have been selected for strongly.
There are vast differences in the types and availability of food resources around the world. During human infancy, childhood, and adolescence, much of the energy provided by nutrients is devoted to growth. Too few calories can result in a reduction in size and a delay in maturity. Too many calories, on the other hand, can result in fat accumulation and acceleration in physical maturity. Neither result is ideal. Inadequate nutrients can impact basic biological processes and lead to disease susceptibility.
Food acquisition changed little through most of the course of human evolution. Humans were hunters and gatherers starting at least 2 million years ago, until the development of agriculture about 12,000 years ago. In general, hunting provided the smaller portion of the calories in a diet and the greater portion came from tubers, fruits, and seeds, but in any case, local environment shaped diet. Groups living near water would exploit fish and seafood while those in arid regions relied on other sources for nutrients. An exception is the Inuit, as having so little vegetation available for consumption resulted in the bulk of their diet coming from meat and fish.
The development of agriculture had a profound impact on human development. The domestication of crops provided an increased concentration of food, which expanded permanent settlements and increased population growth more rapidly. These factors, in turn, increased the spread of disease. Climate, resources, and level of technology all influenced food quality and quantity.
One of the best-known genetic adaptations to diet is illustrated in the adult human ability to digest lactose, the sugar that is found in cow’s milk. The body creates an enzyme called lactase to break down this sugar for digestion. While infants and young children in all human populations can digest milk, the gene encoding to produce lactase shuts off during childhood in some populations. If too much milk is ingested after this happens, it ferments in the large intestine, causing severe gastrointestinal distress. In many African and Asian populations today, most adults are lactose intolerant, but in European and Middle Eastern populations, adults tend toward lactose tolerance.
What would cause this variation? In hunter-gatherer societies throughout the Paleolithic, milk was generally not available after children were weaned. Perhaps the body continuing to produce an unneeded enzyme affected the digestion of the new foods in the growing child’s diet, making it a selective advantage to have the production of lactase turn off after it was no longer needed. Many European populations are lactose tolerant, and are at least partially descended from peoples in the Middle East who also exhibit lactose tolerance. The Middle Eastern groups tended to be pastoral or agricultural, raising cows, goats, and other milk-producing animals. They undoubtedly consumed milk and milk products throughout their lives. Selection pressures in this environment would favor lactose tolerance.
The rise of agricultural societies created an increase in food production, but resulted in more restricted diets. Agriculturalists tend to rely on one or very few staple crops, resulting in less dietary variety which can lead to undernutrition, starvation, or malnutrition. Undernutrition and starvation result from a lack of calorie intake to thrive, but malnutrition results when diets lack a critical vitamin, mineral, or protein. In underdeveloped countries, protein malnutrition is the most common form, resulting in a disease called kwashiorkor, which causes swelling, anemia, hair loss, and general apathy. A related syndrome called marasmus is caused by a combination of protein and calorie deficiency.
Malnutrition and starvation have a profound effect on reproduction. Malnourished mothers suffer from high rates of premature delivery, prenatal mortality, and delivering children with birth defects and low birth weight. Infants that do survive face retarded growth and development, along with decreased resistance to infectious and gastrointestinal diseases.
In modern times, industrial societies are coping with the result of too much food being readily available. Combined with an increasingly sedentary lifestyle, food overabundance is a source of environmental stress. If more calories are ingested than needed to maintain an active and healthy body, the excess is deposited as fat. Obesity is a growing problem in Westernized societies, making people susceptible to heart disease, high blood pressure, and diabetes.
Throughout the course of human evolution, disease has exerted a great deal of pressure on human populations, with a variety of causes and effects. Disease can be hereditary (e.g., sickle-cell anemia), metabolic (e.g., vitamin deficiency), degenerative (e.g., heart disease), from malignant cells (e.g., cancer), or infectious (e.g., malaria). Cultural factors are as critical as physiologic causes in the spread of disease.
Before urbanization, disease impact was limited. Small groups of people were constantly moving around a region with little contact between groups to spread disease. Large settlements with high population densities greatly accelerated the spread of airborne infections such as influenza, smallpox, and measles. The domestication of animals also introduced greater risks. The type of animal raised, sanitary conditions, and the proximity of animals to humans influenced exposure. Cultural behavior also contributes to the spread of disease: Practices such as ritual cannibalism, sharing ceremonial pools, having multiple spouses, and other activities can add to the risks.
Changing the environment also influences the development of disease. Clearing forested land in tropical regions for farming leads to open pools of standing water. These still pools of open water in a warm climate stimulate mosquito breeding, creating the ideal conditions to spread malaria. Crowded, unsanitary urban conditions repeatedly expose large numbers of humans to a host of easily spread infectious diseases. Changing patterns of land use, population density, birth rates, and access to medical care all impact the environment and other species that cohabitate with us. Large numbers of animal species are endangered or extinct as a direct result of human modification of the environment.
While many species suffer from human urbanization, other species—namely viruses, bacteria, and other pathogens—may thrive. In the past, infectious disease would decimate human populations. The bubonic plague swept through Europe in the Middle Ages and wiped out a large percentage of the population. Today, cultural adaptations in the form of modern medicine protect us from past diseases, but sometimes give rise to new pathogens. The overuse of antibiotics and antibacterial health and hygiene products has caused natural selection to occur in bacteria species. Bacteria resistant to these products tends to reproduce, creating very resistant “superbugs” that are very difficult to eradicate.
Human beings are still evolving. Evolution is a process, not a task with a final endpoint or finished result. Evolution is increasingly complex due to our biocultural nature, with human adaptability primarily based on our culture. We can change our behavior to adapt to a new situation faster than we can respond physiologically. We can also direct our cultural evolution while we cannot control the path of natural selection.
When one considers the vastness of geologic time, our population explosion is extremely recent. The implications are complex and little understood. The extent of our agricultural technology assures we have not yet reached the limits of food production. Medical technologies have (on average) reduced infant mortality rates and increased life expectancies, though social and political factors greatly influence both.
What are the potential effects of dramatic population increases? A likely result is increased genetic diversity. As more individuals are born, the rate of new genetic combinations increases. As the individuals mature and reproduce, the gene pool of the species increases in diversity. Increasing diversity increases adaptive ability, which would be beneficial to the species.
However, there are also a number of troublesome consequences. Humans can no longer rely on gathering enough naturally growing plants for food. We are increasingly reliant on highly industrialized agriculture. High-yield crops and farmed livestock, both bred for specific characteristics, are the norm. These engineered crops are often treated with pesticides, hormones, fertilizers, and more. There is also increasing reliance on manufactured goods. The long-term impact of industrialization can only be speculated on though, since evolutionarily, the Industrial Revolution is still in its infancy.
In this chapter, we have defined various types of biological and cultural adaptations to the environment. In the past, human populations were viewed in terms of race. This chapter reviewed the development of evolutionary theory, ending with the modern synthesis of genetics and evolution.
Culture plays a critical role in the human ability to adapt. We have considered physiological and behavioral adaptations to various environmental conditions, including exposure to ultraviolet radiation, heat, cold, high altitude, nutritional availability, and the influence of disease.
Clearly, population growth is one of the leading influences on current human adaptation. One result is increasing population density in inhabited regions. Urban centers have millions of people in close daily contact. Evolutionarily, these situations are brand-new, so it is difficult to predict how we will adapt. Scientists have noted correlations between the level of development of a region and instances of heart disease, hypertension, cancer, and neurological disorders. As generations reproduce, will humans select for characteristics to resist these diseases? Humans will also have to adapt to new environmental stresses they have created themselves. In addition to crowding, noise pollution, and exposure to artificial radiation, the greater consumption of resources leads to waste, pollution, and environmental degradation. Fossil fuels used for energy are affecting the environment. Deforestation also contributes to global warming. Even if the planet were undergoing a normal warming cycle, human activity appears to be tipping the balance toward a catastrophic, global climate change.
It takes a long time for humans to undergo genetic adaptation, and the characteristics that are selected for cannot be controlled. It stands to reason that all humans in our modern global society have to agree to work together to protect and adapt to the always-changing world we inhabit. We must integrate evolutionary, biological, and anthropological knowledge in order to understand ourselves and our place in nature. By using collaborative investigative methods and critical thinking, we surely have the capacity to change the world for the better.