Human Biological Adaptability: Overview (2024)

Inuit (Eskimo) living in a polar desert--an example of the human ability to survive in extreme environments

The human body readily responds to changing environmental stresses in a variety of biological and cultural ways. We can acclimatize to a wide range of temperature and humidity. When traveling to high altitudes, our bodies adjust so that our cells still receive sufficient oxygen. We also are constantly responding in physiological ways to internal and external stresses such as bacterial and viral infections, air and water pollution, dietary imbalance, and overcrowding.

This ability to rapidly adapt to varying environmental conditions has made it possible for us to survive in most regions of the world. We live successfully in humid tropical forests, harsh deserts, arctic wastelands, and even densely populated cities with considerable amounts of pollution. Most other animal and plant species are restricted to one or relatively few environments by their more limited adaptability.

Humans normally respond to environmental stresses in four ways:

The first three are biological responses. The last three occur during our lifetime without further genetic change.

Genetic Change

When an environmental stress is constant and lasts for many generations, successful adaptation may develop through biological evolution. Those individuals who inherit a trait that offers an advantage in responding to particular stressesare more likely to survive longer and pass on more of their genes to the next generation. This is evolution through natural selection.For instance, people whose ancestors have lived in areas that have had endemic malaria for thousands of years often inherit some degree of immunity to this serious disease. The high incidence of sickle-cell trait among the people of Central Africa is largely the result of indirect selection for this trait by malaria. Heterozygous carriers of the sickling gene usually do not have sickle-cell anemia and are sufficiently resistant to the malarial microorganism that they are at a selective advantage. Another example of a genetic solution to an environmental stress is our ability to produce sweat as an aid in cooling our bodies in hot environments. It is not surprising that we have this capability because our immediate prehuman ancestors were tropical animals.

Genetic change in response to environmental stresses usually takes many generations to become widespread in a population. Fortunately, we also have other ways of responding more quickly as individuals during our own lifetime. The word adjustmentsis used here to refer to these shorter term physiological changes that are not inheritable. Thewordadaptations is reserved for inheritable genetic changes developed in a population over a long period of time.

Developmental Adjustment

One of the more powerful types of adjustments to environmental stresses is a change in growth patterns and development. This occurs in childhood and typically results in anatomical and/or physiological changes that are mostly irreversible in adulthood. Such permanent changes are referred to as developmental adjustment or developmental acclimatization.

	X-ray of an early 20th century Chinese woman's bound foot The growth was stunted and the bones were significantly deformed so that they could fit into a tiny pointed slipper.

Among humans, developmental adjustments result from both natural environmental pressures and cultural practices. An example of the latter was the now illegal custom in China of tightly wrapping or binding the feet of young girls with cloth in order to hinder normal growth. While this caused permanent, crippling deformities of the foot bones, it also resulted in extremely tiny feet which were considered to be very attractive. Parents crippled their daughters with good intentions. Small feet would make them more attractive marriage partners for rich important men and save them from a life of drudgery.

It is easy to condemn the old Chinese custom of foot binding as being barbaric. However, it is worth considering that North Americans and Europeans have intentionally altered parts of the bodies of their children and themselves with unpleasant procedures as well. In the late 19th century, tight corsets worn by girls when their bodies were still growing had the effect of deforming lower rib bones dangerously in towards their lungs. Some rich women even had lower ribs surgically removed in order to achieve a stylish "wasp-shaped" waist. A 19 inch circumference was the ideal.

Intentional deformation of parts of the body is not something that only happened in the past. In China today, there is a growing concern among many upwardly mobile middle class men and women that they are too short. Thousands have sought a solution to this problem by having their legs lengthened. This is achieved by a long, painful process involving the surgical breaking of the two lower leg bones in both legs and then using adjustable metal braces that are anchored with steel pins implanted in the bone just below the knees and above the ankles to progressively extend the length by nearly 1/16 inch (about 1 mm.) a day as the bones heal. This widens the gap at the break areas, thereby stimulating new bone growth. As a result, the average patient permanently adds about 3 inches (7-8 cm.) to their height in half a year.

Intentional body deformation is a common practice in North America today as well. It is customary for middle and upper class parents to have the teeth of their children straightened with retainers and braces. This is a long, costly, and somewhat painful experience that alters the alignment of teeth. In part, it is done to preserve and improve their functioning. However, a strong motivation is to enhanceappearance. Nose straightening and other forms of plastic surgery are often done for the same reason, despite the fact that they are painful. It is assumed by parents that these kinds of body alteration will increase the likelihood that their children will grow up to be more successful in life. This was also the motivation of rich Chinese parents in the past who bound the feet of their daughters and of contemporary Chinese who undergo leg lengthening.

	Wearing fashionable high-heeled shoes can cause foot deformities and other orthopedic problems over time

Permanent changes to the shape of body parts may be unintended. For instance, wearing leather shoes that enclose the feet makes them narrower than they would be otherwise. Similarly, the practice of women wearing shoes with pointed toes, high heels, and often too small of a size commonly result in a number of painful orthopedic deformities. Driving this outwardly illogical Western cultural practice is the belief that small feet are attractive for women. The American Academy of Orthopaedic Surgeons has reported that 9 out of 10 women in the United States wear shoes that are too small for their feet, and 7 out of ten subsequently have developed painful bunions, hammertoes, or other foot deformities.

What makes such developmental adjustments to parts of our bodies possible is the fact that humans have a high degree of physiological plasticity . That is to say, we can be physically molded by our environment during the growing process. Adults are the result of genetically inherited traits that were shaped to a certain degree in each of us by our environment as we grew up.

Child with marasmus

Extreme undernourishment or overnourishment in early childhood can result in devastating developmental adjustments. When there is a prolonged shortage of food, as is experienced in a famine situation, people can develop marasmus (from Greek meaning "to waste away"). Symptoms include extreme emaciation, diarrhea, anemia, and apathy. Women with marasmus usually stop ovulating and, subsequently, cannot become pregnant. The loss of insulating body fat makes people with marasmus highly vulnerable to death resulting from a drop in core body temperature when the air falls below 60-65º F. (15-18º C.). Young children who survive marasmus usually end up with short adult stature and some degree of mental retardation. Unfortunately, marasmus is not a rare occurrence around the globe today. Approximately 31 million children die each year from undernutrition, and 178 million are stunted in their growth. About 1 billion people are now undernourished, and essentially the same number are overnourished and suffer the health problems related to obesity.

A lack of specific kinds of nutrients can result in other life threatening health problems. For instance, when babies and very young children have a diet that is extremely low in protein, they will likely develop kwashiorkor . Also contributing to this condition is an inadequate consumption of vitamins A and E as well as the minerals zinc and selenium. Typical symptoms of kwashiorkor include edema (or swelling) due to fluid retention (especially in the abdomen), stick-like legs and arms with little fat or muscle mass, apathy, and loss of hair and skin pigmentation in patches. As in the case of marasmus, children with kwashiorkor are likely to be apathetic and have impaired immune systems which reduces their ability to fight off infections. If a child survives kwashiorkor, they are likely to have their growth retarded.

Even specific vitamin deficiencies alone can result in serious health problems for children despite otherwise adequate diets. For example, a lack of vitamin D can cause the bone disease known as rickets, while inadequate amounts of vitamin A can cause permanent blindness and impair the immune system. Nearly 100 million people in the world today have vitamin A shortages. Most live in Asia. In order to reduce this deficiency, a new strain of genetically modified rice ("golden rice") that has relatively high amounts of vitamin A is now grown extensively in Asia. However, a diet that has too much vitamin A is equally harmful. It can cause birth defects (especially cleft palate) and can interfere with the cells that produce new bone, resulting in a dramatic increase in the risk of fractures.

	Contemporary tall Japanese

Developmental adjustment does not only result in defects and disorders. Dietary changes also can have a positive effect if nutrition is improved. This has been the case in Japan since the end of World War II. The Japanese Education Ministry reported that children have been significantly taller in each generation since then. In 1986, for instance, 14-15 year old Japanese boys averaged 7 inches taller than did comparable aged boys in 1959. A key changing factor in Japanese lifestyle has been diet. It is likely that this was mostly responsible for the increased body size. Between 1961 and 1971, Japanese consumption of animal protein rose 37% while plant food consumption dropped 3%. In the cities of Japan and other increasingly affluent areas of East Asia, food habits have changed dramatically over the last several decades. Hamburgers, pizza, fried chicken, and other high fat Western foods are very popularwith the young and affluent. In Japan today about one fourth of the calories consumed are fat--this is 5 times higher than just after WWII. Lending support to the hypothesis that diet changes of this sort can result in significant developmental adjustments is a recent two year study of children in Kenya. It found that the inclusion of only 60 grams (about two spoonfuls) of meat a day to the diet of young children resulted in the development of 80% greater upper-arm muscle compared to children who were strict vegetarians. A diet that included a comparable amount of milk instead of meat resulted in an increase of 40%. Foods of animal origin are important in the diet of young children because they contain nutrients that are difficult to get from non-meat or non-dairy sources. However, too much animal protein and fat can result in obesity and other health risks.

Acclimatization

All other forms of adjustment to environmental stresses are usually reversible whether they occur in childhood or adulthood. These reversible changes are referred to as acclimatization or acclimatory adjustment. It is useful to consider the different forms of acclimatization in terms of the length of time over which they can occur.

Forms of Acclimatization

	Tanning is a common form of seasonal acclimatization

An example of a long-term acclimatization is people who lose excess body fat and are very slender as a result of mild, long-term undernourishment. If they later increase their diet to a consistent level of excessive calories, they will very likely retain more body fat and eventually become obese. They experience long-term acclimatization when they initially lose body fat and again later when they retain it. In both cases, they are acclimatizing to the available food supply.

Anatomical and/or physiological adjustments also may develop over even shorter time periods. For example, many people acquire dark skin tans during the summer months and lose them during the winter. This change in skin coloration is a seasonal acclimatization to the destructive effects of ultraviolet radiation from the sun.

When skin divers descend into the ocean, they experience rapidly increased water pressure. Within seconds, they can suffer from excruciating pain in their ears due to the unequal pressure inside and outside of their eardrums. They must equalize this pressure by blowing hard through their nose. By doing this, they are making a short-term acclimatization to the changed environment.

Changing water pressure requires short-term acclimatization for skin divers

When traveling to a high altitude, it is common to experience a progressive drop in the ability to hear due to a pressure inequality from one side of the ear drum to the other causing it to slightly balloon out and become less flexible. This difference in pressure experienced in mountains can usually be cancelled out by yawning, swallowing, or chewing gum. However, if someone has a stuffy nose, it is often difficult to equalize the pressure when they ascend to high altitude. As a result, they are liable to have considerable pain in their ears. This was a major problem for flyers prior to the time when airplanes were pressurized.

The difference between the kinds of acclimatization is not only in the amount of time it takes for the adjustment to initially occur. Usually, the shorter the time for acclimatization, the quicker it is also in reversing once the environmental stress is no longer present.

Combined Effects

Genetic adaptation and the three types of adjustments to environmental stresses are not always distinct phenomena. Acclimatization occurring in childhood may result in permanent anatomical changes, as is often the case with malnutrition. When an acclimatization is successful in providing good health and longevity, it can give individuals a selective advantage in passing on their genes to the next generation. This can have a strong determinant effect on the direction of evolution. In turn, genetic change can play a significant role in adjustment since the ability to acclimatize is ultimately dependent on genetic makeup.

Adaptability to specific environmental stresses varies from person to person and from population to population. We are not all biologically equal. For instance, some groups of people are more successful in adjusting to high altitudes. Others can better handle intense heat and high humidity. Adaptive responses tend to occur in spatial clusters around the world. Usually, the most efficient adaptations for specific environmental stresses are found in areas where those stresses are most common. This is evidence that natural selection has occurred in the successfully adapting population.

Cultural Practices and Technology

It is important to remember that humans do not only interact with their environments biologically. We use culture as well. Over the last half million years at least, we invented technological aids that allowed us to occupy new environments without having to first evolve biological adaptations to them. Houses, clothing, and fire permitted us to live in temperate and, ultimately, arctic regions despite the fact we still essentially have the bodies of tropical animals.

FA-18 fighter pilot using cultural technology to adapt to high altitude

This does not mean, however, that human-made technology eliminates the biological adaptive advantages of particular individuals or groups. People who have thicker layers of fat insulation under their skin still usually survive better in cold climates, while people who are slender do better in hot ones.

In the next four sections of the tutorial you will learn how our bodies respond to several common kinds of environmental stresses.