Old Dominion University

Human Adaptation and Variation

Alyssa Carlson
01005378
ANTR 110S_10772
Dr. Elva Smith
24 November 2014 Human biological diversity encompasses much more than the characteristics seen by the naked eye. Our species walked new and unique paths to arrive at our present existence. Our journey resulted in the achievement of many innovative and unique physiognomies allowing us to survive and reproduce. What characteristics have our species evolved and which most greatly shaped our evolution by conferring additional advantages in survivorship? Although there are many remarkable characteristics that have evolved the most striking variance and adaptation amongst the human population is variation in skin color. The fascination lies in the fact that for centuries the human body has used continuing adaptation to protect humans in different ways such as disease and protection from the sun. Scientists have made considerable progress in explaining variation in human skin color, along with many other features of biological and genetic diversity. Diversities of skin color are a result of natural selection. Over periods of time, adaptation has allowed the favored types to survive by natural selection. In addition, the color of one’s skin goes well beyond the surface as a result of geographic distribution. With years of struggle against nature behind us, human bodies have acclimated to their surroundings and allow us to continue living in all corners of the world. The purpose of this paper is to discuss the adaptations and variations of skin color. To achieve this I will begin by explaining adaptations such as climate and disease that reflect human biological diversity. It will also include how cultures around the world make an impact. After this criterion has been established, I will expand on genetic adaptations and reflect on the human body experiences. I will conclude with the advantages and disadvantages of skin pigment.

As Charles Darwin once said, “It is not the strongest or the most intelligent that will survive but those who can best manage change.” Natural selection provides the best answer to geographical distribution of human skin color. Natural selection is the process by which the forms most fit to survive and reproduce in a given environment do so in greater numbers. Over the generations, the least fit organisms die out, and the favored types survive by producing more offspring. The role of natural selection in producing variation in skin color will elucidate the explanatory approach to human biological diversity.
Essential to understanding natural selection are phenotype and genotype. Phenotype is the organism’s evident biological characteristics. Over the years, this develops as the organism is influenced by the environment. Genotype refers to the hereditary factors which includes chromosomes and genes. In continuance, traditional racial classifications assumed that biological characteristics were determined by heredity. Contemporary findings, however, prove otherwise.
Skin is a complex biological trait and is influenced by several genes. One of the primary determinants of human skin color is melanin. Melanin is a chemical substance manufactured in the epidermis, or outer layer of skin. To have a better understanding of variation of skin color, pigment is determined by the amount and size of melanin. For example, darker skinned people produce more and larger granules of melanin than light skinned people. Melanin protects from an assortment of maladies and is offered by screening out radiation from the sun. With the amount of sun absorbed by the body and the volume of melanin produced it is important to incorporate location when comparing skin color. With geographical distribution in mind, it combines with natural selection to play a role in the vast variety of skin pigments around the world to accompany survival in different climates.
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. The nature of skin color distribution suggests an association with environmental factors varying with latitude. The association between dark skin color and a tropical habitat existed throughout the Old World due to the majority of the world’s dark skinned population residing in the tropics. The tropics are 23 degrees north and south of the equator. Humans and their ancestors have been inhabitants of that land for thousands of years until migration to all corners of the globe started to occur. As migration continued so did natural selection. Outside of the tropics, in locations such as the poles, skin variation is different due to the lack of sun and is quite obvious. Adaptation to lighter pigmentation may have been driven by a need to increase UV absorption for vitamin D synthesis at high latitudes. In addition, the pigment of the poles inhabitants did not change rapidly. Instead it was done by becoming acclimated to the environment. Acclimatization is another kind of physiological response to environmental conditions, and it can be short-term, long-term, or even permanent. Acclimatization is the physiological responses to changes in the environment that occur during an individual’s lifetime. These responses to environmental factors are partially influenced by genes, but some can also be affected by the duration and severity of the exposure. After lifetimes of generations are exposed to it the human body biologically adapts to the environment so it is sustainable to live there.
Furthermore, natural selection has favored dark skin in areas nearest the equator, where the sun’s rays are most direct and thus where exposure to UV light is most intense. For dark skinned people, they are able to live in the tropics because they have biologically adapted to direct sunlight and high temperatures. In considering the cancer-causing effects of UV radiation from an evolutionary perspective, three points must be kept in mind: 1. Early hominids lived in the tropics, where solar radiation is more intense than in temperate areas to the north and south. 2. Unlike modern city dwellers, early hominids spent their days outdoors. 3. Early hominids didn’t wear clothing that would have protected them from the sun. Given these conditions, UV radiation was probably a powerful agent selecting for high levels of melanin production in early humans.
In continuance, being out in high temperatures can pose risks on the body such as sunburn. Heat exposure can damage the sweat glands and can reduce the body’s ability to perspire and thus regulate its own body temperature, best known as homeostasis. As a response to overexposure to sun, the body produces a natural sunscreen, melanin.
For light skinned people, the tropics are manageable to live in because of the available cultural alternatives that are accessible for use such as sunscreen, umbrellas, and lotions. However, although they are manageable, they still conduct a higher risk for light skinned people to get cancer. When it comes to cancer, it is less likely to be found in dark skinned people living in tropical areas. This is a result of their constant exposure and biological adaptation to the sun. Equally important, diseases can occur when the amount of sunlight is not sufficient enough. Naturally, the body can produce its own vitamin D when exposed to adequate sunlight. However, for people in cold environments who protect themselves with layers of clothes, they are shortening their vitamin D supply because it interferes with the body’s self-production. The shortage of vitamin D diminishes the absorption of calcium in the intestines. From this develops a national disease known as rickets. As a result, the bones begin to soften and deform. For women, this could mean interference with childbirth. In addition, another disease that stems from the explanation of geographical location and skin pigment involves the effects of UV on folate. Folate is an essential nutrient that the human body manufactures from folic acid. Folate is vital for cell division and the production of new DNA. With that being said the connection between folate and women who are pregnant, it strengthens the cell division and reproductive success. As a result of a weak connection, when a pregnant women has folate deficiency it leads to neural tube defects. Neural tube defects are incomplete closure of the spinal cord and cause the spinal cord to develop incompletely. For this reason, infants can be born with defects such as anencephaly, exposure of the brain mass, or spina bifida, incomplete formal of the spinal cord. Consequently, both defects can lead to death.
In a like manner, genes and diseases are also biologically adaptable. Infectious disease has exerted enormous selective pressures on populations and consequently has influenced the frequency of certain alleles that affect the immune response. In fact, it would be difficult to overstate the importance of infectious disease as an agent of natural selection in human populations. In the cases of diseases for which there are no cures, genetic resistance maintains its significance. For example, AIDS has no genetic susceptibility. Those with this or exposed to it run the risks of becoming infectious in which it would progress from there. However, there are some places around the world that experience it more than others. As a result of that, AIDS could cause large shifts in human gene frequencies-again illustrating the ongoing operation of natural selection.
As was previously stated, there are advantages and disadvantages to adapting to a variety of climates and having a certain skin as a result of where you live. To start, people who have darker skin have a better advantage in health than someone with light skin. Due to the amount of melatonin produced by the body in dark skinned people, they never have to worry about overexposure to the sun, as it acts as a natural sunscreen. As been mentioned, when people are not over exposed to the sun, they do not have to worry about folate destruction and the diseases that follow. Another advantage to have dark skin is disease susceptibility. Due to the exposure of the sun for thousands of years and adaptions that developed from it, people with dark skin are at a lower risk of cancer. Cancer is the overgrowth of cells and since their cells are producing more melanin to give their skin pigmentation enough color to provide protection from their environment, it is an advantage for dark skinned people.
Aside from the abundance of advantages dark skinned people have, having dark skin does have its disadvantages. Although being protected from UV rays is an advantage, there is a drawback too. With the plentiful protection from the sun, those with dark skin color have reduced the disability of UV absorption. As stated before, UV absorption is important to the body because it is where the majority of human’s vitamin D comes from. Without vitamin D, the risk of developing rickets or osteoporosis increases. There are cultural alternatives such as foods and vitamin D supplements but they will not give a person a sufficient amount of the vitamin need and they still open themselves up for susceptibility.
On the opposing side of dark skinned people are people with light skin. Light skin people face more disadvantages. To illustrate, light skinned people are able to absorb more rays from the sun because they do not have as much melanin. In return, the amount of vitamin D that is absorbed prevents rickets and osteoporosis. Dissimilar from dark skin, people with light skin obtain a higher risk of developing diseases and skin cancer. Conversely, light skinned people obtain a larger amount of disadvantages in comparison to dark skin.
To begin, one of the disadvantages is impaired spermatogenesis, the process of sperm production. It also increases the susceptibility to folate destruction and thus to NTDs, which includes spina bifida. To continue, overexposure of UV rays to light skin can increase susceptibility to sunburn and thus to impaired sweating and poor thermoregulation. Dissimilar from dark skin, light skinned people obtain a higher risk for developing diseases and skin cancer. With that being said, it can be concluded that people with light skin run a higher risk for disease and developing health problems from exposure to the sun. In conclusion, our species walked new and unique paths to arrive at our present existence. Our journey resulted in the achievement of many innovative and unique physiognomies allowing us to survive and reproduce. We have been able to biologically adapt to our surrounding environments after years of exposure. Humans have also been able to continue to expand and inhabit more areas of the earth. Our skin color is a result of natural selection and helps us to survive. As a result of natural selection and skin color, some humans can withstand heat exposure for periods of time and feel okay. Others have to bundle up with layers of clothes just to feel warm but not get the exposure to vitamin D like they should. The environment we live in plays a role in our biological adaptations. Because of the environments some are at a higher advantage of not developing diseases or cancer while the lighter skinned people are. In addition to skin color, biological adaptation has allowed our bodies to respond better to environmental stresses such as disease. As we continue to explore and walk down unique paths, humans will continue to develop and create innovate characteristics needed for survival. As time continues, these adaptations will gradually advance and they will be passed down the lineages.
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