Introduction to Paleoanthropology/Modern Humans/Population Variation
HUMAN VARIATION AND ADAPTATION
One of the notable characteristices of the human species today is its great variability. Human diversity has long fascinated people, but unfortunately it also has led to discrimination. In this chapter we will attempt to address the following questions:
- What are the causes of physical variability in modern animals?
- Is the concept of race useful for studying human physical variation?
- Are there differences in intelligence from one population to another?
Variation and evolution
Human genetic variation generally is distributed in such a continuous range, with varying clusters of frequency.
- Ex. Our hair is curly or straight, our skin is lightly to heavily pigmented, and in height we range from short to tall.
The significance we give our variations, the way we perceive them (in fact, whether or not we perceive them at all) is determined by our culture.
Many behavioral traits are learned or acquired by living in a society; other characteristics, such as blue eyes, are passed on physically by heredity. Environment affects both.
The physical characteristics of both populations and individuals are a product of the interaction between genes and environments.
- Ex. One's genes predispose one to a particular skin color, but the skin color one actually has is strongly affected by environmental factors such as the amount of solar radiation.
For most characteristics, there are within the gene pool of Homo sapiens variant forms of genes, known as alleles.
- In the color of an eye, the shape of a hand, the texture of skin, many variations can occur.
This kind of variability, found in many animal species, signifies a rich potential for new combinations of characteristics in future generations. A species faced with changing environmental conditions has within its gene pool the possibility of producing individuals with traits appropriate to its altered life. Many may not achieve reproductive success, but those whose physical characteristics enable them to do well in the new environment will usually reproduce, so that their genes will become more common in subsequent generations. Thus, humankind has been able to occupy a variety of environments.
A major expansion into new environments was under way by the time Homo erectus appeared on the scene. Populations of this species were living in Africa, Southeast Asia, Europe and China. The differentiation of animal life is the result of selective pressures that, through the Pleistocene, differed from one region to another. Coupled with differing selective pressures were geographical features that restricted or prevented gene flow between populations of different faunal regions.
- Ex. The conditions of life were quite different in China, which lies in the temperate zone, than they were in tropical Southeast Asia.
Genetic variants will be expressed in different frequencies in these geographically dispersed populations.
- Ex. In the Old World, populations of Homo sapiens living in the tropics have a higher frequency of genes for dark skin than do those living in more northerly regions.
In blood type, H. sapiens shows four distinct groups (A, B, O or AB):
- The frequency of the O allele is highest in Native Americans, especially
among some populations native to South America;
- The highest frequencies of the allele for Type A tend to be found among
certain European populations;
- The highest frequencies of the B allele are found in some Asian populations.
The Meaning of Race
Early anthropologists tried to explore the nature of human species by systematically classifying H. sapiens into subspecies or races, based on geographic location and physical features such as skin color, body size, head shape and hair texture. Such classifications were continually challenged by the presence of individuals who did not fit the categories.
The fact is, generalized references to human types such as "Asiatic" or "Mongoloid", "European" or "Caucasoid", and "African" or "Negroid" were at best mere statistical abstractions about populations in which certain physical features appeared in higher frequencies than in other populations.
- No example of "pure" racial types could be found.
These categories turned out to be neither definitive nor particularly helpful. The visible traits were found to occur not in abrupt shifts from population to population, but in a continuum that changed gradually. Also one trait might change gradually over a north-south gradient, while another might show a similar change from east to west.
- Human skin color becomes progressively darker as one moves from northern Europe to central Africa, while blood type B becomes progressively more common as one moves from western to eastern Europe.
Race as a biological concept
To understand why the racial approach to human variation has been so unproductive, we must first understand the race concept in strictly biological terms.
In biology, a race is defined as a population of a species that differs in the frequency of different variants of some gene or genes from other populations of the same species. Three important things to note about this definition:
- it is arbitrary. There is no agreement on how many genetic differences it takes to make a race. For some, different frequencies in the variants of one gene are sufficient; for others, differences in frequencies involving several genes were necessary. The number of genes and precisely which ones are the more important for defining races are still open to debate;
- it does not mean that any race has exclusive possession of any particular variant of any gene or genes. In human terms, the frequency of the allele for blood group O may be high in one population and low in another, but it is present in both. Races are genetically "open", meaning that gene flow takes place between them. Thus one can easily see the fallacy of any attempt to identify "pure" races: if gene flow cannot take place between two populations, either directly or indirectly through intermediate populations, then they are not races, but are separate species;
- individuals of one race will not necessarily be distinguishable from those of another. In fact, the differences between individuals within a population are generally greater than the differences between populations.
The concept of human races
As a device for understanding physical variation in humans, the biological race concept has serious drawbacks:
- the category is arbitrary, which makes agreement on any given classification difficult, if not impossible. For example, if one researcher emphasizes skin color, while another emphasizes blood group differences, they will not classify people in the same way. What has happened is that human populations have grown in the course of human evolution, and with this growth have come increased opportunities for contact and gene flow between populations. Since the advent of food production, the process has accelerated as higher birth rates and periodic food shortages have prompted the movement of farmers from their homelands to other places;
- things are complicated even more because humans are so complicated genetically;
- "race" exists as a cultural, as well as a biological, category. In various ways, cultures define religious, linguistic and ethnic groups as races, thereby confusing linguistic and cultural traits with physical traits;
- to make the matter even worse, this confusion of social with biological factors is frequently combined with attitudes (racism) that are then taken as excuses to exclude whole categories of people from certain roles or positions in society. In the United States, for example, the idea of race originated in the 18th century to refer to the diverse peoples - European settlers, conquered Indians, and Africans imported as slaves - that were brought together in colonial North America. This racial worldview assigned some groups to perpetual low status on the basis of their supposedly biological inferiority, while access to privilege, power and wealth was reserved for favored groups of European descent.
There has been a lot of debate not just about how many human races there may be, but about what "race" is and is not. Often forgotten is the fact that a race, even if it can be defined biologically, is the result of the operation of evolutionary process. Because it is these processes rather than racial categories themselves in which we are really interested, most anthropologists have abandoned the race concept as being of no particular utility. Instead, they prefer to study the distribution and significance of specific, genetically based characteristics, or else the characteristics of small breeding populations that are, after all, the smallest units in which evolutionary change occurs.
Not only have attempts to classify people into races proven counterproductive, it has also become apparent that the amount of genetic variation in humans is relatively low, compared to that of other primate species.
Nonetheless, human biological variation is a fact of life, and physical anthropologists have learned a great deal about it. Much of it is related to climatic adaptation. A correlation has been noted between body build and climate:
- Generally, people native to regions with cold climates tend to have greater body bulk (not to be equated with fat) relative to their extremities (arms and legs) than do people native to regions with hot climates, who tend to be long and slender. Interestingly, these differences show up as early as the time of Homo erectus.
Certain body builds are better suited to particular living conditions than others.
- People with larger body bulk and shorter extremities may suffer more from summer heat than someone whose extremities are long and whose body is slender. But they will conserve needed body heat under cold conditions. The reason is that a bulky body tends to conserve more heat than a less bulky one, since it has less surface relative to volume;
- People living in hot, open country, by contrast, benefit from a body build that can get rid of excess heat quickly so as to keep from overheating; for this, long extremities and a slender body, which increase surface area relative to volume, are advantageous.
Anthropologists have also studied such body features as nose, eye shape, hair textures and skin color in relation to climate.
- Ex. Subject to tremendous variation, skin color is a function of four factors: transparency or thickness of the skin, distribution of blood vessels, and amount of carotene and melanin in a given area of skin. Exposure to sunlight increases the amount of melanin, darkening the skin. Natural selection has favored heavily pigmented skin as protection against the strong solar radiation of equatorial latitudes. In northern latitudes, natural selection has favored relatively depigmented skins, which can utilize relatively weak solar radiation in the production of Vitamin D. Selective mating, as well as geographical location, plays a part in skin color distribution.
Continuing human biological evolution
In the course of their evolution, humans in all parts of the world came to rely on cultural rather than biological adaptation for their survival. Nevertheless, as they spread beyond their tropical homeland into other parts of the world, they did develop considerable physical variation from one population to another.
The forces responsible for this include:
- genetic drift, especially at the margins of their range where small populations were isolated for varying amounts of time;
- biological adaptation to differing climates.
Although much of this physical variation can still be seen in human populations today, the increasing effectiveness of cultural adaptation has often reduced its importance. Cultural practices today are affecting the human organism in important, often surprising, ways.
The probability of alterations in human biological makeup induced by culture raises a number of important questions. By trying to eliminate genetic variants, are we weakening the gene pool by allowing people with hereditary diseases and defects to reproduce? Are we reducing chances for genetic variation by trying to control population size? We are not sure of the answers to these questions.