Discussion : Human Variation, "Race", and DNA Testing

Human Variation To help us understand modern humans from the perspective of physical anthropology, we

should remind ourselves that humans are a uniquely biocultural species; we have adapted

in both physical and cultural ways to nearly every environment on earth. To understand our

species further, one of the main topics we will explore is human variation: looking at how

physical anthropology can explain some of the differences we see in the range of humans

around the world.

Biggest height difference ever in the NBA, Muggsy Bogues (5’3”) and Gheorghe Muresan (7’7”) (tied with Manute Bol for tallest).

The above image gives us a good idea of the types of differences that we tend to notice

about people. They consist of external characteristics, such as height, skin color, eye color,

hair color and texture. But we have to understand that the external phenotypes that

preoccupy our categorizations are really just a drop in the bucket of human variation. We

need to review the sources of variation that we have learned about and add an additional

source. One source of variation is clearly our genes. What we inherit from our parents

directly contributes to our phenotype through the genetic combinations we receive from our

mother and father--we just learned about this in the last module. In some cases, these

traits are monogenic and recessive; offspring can end up appearing quite different from

their parents. We know that the genetic sources of variation can be considerably more

complex. Many traits are polygenic or controlled by more than one gene.

Albinism, a lack of pigmentation of hair, eyes and skin, can be caused by just one pair of recessive genes. The recessive allele must be inherited from both parents. Note that albinism can affect individuals of any ancestry. Leucism, which is similar to albinism, can affect just about any species.

Going beyond our genes, we know that we are affected by the environment as well. An

important component of the human environment is culture. Human existence is biocultural

in that we depend on many cultural innovations to help us survive. The tools, technologies

and practices that we have developed have had an impact on our bodies in many ways.

We can see the impact of culture in a simple example. When immigrants from Mexico and

Central American nations come to the United States, the children of those immigrants end

up taller, on average, than those in the home countries. This pattern only holds, however,

for those whose economic conditions improve in the U.S. What accounts for these

differences? Genes have not changed, but better access to health resources and nutrition

is the likely sources of change. In this case, cultural environments may cause better or

worse rates of development for children.

From Acclimatization to Adaptation

Let’s turn to patterns of variation found in humans. We know that humans are not all

exactly the same. Are any of these differences among humans explainable through the

lens of physical anthropology? First we need to revisit the definition of adaptation. An

adaptation is a trait that has arisen due to processes of natural selection. That is, an

adaptation is a trait that confers an advantage on a species, so the genes that control that

trait were more likely to be passed down.

Every day, our bodies are reacting to our environments in order to preserve homeostasis.

Depending on the environment, coupled with the activities we pursue, our bodies may

experience a degree of stress in the attempt to keep ourselves operating within normal

parameters.

In order to cope, humans (and other species) have certain abilities that allow a short-term

response to changing environments. Acclimatization effects are physiological changes

that may be simple, short-term, seasonal, long-term, or developmental. An example of an

acclimatization effect in non-humans would be animals that develop a thick coat in the

winter that is shed when winter is over. You are also probably aware that mountain climbers

have to acclimate to high altitude.

Let’s take a look at three different types of extreme environments that can stress human

bodies and compare how variation has affected different groups of people with ancestry in

different parts of the world. We will look at hot environments, cold environments and high

altitude environments.

Hot, dry environments are some of the most difficult to deal with for any mammal. All

humans have the capability to adjust through two physiological responses. The first

is perspiration. As we lose water through the sweat glands that cover much of our body,

the act of evaporation actually cools us. Did you know that very few mammals cool

themselves by sweating profusely? Humans and horses do, but the pig does not. The

phrase “sweating like a pig” is therefore completely meaningless, since pigs do not sweat.

Humans also experience vasodilation, where the small blood vessels close to the skin’s

surface dilate to bring more blood to the surface so it will cool faster. If you “flush” or turn

red when you are exerting yourself, you are vasodilating. There are limits to these

responses, however. One risks dehydration and heat stroke if too much time is spent in a

very hot environment. How has natural selection dealt with this?

Heat stroke is not particularly fun. Avoiding dehydration is key, particularly in dry environments.

The size shape of a body, either human or another animal, can affect how well heat is

tolerated. In some cases, human bodies that have been subject to hot environments for

long times show a body shape that is adapted more specifically for that

environment. Bergmann’s rule shows that larger bodies with less surface area are more

adept at maintaining heat, while less mass with more surface area loses heat

faster. Allen’s rule illustrates that appendages (arms and legs) that are shorter retain more

heat, and longer arms and legs will shed heat faster. Generally speaking, a thinner body

with longer arms and legs will have more surface area compared to overall body mass.

This shape of a body is more conducive to staying cool, and is found in parts of sub-

Saharan Africa where environment is typically hot and dry.

Allen's Rule: Note how these two shapes above have the same volume, but a difference in the distribution of mass changes the surface area. The short, square shape has less surface area and the tall, thin shape has more surface area. This translates into better heat retention for shorter, thicker appendages and better cooling properties for long, thin appendages.

Cold environments have their own challenges. All humans will shiver in response to the

cold, bringing up the body’s metabolism. And vasoconstriction is when the blood cells

constrict in an effort to warm the body’s core at the expense of the extremities. In freezing

temperatures, the risks of experiencing vasoconstriction for too long are substantial—

frostbite can cause the loss of extremities. Again, for groups that have traditionally lived in

cold areas, such as the Inuit who live in sub-Arctic regions, we see more effective

permanent responses. These include a higher constant metabolic rate overall, and a

shorter body with shorter limbs. This pattern is predicted by Bergmann’s and Allen’s rules,

and was also seen among the shorter, stockier Neandertals of the Middle Pleistocene.

Inuit hunters in the Arctic deal with sub-freezing temperatures daily. These hunters are wearing traditional dress.

High altitude environments provide a different challenge for the human body. The main

issue faced by humans is that we have a harder time oxygenating our bodies, due to low

pressure. If you have experienced generalized hypoxia (lack of oxygen), you may have

felt shortness of breath, increased heart rate, a feeling of “pins and needles”, nausea or

dizziness. More serious symptoms of altitude sickness can include the build-up of fluid in

the lungs or swelling of the brain. Additionally, there are particular reproductive issues

related to high altitudes, most revolving around the fetus receiving adequate blood supply

from the mother.

Responses to high altitudes are quite different from those from low elevations, those newly

born in high elevations, and those fully adapted to high elevations. Eventually, a low-lander

can acclimatize to a high elevation environment because their heart rate, breathing rate,

and production of oxygen-carrying red blood cells will all increase. It takes several days for

this to occur, which is why high-altitude mountain climbers spend up to several weeks at

basecamps and later at intermediate-elevation camps in order to continue their climbs. If a

child is born at a high altitude to lowland parents, they will experience developmental

acclimation. They will have greater heart and lung capacities and their bodies diffuse

oxygen to cells more efficiently. However, only those who have been living at high altitudes

for a very long period of time, such as the native populations of the Himalayas or the

Andes, show the most thorough adaptation to the environment. This is most notable in the

fact that they do not experience the same reproductive problems faced by others.

Nepalese sherpas (porters), Mount Everest. They have considerably fewer issues with high elevation than the Western mountaineers they work for.