Evolution And History

Human Adaptation to Natural Environmental Stressors 303

above sea level. Physiological adaptation to the lower par-

tial pressure of oxygen in the environment has rendered

their body tissues resistant to oxygen deprivation.

Typical lowlanders can make both short- and long-

term physiological adjustments to high altitude. In general,

short-term changes help an individual avoid an immediate

crisis, but the poor efficiency of these changes makes them

difficult to sustain. Instead, long-term responses take over

as the individual’s physiological responses attain equi-

librium with the environment. This process is known as

acclimatization. Most lowlanders stepping off an airplane

in Lima, Peru, will experience increased respiratory rate,

cardiac output, and pulse rate. Their arteries will expand

as blood pressure increases in order to get oxygen to the

tissues. This kind of response cannot be maintained in-

definitely. Instead, lowlanders acclimatize as their bodies

begin to produce more red blood cells and hemoglobin in

and fertility levels of women in high-powered careers.

These women tend to have a more cylindrical shape,

which could be because their bodies produce relatively

more androgen than hourglass-shaped women. A higher

androgen level, however, may be the reason for lower

fertility among these women. High androgen levels may

also represent a biological response to a specific work en-

vironment that ultimately impedes the fertility of women

in high-powered careers.^4 The bottom line is that human

hormonal systems are highly sensitive to a variety of envi-

ronmental stimuli.

Over the past fifty years a secular trend of lower age

at menarche has become evident in North America.

Whether this secular trend is attributable to healthy or

problematic environmental stimuli (such as childhood

obesity or hormones in the environment) has yet to be

determined. Biological anthropologist Peter Ellison works

extensively on the connections between hormones and

the environment—a subspecialty defined as reproductive

ecology (see Anthropologist of Note).

While genetic and developmental adaptations are

permanent parts of an adult’s phenotype, physiological

adaptations, short-term changes in response to a specific

environmental stimulus, come and go. Along with cultural

adaptations, these various biological mechanisms allow

humans to be the only primate species to inhabit the entire

globe. Over the course of our evolutionary history, most

environmental stressors were climatic and geographic.

Today, humans face a series of new environmental stres-

sors of their own making.

Adaptation to High Altitude

High altitude differs from other natural environmental

stressors because it is the least amenable to cultural adap-

tation. The major challenge of high altitude is the reduced

availability of oxygen. Before the invention of oxygen

masks and pressurized cabins in airplanes, there was no

way to modify this environmental stressor.

When people speak of the air being “thinner” at high

altitude, they are referring to the concentration (partial

pressure) of oxygen available to the lungs, and so to the

circulatory system. At high altitudes, the partial pressure

of oxygen is sufficiently reduced so that most lowlanders

experience severe oxygen deprivation (Figure 13.2).

Populations that have lived at high altitudes for genera-

tions, such as the Quechua Indians of the highlands of Peru

and the Sherpa native to the Himalaya Mountains, possess

a remarkable ability to tolerate oxygen deprivation, living

and working at altitudes as high as 17,000 to 20,000 feet

physiological adaptation A short-term physiological

change in response to a specific environmental stimulus. An

immediate short-term response is not very efficient and is grad-

ually replaced by a longer term response (see acclimatization).

acclimatization Long-term physiological adjustments made

in order to attain an equilibrium with a specific environmental

stimulus.

10,000 ft

Sea level

Figure 13.2 The amount of atmosphere above us determines

the amount of pressure being exerted on oxygen molecules

in the air. At sea level, the pressure of the atmosphere packs

oxygen molecules more tightly together compared to the density

of oxygen molecules at higher altitudes. This in turn impacts

the ease at which oxygen can enter the lungs when we breathe.

(^4) Cashdan, E. (2008). Waist-to-hip ratio across cultures: Trade-offs between
androgen- and estrogen-dependent traits. Current Anthropology 49 (6).
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