As you know, when biological anthropologists study human variation, they consider all evolutionary factors. But natural selection favoring adaptive traits was the most important mechanism that produced the variation we see today. We must also bear in mind that to accommodate differences in climate, terrain, and available resources, humans had to adopt lifestyles that differed with regard to technology and diet. As time passed, and especially after the domestication of plants and animals beginning around 14,000 ya, cultural changes, including dietary practices, exerted an even greater degree of selective pressure. Thus, as populational differences in lactose tolerance demonstrate, the interaction between culture and biology became ever more important to human adaptive responses, and this interaction was responsible for changes in the frequencies of many alleles. Since the sequencing of the human genome in 2003, geneticists, armed with an array of new technologies, have been looking at the genes that govern adaptive traits in many populations. Specifically, they’ve been focusing on single nucleotide polymorphisms, or SNPs, studying how differences in single DNA bases alter gene action and how their frequencies vary between populations. Within the next few years, our understanding of many aspects of human adaptation will increase dramatically, owing to advances in genetic research that will allow the testing not only of long-held hypotheses but also of new ones.
To survive, all organisms need to maintain the normal functions of internal organs, tissues, and cells. What’s more, they must accomplish this task in the context of an everchanging environment. Even during the course of a single, seemingly uneventful day, there are numerous fluctuations in temperature, wind, solar radiation, humidity, and so on. Physical activity also places stress on physiological mechanisms. The body must accommodate all these changes in order to maintain internal constancy, or homeostasis, and all life forms have evolved physiological mechanisms that, within limits, achieve this goal.
Physiological responses to environmental change are influenced by genetic factors. We’ve already defined adaptation as a response to environmental conditions in populations and individuals. In a broader sense, adaptation refers to long-term evolutionary (that is, genetic) changes that characterize all individuals within a population or species.
Examples of long-term adaptations in humans include physiological responses to heat (sweating) or excessive levels of UV light (deeply pigmented skin near the equator). These characteristics are the results of evolutionary change in our species and they don’t vary because of short-term environmental change. For example, the ability to sweat isn’t lost in people who spend their lives in predominantly cool areas. Likewise, individuals born with dark skin won’t become pale, even if they’re never exposed to sunlight. Acclimatization is another kind of physiological response to changing environmental conditions in individuals. Most forms of acclimatization are temporary and last only until environmental conditions return to their former state. The physiological responses to environmental stressors are at least partially influenced by genetic factors, but some can also be affected by the duration and severity of the exposure, technological buffers (such as shelter or clothing), individual behavior, weight, and overall body size.
The simplest form of acclimatization is a temporary and rapid adjustment to an environmental change (for example, tanning). Another example is one you may not know about although you’ve probably experienced it. This is the rapid increase in hemoglobin production that occurs in low-altitude residents who travel to higher elevations. (It’s happened in your own body if you’ve spent a few days in the mountains.) In both these examples, the physiological changes are temporary. Tans fade when exposure to sunlight is reduced, and hemoglobin production drops to original levels after returning to lower elevations.
Another type of acclimatization, called developmental acclimatization, results from exposure to an environmental challenge during growth and development. Because this kind of acclimatization is incorporated into an individual’s physiology, it isn’t reversible. Certain physiological responses seen in lifelong residents of high altitude are examples of developmental acclimatization.
In this section, we present some of the many examples of how humans respond to environmental challenges. Some of these examples describe adaptations that characterize our entire species; others are shared by most or all members of only certain populations.