Humans, all other organisms, and even ecological systems, live in an environment of constant change. The persistently shifting, modulating, and changing milieu would not permit survival, if it were not for the capacity of biological systems to respond to this constant flux by maintaining a relatively stable internal environment. An example taken from mammalian biology is temperature which appears to be "fixed" at approximately 98.6°F (37°C). While humans can be exposed to extreme summer heat, and arctic mammals survive intense cold, body temperature remains constant within vary narrow limits. Homeostasis is the sum total of all the biological responses that provide internal equilibrium and assure the maintenance of conditions for survival.

The human species has a greater variety of living conditions than any other organism. The ability of humans to live and reproduce in such diverse circumstances is due to a combination of homeostatic mechanisms coupled with cultural...