Cold and heat adaptations in humans
Cold and heat adaptations in humans are a part of the broad adaptability of Homo sapiens. Adaptations in humans can be physiological, genetic, or cultural, which allow people to live in a wide variety of climates. There has been a great deal of research done on developmental adjustment, acclimatization, and cultural practices, but less research on genetic adaptations to cold and heat temperatures.
The human body always works to remain in homeostasis. One form of homeostasis is thermoregulation. Body temperature varies in every individual, but the average internal temperature is 37.0 °C (98.6 °F). Stress from extreme external temperature can cause the human body to shut down. When the body becomes hypothermic, the core temperature drops to 35 °C (95 °F). Hyperthermia results when the core body temperature rises above 37.5-38.3 °C (99.5-100.9 °F). These temperatures commonly result in mortality. Humans have adapted to living in these extreme climates primarily through culture and technology, such as the use of clothing and shelter.
Origin of cold and heat adaptations
Modern humans emerged from Africa approximately 40,000 years ago. Evidence suggests that modern culture may have originated then, too. Modern humans spread into Europe and outcompeted Neanderthals, which suggests that early modern humans were adaptable enough to live in various climates. This is supported in the variability selection hypothesis proposed by Richard Potts, which says that human adaptability came from environmental change over the long term.
Bergmann’s rule states that endothermic animal subspecies living in colder climates have larger bodies than that of the subspecies living in warmer climates. Individuals with larger bodies are better suited for colder climates because larger bodies produce more heat due to having more cells, and have a smaller surface area compared to smaller individuals, which reduces heat loss. A study by Frederick Foster and Mark Collard found that Bergmann’s rule can be applied to humans when the latitude and temperature between groups differ widely.
Allen’s rule is a biological rule that says the limbs of an endotherm is either shorter in cold climates or longer in hot climates. Limb length affects the body’s surface area, which helps with thermoregulation. Shorter limbs help to conserve heat, while longer limbs help to dissipate heat. Marshall T. Newman argues that this can be observed in Eskimo, who have shorter limbs than other people and are laterally built.
Ambient air temperature affects how much energy investment the human body must make. The temperature that requires the least amount of energy investment is 21 °C (69.8 °F). The body controls its temperature through the hypothalamus. Thermoreceptors in the skin send signals to the hypothalamus, which indicate when vasodilation and vasoconstriction should occur.
The human body has two methods of thermogenesis, which produces heat to raise the core body temperature. The first is shivering, which occurs in an unclothed person when the ambient air temperature is under 25 °C (77 °F). It is limited by the amount of glycogen available in the body. The second is non-shivering, which occurs in brown adipose tissue.
The only mechanism the human body has to cool itself is by sweat evaporation. Sweating occurs when the ambient air temperatures is above 28 °C (80 °F) and the body fails to return to the normal internal temperature. The evaporation of the sweat helps cool the blood beneath the skin. It is limited by the amount of water available in the body, which can cause dehydration.
When humans are exposed to certain climates for extended periods of time, physiological changes occur to help the individual adapt to hot or cold climates. This helps the body conserve energy.
The Inuit have more blood flowing into their extremities, and at a hotter temperature, than people living in warmer climates. A 1960 study on the Alacaluf Indians shows that they have a resting metabolic rate 150 to 200 percent higher than the white controls used. Lapps do not have an increase in metabolic rate when sleeping, unlike non-acclimated people. Australian aborigines undergo a similar process, where the body cools but the metabolic rate does not increase.
Humans in Central Africa have been living in similar tropical climates for at least 40,000 years, which means that they have similar thermoregulatory systems.
A study done on the Bantus of South Africa showed that Bantus have a lower sweat rate than that of acclimated and nonacclimated whites. A similar study done on Australian aborigines produced similar results, with aborigines having a much lower sweat rate than whites.
Social adaptations enabled early modern humans to occupy environments with temperatures that were drastically different from that of Africa. (Potts 1998). Culture enabled humans to expand their range to areas that would otherwise be uninhabitable.
Humans have been able to occupy areas of extreme cold by creating microclimates, such as clothing, buildings, and manipulation of fire. Using energy to create technology such as furnaces has further enabled the occupation of cold environments.
Australian aborigines only wear genital coverings for clothes, but studies have shown that the warmth from the fires they build is enough to keep the body from fighting heat loss through shivering. Eskimos use well-insulated houses that are designed to transfer heat from an energy source to the living area, which means that the average indoor temperature for coastal Eskimos is 10 to 20 °C (50-68 °F).
Humans inhabit hot climates, both dry and humid, and have done so for thousands of years. Selective use of clothing and technological inventions such as air conditioning allows humans to thrive in hot climates.
One example is the Chaamba Arabs, who live in the Sahara Desert. They wear clothing that traps air in between skin and the clothes, preventing the high ambient air temperature from reaching the skin.
There has been very little research done in the genetics behind adaptations to heat and cold stress. Data suggests that certain parts of the human genome have only been selected for recently. Research on gene-culture interaction has been successful in linking agriculture and lactose tolerance. However, most evidence of links between culture and selection has not been proven.
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