Cold and heat adaptations in humans

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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 colder and hotter temperatures.

Contents

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). [1] Sufficient stress from extreme external temperature may cause injury or death if it exceeds the ability of the body to thermoregulate. Hypothermia can set in when the core temperature drops to 35 °C (95 °F). [2] Hyperthermia can set in when the core body temperature rises above 37.5–38.3 °C (99.5–100.9 °F). [3] [4] Humans have adapted to living in climates where hypothermia and hyperthermia were common primarily through culture and technology, such as the use of clothing and shelter. [5]

Origin of cold and heat adaptations

Modern humans emerged from Africa approximately 70,000 years ago during a period of unstable climate, leading to a variety of new traits among the population. [6] [5] When modern humans spread into Europe, they outcompeted Neanderthals. Researchers hypothesize that this suggests early modern humans were more evolutionarily fit to live in various climates. [7] [8] 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. [9]

Ecogeographic rules

Bergmann's rule states that endothermic animal subspecies living in colder climates have larger bodies than those of the subspecies living in warmer climates. [10] 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 to volume ratio 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. [11]

Allen's rule is a biological rule that says the limbs of endotherms are shorter in cold climates and 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. [12] Marshall T. Newman argues that this can be observed in Eskimo, who have shorter limbs than other people and are laterally built. [13]

Physiological adaptations

Origins of heat and cold adaptations can be explained by climatic adaptation. [14] [15] 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). [5] 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.

Cold

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). [16] It is limited by the amount of glycogen available in the body. [5] The second is non-shivering, which occurs in brown adipose tissue. [17]

Population studies have shown that the San tribe of Southern Africa and the Sandawe of Eastern Africa have reduced shivering thermogenesis in the cold, and poor cold-induced vasodilation in fingers and toes compared to that of Caucasians. [5]

Heat

The only mechanism the human body has to cool itself is by sweat evaporation. [5] Sweating occurs when the ambient air temperature is above 35 °C (95 °F) and the body fails to return to the normal internal temperature. [16] 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. [5]

Humans adapted to heat early on. In Africa, the climate selected for traits that helped them stay cool. Also, humans had physiological mechanisms that reduced the rate of metabolism and that modified the sensitivity of sweat glands to provide an adequate amount for cooldown without the individual becoming dehydrated. [15] [18]

There are two types of heat the body is adapted to, humid heat and dry heat, but the body has adapted to both in the same way. Humid heat is characterized by warmer temperatures with a high amount of water vapor in the air. Humid heat is dangerous as the moisture in the air prevents the evaporation of sweat. Regardless of acclimatization, humid heat poses a far greater threat than dry heat; humans cannot carry out physical outdoor activities at any temperature above 32 °C (90 °F) when the ambient humidity is greater than 95%. When combined with this high humidity, the theoretical limit to human survival in the shade, even with unlimited water, is 35 °C (95 °F) – theoretically equivalent to a heat index of 70 °C (158 °F). [19] [20] Dry heat is characterized by warmer temperatures with little to no water vapor in the air, such as desert conditions. Dry heat is also very dangerous as sweat will tend to evaporate extremely quickly, causing dehydration. Both humid heat and dry heat favor individuals with less fat and slightly lower body temperatures. [21]

Acclimatization

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. [17]

Cold

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. The Sami do not have an increase in metabolic rate when sleeping, unlike non-acclimated people. [13] Aboriginal Australians undergo a similar process, where the body cools but the metabolic rate does not increase. [16]

Heat

Humans and their evolutionary predecessors in Central Africa have been living in similar tropical climates for millions of years, which means that they have similar thermoregulatory systems. [5]

A study done on the Bantus of South Africa showed that Bantus have a lower sweat rate than that of acclimated and nonacclimated white people. A similar study done on Aboriginal Australians produced similar results, with Aboriginals having a much lower sweat rate than white people. [16]

Culture

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. [16]

Cold

Humans have been able to occupy areas of extreme cold through clothing, buildings, and manipulation of fire. Furnaces have further enabled the occupation of cold environments. [16] [17]

Historically many Indigenous Australians wore only genital coverings. Studies have shown that the warmth from the fires they build is enough to keep the body from fighting heat loss through shivering. [16] Inuit 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 Inuit is 10 to 20 °C (50-68 °F). [16]

Heat

Humans inhabit hot climates, both dry and humid, and have done so for millions 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, 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. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Humidity</span> Concentration of water vapour in the air

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.

<span class="mw-page-title-main">Dew point</span> Temperature at which air becomes saturated with water vapour during a cooling process

The dew point of a given body of air is the temperature to which it must be cooled to become saturated with water vapor. This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs through the air's contact with a colder surface, dew will form on that surface.

<span class="mw-page-title-main">Hyperthermia</span> Elevated body temperature due to failed thermoregulation

Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme temperature elevation occurs, it becomes a medical emergency requiring immediate treatment to prevent disability or death. Almost half a million deaths are recorded every year from hyperthermia.

<span class="mw-page-title-main">Perspiration</span> Fluid secreted from sudoriferous glands

Perspiration, also known as sweat, is the fluid secreted by sweat glands in the skin of mammals.

<span class="mw-page-title-main">Hypothermia</span> Human body core temperature below 35.0 °C (95.0 °F)

Hypothermia is defined as a body core temperature below 35.0 °C (95.0 °F) in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe hypothermia, there may be hallucinations and paradoxical undressing, in which a person removes their clothing, as well as an increased risk of the heart stopping.

<span class="mw-page-title-main">Heat wave</span> Prolonged period of excessively hot weather

A heat wave, sometimes known as extreme heat, is a period of abnormally hot weather. High humidity often accompanies heat waves. This is especially the case in oceanic climate countries. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be called a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s. This is due to climate change.

<span class="mw-page-title-main">Endotherm</span> Organism that maintains body temperature largely by heat from internal bodily functions

An endotherm is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as shivering, and uncoupled oxidative metabolism, such as within brown adipose tissue.

<span class="mw-page-title-main">Thermoregulation</span> Ability of an organism to keep its body temperature within certain boundaries

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature as its own body temperature, thus avoiding the need for internal thermoregulation. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. Humans may also experience lethal hyperthermia when the wet bulb temperature is sustained above 35 °C (95 °F) for six hours. Work in 2022 established by experiment that a wet-bulb temperature exceeding 30.55°C caused uncompensable heat stress in young, healthy adult humans. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It results when the homeostatic control mechanisms of heat within the body malfunction, causing the body to lose heat faster than producing it. Normal body temperature is around 37°C(98.6°F), and hypothermia sets in when the core body temperature gets lower than 35 °C (95 °F). Usually caused by prolonged exposure to cold temperatures, hypothermia is usually treated by methods that attempt to raise the body temperature back to a normal range. It was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained. It was then found that local differences were present, since heat production and heat loss vary considerably in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to identify the parts of the body that most closely reflect the temperature of the internal organs. Also, for such results to be comparable, the measurements must be conducted under comparable conditions. The rectum has traditionally been considered to reflect most accurately the temperature of internal parts, or in some cases of sex or species, the vagina, uterus or bladder.

<span class="mw-page-title-main">Bergmann's rule</span> Biological rule stating that larger size organisms are found in colder environments

Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments. Bergmann's rule only describes the overall size of the animals, but does not include body proportions like Allen's rule does.

<span class="mw-page-title-main">Evaporative cooler</span> Device that cools air through the evaporation of water

An evaporative cooler is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning systems, which use vapor-compression or absorption refrigeration cycles. Evaporative cooling exploits the fact that water will absorb a relatively large amount of heat in order to evaporate. The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation). This can cool air using much less energy than refrigeration. In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants.

<span class="mw-page-title-main">Allen's rule</span> Relation of habitat temperature and limb length

Allen's rule is an ecogeographical rule formulated by Joel Asaph Allen in 1877, broadly stating that animals adapted to cold climates have shorter and thicker limbs and bodily appendages than animals adapted to warm climates. More specifically, it states that the body surface-area-to-volume ratio for homeothermic animals varies with the average temperature of the habitat to which they are adapted.

<span class="mw-page-title-main">Wet-bulb temperature</span> Temperature read by a thermometer covered in water-soaked cloth

The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature ; at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.

<span class="mw-page-title-main">Thermal comfort</span> Satisfaction with the thermal environment

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation. The human body can be viewed as a heat engine where food is the input energy. The human body will release excess heat into the environment, so the body can continue to operate. The heat transfer is proportional to temperature difference. In cold environments, the body loses more heat to the environment and in hot environments the body does not release enough heat. Both the hot and cold scenarios lead to discomfort. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

Normal human body temperature is the typical temperature range found in humans. The normal human body temperature range is typically stated as 36.5–37.5 °C (97.7–99.5 °F).

<span class="mw-page-title-main">Effects of high altitude on humans</span> Environmental effects on physiology and mental health

The effects of high altitude on humans are mostly the consequences of reduced partial pressure of oxygen in the atmosphere. The medical problems that are direct consequence of high altitude are caused by the low inspired partial pressure of oxygen, which is caused by the reduced atmospheric pressure, and the constant gas fraction of oxygen in atmospheric air over the range in which humans can survive. The other major effect of altitude is due to lower ambient temperature.

<span class="mw-page-title-main">Eurytherm</span> Organism tolerant of a wide temperature range

A eurytherm is an organism, often an endotherm, that can function at a wide range of ambient temperatures. To be considered a eurytherm, all stages of an organism's life cycle must be considered, including juvenile and larval stages. These wide ranges of tolerable temperatures are directly derived from the tolerance of a given eurythermal organism's proteins. Extreme examples of eurytherms include Tardigrades (Tardigrada), the desert pupfish, and green crabs, however, nearly all mammals, including humans, are considered eurytherms. Eurythermy can be an evolutionary advantage: adaptations to cold temperatures, called cold-eurythemy, are seen as essential for the survival of species during ice ages. In addition, the ability to survive in a wide range of temperatures increases a species' ability to inhabit other areas, an advantage for natural selection.

<span class="mw-page-title-main">Xerocole</span> Any animal adapted to live in the desert

A xerocole, is a general term referring to any animal that is adapted to live in a desert. The main challenges xerocoles must overcome are lack of water and excessive heat. To conserve water they avoid evaporation and concentrate excretions. Some are so adept at conserving water or obtaining it from food that they do not need to drink at all. To escape the desert heat, xerocoles tend to be either nocturnal or crepuscular.

Endothermic organisms known as homeotherms maintain internal temperatures with minimal metabolic regulation within a range of ambient temperatures called the thermal neutral zone (TNZ). Within the TNZ the basal rate of heat production is equal to the rate of heat loss to the environment. Homeothermic organisms adjust to the temperatures within the TNZ through different responses requiring little energy.

Thermal Work Limit (TWL) is defined as the limiting sustainable metabolic rate that well-hydrated, acclimatized individuals can maintain in a specific thermal environment, within a safe deep body core temperature and sweat rate. The index is designed for self-paced workers and does not rely on estimation of actual metabolic rates, a process that is difficult and subject to considerable error. The index has been introduced into the United Arab Emirates and Australia, resulting in a substantial and sustained fall in the incidence of heat illness in the latter.

As in other mammals, thermoregulation in humans is an important aspect of homeostasis. In thermoregulation, body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles. Humans have been able to adapt to a great diversity of climates, including hot humid and hot arid. High temperatures pose serious stress for the human body, placing it in great danger of injury or even death. For humans, adaptation to varying climatic conditions includes both physiological mechanisms resulting from evolution and behavioural mechanisms resulting from conscious cultural adaptations.

References

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  2. ↑ Brown, Douglas J. A.; Brugger, Hermann; Boyd, Jeff; Paal, Peter (2012-11-15). "Accidental Hypothermia". The New England Journal of Medicine . 367 (20): 1930–38. doi:10.1056/NEJMra1114208. ISSN   0028-4793. PMID   23150960. S2CID   205116341.
  3. ↑ Axelrod, Yekaterina K.; Diringer, Michael N. (2008). "Temperature Management in Acute Neurologic Disorders". Neurologic Clinics . 26 (2): 585–603. doi:10.1016/j.ncl.2008.02.005. PMID   18514828.
  4. ↑ Laupland, Kevin B. (2009). "Fever in the critically ill medical patient". Critical Care Medicine . 37 (Supplement): S273–78. doi:10.1097/ccm.0b013e3181aa6117. PMID   19535958. S2CID   21002774.
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  6. ↑ Krajick, Kevin (2017-10-05). "Ancient Humans Left Africa to Escape Drying Climate, Says Study". State of the Planet. Columbia University. Retrieved 4 December 2018.
  7. ↑ Daley, Jason. "Climate Change Likely Iced Neanderthals Out Of Existence". Smithsonian Magazine. Smithsonian. Retrieved 4 December 2018.
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  9. ↑ Potts, Richard (1998-01-01). "Environmental hypotheses of hominin evolution". American Journal of Physical Anthropology. 107 (S27): 93–136. doi:10.1002/(sici)1096-8644(1998)107:27+<93::aid-ajpa5>3.0.co;2-x. ISSN   1096-8644. PMID   9881524.
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  11. ↑ Foster, Frederick; Collard, Mark (2013-08-28). "A Reassessment of Bergmann's Rule in Modern Humans". PLOS ONE. 8 (8): e72269. Bibcode:2013PLoSO...872269F. doi: 10.1371/journal.pone.0072269 . ISSN   1932-6203. PMC   3756069 . PMID   24015229.
  12. ↑ Holliday, Trenton W.; Hilton, Charles E. (2010-06-01). "Body proportions of circumpolar peoples as evidenced from skeletal data: Ipiutak and Tigara (Point Hope) versus Kodiak Island Inuit". American Journal of Physical Anthropology. 142 (2): 287–302. doi:10.1002/ajpa.21226. ISSN   1096-8644. PMID   19927367.
  13. 1 2 Newman, Marshall T. (1961-06-01). "Biological Adaptation of Man to His Environment: Heat, Cold, Altitude, and Nutrition". Annals of the New York Academy of Sciences. 91 (3): 617–633. Bibcode:1961NYASA..91..617N. doi: 10.1111/j.1749-6632.1961.tb31093.x . ISSN   1749-6632. PMID   13728673.
  14. ↑ The Editors of Encyclopaedia Britannica. "Climatic adaptation". Encyclopædia Britannica. Encyclopædia Britannica, inc. Retrieved 4 December 2018.{{cite web}}: |last1= has generic name (help)
  15. 1 2 Smithsonian Institution. "Climate Effects on Human Evolution" . Retrieved 20 May 2023.
  16. 1 2 3 4 5 6 7 8 9 Frisancho, A. Roberto (1993-01-01). Human adaptation and accommodation. University of Michigan Press. OCLC   26763611.
  17. 1 2 3 Kaciuba-Uscilko, Hannah; Greenleaf, John (April 1, 1989). "Acclimatization to cold in humans" (PDF). ntrs.nasa.gov/. United States. Retrieved 2016-12-15.
  18. ↑ Taylor, N.A. (2014). "Human Heat Adaptation". Comprehensive Physiology. 4 (1): 325–365. doi:10.1002/cphy.c130022. ISBN   9780470650714. PMID   24692142.
  19. ↑ A Dictionary of Weather. Oxford Reference. 2008. ISBN   978-0-19-954144-7.
  20. ↑ "Potentially fatal combinations of humidity and heat are emerging across the globe".
  21. ↑ "Climatic adaptation | physical anthropology | Britannica". www.britannica.com. Retrieved 2023-02-08.
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