Human Adaptations to Extreme Cold
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Humans have successfully adapted to environmental stresses, including extreme cold. A review of existing literature examining archaeological data, historical data, and current populations regarding human adaptation to cold stress (average annual temperature of ~ -14°C / 7°F) yields evidence supporting distinctive morphological, physiological and behavioral traits that compensate for the stress. Morphological adaptations can be seen in elongated and narrowed nasal passages (long narrow noses), which help warm and hydrate the air before it passes into the lungs; and a decreased surface area to volume ratio and allowing the individuals to more easily maintain a normal core body temperature. Physiological adaptations include: increased basal metabolic rate, which increases the amount of body heat produced; and a higher prevalence of type 1 diabetes, which may be a genetic adaptation that protects cells from freezing. Additionally behavioral adaptations can be observed in agriculture, awareness of fickle environmental factors, and clothing. Traditional populations in sub-polar regions tend to be hunter foragers; agriculture is limited to what can be cultivated in the short growing season. Some populations demonstrate acute awareness of weather patterns, modifying their behaviors to minimize exposure to dangerous conditions while taking full advantage of more temperate periods. The author finds ample evidence of widely varied human adaptations to extremely cold environments which allow sub-arctic populations to survive more easily in their specific environments.
Human Adaptations to Extreme Cold
Humans have successfully adapted morphologically, physiologically, and behaviorally to environmental stress, including extreme cold. As a species, humans have survived Ice Ages which brought the intensely cold environment, normal for our polar and sub-polar regions, across much of the planet. To understand the wide variety of adaptations which have proven successful, the author explored existing literature analyzing data from archaeological, historical, and anecdotal sources, as well as from current populations. Because more information involving human populations exists for inhabitants of sub-polar regions than for any other extremely cold environments (except high-altitude locales where oxygen level is a significant contributing factor for adaptation and would complicate conclusions which might be drawn about adaptations to cold stress) the author focused there. 'Extreme cold' is defined, in terms of environment, by examining monthly mean temperature charts provided online by the U.S. Navy (Guest, 2000). These data show that monthly mean temperatures at sub-polar locations, both north and south, range from -30°C (-22°F) in January to +5° C (+41°F) in July, with daily variations from -40°C (-40°F) to +30°C (+86°F) yielding average annual temperatures ~ -14°C (7°F). Ample evidence exists to support conclusions that morphological, physiological, and behavioral adaptations have occurred in response to the stress of existence in extremely cold environments.
Morphological adaptations can be seen in elongated and narrowed nasal passages, broad pelves, and relatively short, stocky bodies. (Kennedy 2007; Hernandez, Fox, Garcia-Moro 1997). "Fueguians and the Eskimos are the human groups with the narrowest and highest nasal apertures, displaying a combination of large nasal height and low nasal breadth values, while groups from equatorial areas have low, wide nasal passages" (Herná, et al. 1997). Both groups lived and/or live in the sub-polar regions (one nearer the southern pole, one nearer the northern). Fueguians inhabited Tierra del Fuego, the southernmost tip of South American after the ice sheets receded, ~ 10,000 to 12,000 BP (before present). Unlike the sub-Arctic environment, which is cold and dry, the climate of Tierra del Fuego is extremely cold, rainy, foggy, and windy. Average temperatures are in line with our definition of 'extreme cold,' but in addition the area receives ~3000 mm (118 in) of rain each year and strong, persistent winds that blow off the glaciers, inducing a significant windchill affect. (Herná, et al. 1997, and references therein) took craniometric (measurments of the skull) measurements of 180 skulls from three distinct tribal groups of the area and analyzed them in relation to Howells' 28 craniometric series in order to increase the statistical significance of the sample. When all the measurements were plotted on a climate map, a strong correlation between increased nasal height combined with narrow breadth and extremely low temperatures is apparent. Researchers postulate that high, narrow nasal openings allow frigid air to be warmed by the mucous membranes lining the nasal cavity to prevent damage to delicate lung tissue, and enhance "the recovery of heat and moisture from expired air." (Herná, et al. 1997)
Another morphological adaptation supported by existing studies is a short, stocky body structure. "Body proportions of humans [and other endothermic (i.e., ''warm-blooded'') species] have long been known to show significant correlations with climatic variables and their proxies. Specifically, two empirically derived ecogeographical rules, those of Bergmann (1847) and Allen (1877), state that within a widespread endothermic species, those in colder regions will tend to weigh more (Bergmann's rule) and be characterized by shorter appendages (Allen's rule) than their conspecifics [members of the same species] in warmer climes." (Holliday and Hilton, 2010 and references therein). They also put forward "colder-climate groups being characterized by broader pelves," and reference C.B Ruff's work from the early 1990s. Holliday and Hilton (2010) examine skeletal data from the Point Hope Inuit (another name for Eskimo) of North America. A total of 173 individuals, 127 from the Tigara period (13th to 17th century AD) and 46 from the Ipiutak period (~100 BC to 500 AD) were measured and analyzed relative to other Native North Americans, and samples from Europe, North Africa, Sub-Saharan Africa (from similar periods). Based on results from previous studies referenced, Holliday and Hilton concentrated their effort on measurements which have already been determined to vary with climate, "specifically limb bones from the four major limb segments, femoral head diameter, skeletal trunk height (the summed dorsal body heights of T1-L5 plus sacral ventral length), and bi-iliac breadth [pelvic width]." (Holliday and Hilton, 2010). From the basic measurements, the authors computed seven ratios which are identified as indices for comparison. Results show that African samples provide the lowest indices while circumpolar populations show the highest, with European numbers in the middle. Neither of the groups measured specifically for this study (nor the third Native North American sample) is significantly different from the other, but marked variations exist between these groups and both of the African groups. Interestingly, results do not support the authors' expectation that the Inuit and Europeans would show a discernible variation using the specific indices studied. However, the bi-iliac relative breadth index (pelvic breadth compared to assumed trunk height) did separate these two groups distinctly. As a counter-point, it is noted that there are other factors which can affect overall stature, such as under-nutrition. In a harsh environment, maintaining sufficient nutritional intake is likely compromised, and so the shorter body may not be simply an adaptation to the extremely cold environment.
Popular rhetoric holds that a layer of body fat helps keep humans, and other mammals, warm. In his 2007 American Journal of Human Biology article, "Human cold adaptation: An unfinished agenda" Steegmann does not disagree; he says, "Fat insulates better than muscle per unit of thickness. However, in a fit person, muscle layers are usually much thicker than subcutaneous fat and consequently have higher absolute insulative value." Studies in the 1950s and 1960s (referenced in Elsner (1963): LeBlanc, 1954; Baker and Daniels, 1956; Daniels, et al, 1961) demonstrated that Caucasians with a thicker layer of body fat, as measured by skinfold, maintained core temperature, skin temperature, and metabolic rate more reliably when exposed to 15° C (59°F) for two hours. However, in a similar study (Elsner, 1963) compared the skinfold thickness of eight hunter-gatherer groups (aborigines of central and northern Australia, Inuit of Canada, Eskimos, Alacaluf Indians of southern Chile, Lapps, Peruvian Indians, and Kalahari bushmen), and cold-acclimatized Norwegian students, with urban Caucasians as a control. Skinfold thickness was measured at ten locations: abdomen, back (subscapular), calf, cheek, chin, iliac crest, knee, pectoral, upper arm,and side. The urban Caucasian control group had higher values across the board, except for the cheek measurement. Of particular interest, Canadian Inuit, and Eskimos had amongst the lowest values; not what was expected from populations that acquire 70-75% of their caloric intake (see above) from animal fat. Additionally he measured the rectal temperature, metabolic rate, and skin temperature of his subjects during an eight-hour sleep period with ambient room temperature of 0° - 5°C (32° - 41°F) during which time they had only one thin blanket to wrap up in. Elsner reports that there was poor correlation between skinfold thickness and the measurements of interest during the overnight study. In support of these findings, from another study, Steegman (2007) reports results which demonstrate that Inuit "traditionally had high muscle mass and high work capacity, but low body fat." Aside from the subjective observation that the "primitive" groups had "better sleep" than the control group, three sets of reactions emerged from Elsner's study: 1) Canadian Inuit, Eskimos, and Alacaluf Indians, and cold-acclimatized Norwegian students demonstrated high metabolic rates (measurement technique not defined) and warm extremities; 2) Kalahari bushmen and aborigines from central Australia had stable or falling metabolic rate and cooler skin; and 3) Peruvian Indians and Lapps had low rectal temperatures and higher extremity temperatures. So, while a thicker layer of body fat does not seem to be a human adaptation for survival in extremely cold environments, increased metabolic rate and some protective mechanism to keep extremities warm both appear likely. (Makinen, 2007)
Physiological adaptations include: increased basal metabolic rate, high protein/high fat/low carbohydrate nutritional requirements, and some evidence of variations in blood chemistry. (Westerterp-Plantenga 1999; Srivastava, Kumar 1991; Moalem, Storey, Percy, Peros, Perl 2004)
"...An inverse relationship between BMR and mean annual temperature has been documented, which holds true even when controlled for differences in body size." (Snodgrass, et all 2005) In fact, Snodgrass, et al (2005) conducted extensive research among the Yakut population in Siberia (sub-polar Asia) which supports the claim that increased basal metabolic rate is an important human adapation to the stress of an extremely cold environment. With a thorough and well-documented scientific process, participants in the Snodgrass study underwent measurements of core temperature, oxygen consumption, carbon dioxide production, and heart rate in a thermoneutral (23° - 27°C) environment after a 12-hour fast. Results for basal metabolic rate (BMR) were predicted based on three standards drawn from a European population: fat-free mass (FFM), surface area (SA), and body mass. In all three cases, for males and females, the Yakut BMR measured significantly higher than predicted values. The BMR of Yakut men and women were demonstrably elevated over their more southern-dwelling, European counterparts. Another metabolic adaptation might be seen in the increased incidence of Type 1 diabetes mellitus among northern Europeans. Moalem, et al (2004) "Recent animal research has uncovered the importance of the generation of elevated levels of glucose, glycerol and other sugar derivatives as a physiological means for cold adaptation. High concentrations of these substances depress the freezing point of body fluids and prevent the formation of ice crystals in cells through supercooling, thus acting as a cryoprotectant or antifreeze for vital organs as well as in their muscle tissue." Citing the example of cystic fibrosis conferring immunity to typhoid (salmonella typhi), the authors suggest that elevated blood glucose levels, such as are seen when the body does not produce insulin, may be the result of genetic mutation which gave an evolutionary advantage to inhabitants of cold climates about 14,000 years ago when world-wide temperatures dropped dramatically. Life expectancies then were short, so genetic adaptations that enhanced survival would have favored changes in the short term. Now that our life expectancies have increased to 70+ years, we can observe that such changes might have been beneficial then, but currently are causing dangerous health issues within the aging population.
Traditional dietary intake of these populations of cold-dwellers depends completely on what is available at any given time. In 2004 Patricia Cochran, a native Inuit Alaskan, wrote on the traditional diet for Discovermagizine.com. "Our meat was seal and walrus, marine mammals that live in cold water and have lots of fat. We used seal oil for our cooking and as a dipping sauce for food. We had moose, caribou, and reindeer. We hunted ducks, geese, and little land birds like quail, called ptarmigan. We caught crab and lots of fish-salmon, whitefish, tomcod, pike, and char. Our fish were cooked, dried, smoked, or frozen. We ate frozen raw whitefish, sliced thin. The elders liked stinkfish, fish buried in seal bags or cans in the tundra and left to ferment. And fermented seal flipper, they liked that too." She reports that in the short summers the villagers would forage for roots, greens, and berries.. "What the diet of the Far North illustrates," says Harold Draper, a biochemist and expert in Eskimo nutrition, "is that there are no essential foods-only essential nutrients. And humans can get those nutrients from diverse and eye-opening sources." Inhabitants of extremely cold climates do not live to eat, they eat to live. The traditional Inuit diet, which seems to a Westerner to be sorely lacking in fruits and vegetables, which the U.S. government insists are necessary for wellness, supplies all they need to maintain health in their sub-polar climate. Vitamin C, which is a vital component for healthy connective tissue, is found in raw animal organs, raw kelp, and even muktuk, which is as rich in Vitamin C as orange juice, gram for gram.Fat-soluble vitamins A and D are metabolically mined from cold-water fish and mammal fats and livers. Not surprising, then, that the traditional Inuit diet comprised 90% of its caloric intake from meat and fish, 50-70% of its calories specifically from wild animal fat - fat is the source of not only calories but also necessary nutrients.
This traditional Inuit diet based wholly on what food is available from hunting, fishing and forage-harvesting might be a behavioral/cultural adaptation to the climate, while also encompassing metabolic/digestive adaptations. While morphological and physiological adaptations to environment take eons to manifest, some cultural and social adaptations may be apparent on a far shorter time scale. Steegmann (2007, and references therein) speaks about Richard K. Nelson's comparison of Kutchin natives of east-central Alaska to Eskimos, explaining Nelson's observation that Kutchin hunters keep moving if they lose their way, afraid if they stop they will sleep and freeze. Eskimo rest as needed and only move to stay warm. He also noted that Eskimo had a complex understanding of weather prediction and were better equipped to plan accordingly and keep themselves safe. "In both cases, Eskimos seem to practice higher survival skills and both behaviors are strongly directed by cultural traditions." Two very different responses to the same stimuli in similar environments, with potentially diametrically opposed results: survival and death. Another surprising and non-intuitive variation in responses to the extreme cold of sub-polar life can be found in the clothing styles of arctic and some sub-arctic populations. According to Herná, et al. (1997) arctic inhabitants, such as the Inuit, wear clothing designed to protect them from the harsh cold, whereas the three Fuegian tribes they study, who lived at the southern tip of South America, are anecdotally described as "almost naked throughout their lives." The Fuegian tribes are extinct, so no opportunities to explore their cultural adaptations to their extreme environment.
Human adapation to the stress of an extremely cold environment, such as those of sub-polar regions, can be seen in morphological changes, physiological changes, and behavioral/cultural developments. Morphological changes include long, narrow nasal passages, to pre-warm icy air and protect fragile lung tissues and short, stocky body structure, which increases the body mass to surface area ratio, conserving body heat. Physiologically, increased basal metabolic rate is strongly supported as an adaptation, in a contemporary population, to the extremely cold climate of Siberia. An increased incidence of Type 1 diabetes in cold climates is suggested as a favorable mutation during the rapid onset of a mini Ice Age, but more studies would be needed to prove this as a lasting adaptation. Changes in metabolism and digestion in order to extract necessary nutrients from the limited food sources available in a sub-polar climate may be a physiological adaptation, but without studies to demonstrate a change in how the Inuit (or other sub-polar inhabitant) body processes food in order to extract necessary nutrients, it should be categorized as a behavioral/cultural adaptations. They eat to live, utilizing all food sources available. Other behavioral adaptations can be observed in a more precise 'weather awareness,' perhaps, and clothing styles.
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