Humans are adapted to numerous ecosystems on earth. This is only possible due to how our physiological systems interact with the environment in a homeodynamic way. This allowed us to spread across the globe, far away from our ancestral home of Africa, and thusly certain adaptations evolved in those populations—which was driven by our intelligent physiology. I will touch on human cold and hot adaptations, how physiology adapts to the two climates and what this means for the populations that make up Mankind.
Physiological adaptations to Arctic climates
The human body is one of the most amazing and complex biological systems on earth. The human body lives and dies on its physiology and how it can adapt to novel environments. When Man first trekked out of Africa into novel environments, our physiology adapted so we could survive in novel conditions. Over time, our phenotypes adapted to our new climates and humans began looking different from one another due to the climatic differences in their environments.
There is a large body of work on human cold adaptation. Thermal balance in humans is maintained by “vasodilation/vasoconstriction of the skin and peripheral tissues within the so-called thermo-neutral zone” (Daanen and Lichtenbelt, 2016). Two other adaptations occur in the cold: shivering thermogenesis (ST) and non-shivering thermogenesis (NST) and one in the heat (the evaporation of sweat). Humans are not Arctic animals by nature, so, therefore, venturing into novel environments would incur new physiological adaptations to better deal with the cold.
Heat is induced by the body in cold climates by shivering (Tikuisis, Bell, and Jacobs, 1991; Daanen and Lichtenbelt, 2016). So, therefore, people in colder climates will have higher metabolisms than people in tropical environments, to generate more body heat for vital functioning. People living in Arctic environments have fewer sweat glands than people who live in the tropics. Sweating removes heat from the body, so having more sweat glands in colder climates would not be conducive for survival.
People who evolved in Arctic climates would also be shorter and have wider pelves than people who evolved in the tropics. This is seen in Neanderthals and is an example of Cold adaptations also show up in the Greenlandic Inuit due to extinct hominins like the Denisova (Fumagalli et al, 2015).
We can see natural selection at work in the Inuits, due to adaptation to Arctic climates (Galloway, Young, and Bjerregaard, 2012; Cardona et al, 2014; Ford, McDowell, and Pierce, 2015; NIH, 2015; Harper, 2015; Tishkoff, 2015). Climate change is troubling to some researchers, with many researchers suggesting that global warming will have negative effects on the health and food security of the Inuit (WHO, 2003; Furgal and Seguin, 2006; Wesche, 2010; Ford, 2009, 2012; Ford et al, 2014, 2016; McClymont and Myers, 2012; Petrasek, 2014; Petrasek et al, 2015; Rosol, Powell-Hellyer, and Chan, 2016). This Inuit are the perfect people to look to to see how humans adapt to novel climates—especially colder ones. They have higher BMIs which is better for heat retention, and larger brains with wider pelves and a shorter stature.
Metabolic adaptations also occur due to BMI, which would occur due to diet and body composition. Daanen and Lichtenbelt, (2016) write:
Bakker et al.,48 however, showed that Asians living in Europe had lower BAT prevalence and exhibited a poorer shivering and non-shivering response to cold than Caucasians of similar age and BMI. On the other hand, subjects living in polar regions have higher BMI, and likely more white fat for body energy reserves and insulation.49 This cannot be explained by less exercise,50 but by body composition51 and food intake.49
Basal metabolic rate (BMR) also varies by race. Resting metabolic rate is 5% higher in white women when compared to black women (Sharp et al, 2002). Though low cardiovascular fitness explains 25 percent of the variance in RMR differences between black and white women (Shook et al, 2014). People in Arctic regions have a 3-19 higher BMR than predicted on the basis of the polar climates they lived in (Daanen and Lichtenbelt, 2016). Further, whites had a higher BMR than Asians living in Europe. Nigerian men were seen to have a lower BMR than African-American men (Sharp et al, 2002). So, whites in circumpolar locales have a higher BMR than peoples who live closer to the equator. This has to do with physiologic and metabolic adaptations.
Blacks also show slower and lower cold induced vasodilation (CIVD) than whites. A quicker CIVD in polar climates would be a lifesaver.
However, just our physiologic mechanisms alone aren’t enough to weather the cold. Our ingenuity when it comes to making clothes, fire, and finding and hunting for food are arguably more important than our bodies physiologic ability to adapt to its present environment. Our behavioral plasticity (ability to change our behavior to better survive in the environment) was also another major factor in our adaptation to the cold. Then, cultural changes would lead to genetic changes, and those cultural changes—which were due to the cold climates—would then lead to more genetic change and be an indirect effect of the climate. The same, obviously, holds for everywhere in the world that Man finds himself in.
Physiologic changes to tropical climates
Physiologic changes in tropical climates are very important to us as humans. We needed to be endurance runners millions of years ago, and so our bodies became adapted for that way of life through numerous musculoskeletal and physiologic changes (Lieberman, 2015). One of the most important is sweating.
Sweating is how our body cools itself and maintains its body temperature. When the skin becomes too hot, your brain, through the hypothalamus, reacts by releasing sweat through tens of millions of eccrine glands. As I have covered in my article on the evolution of human skin variation, our loss of fur (Harris, 2009) in our evolutionary history made it possible for sweat to eventually cool our body. Improved sweating ability then led to higher melanin content and selection against fur. Another hypothesis is that when we became bipedal, our bodies were exposed to less solar radiation, selecting against the need for fur. Yet another hypothesis is that trekking/endurance running led to selection for furlessness, selecting for sweating and more eccrine glands (Lieberman, 2015).
Anatomic changes include long and thin bodies with longer limbs as heat dissipation is more efficient. People who live in tropical environments have longer limbs than people who live in polar environments. These tall and slender bodies are what is useful in that environment. People with long, slender bodies are disadvantaged in the cold. Further, longer, slender bodies are better for endurance running and sprinting. They also have narrower hips which helps with heat dissipation and running—which means they would have smaller heads than people in more northerly climes. Most adaptations and traits were once useful in whichever environment that organism evolved in tens of thousands of years ago. And certain adaptations from our evolutionary past are still evident today.
Since tropical people have lower BMRs than people at more northerly climes, this could also explain why, for instance, black American women, have higher rates of obesity than women of other races. They have a lower BMR and are sedentary and eat lower-quality food so food insecurity would have more of an effect on that certain phenotype. Africans wouldn’t have fast metabolisms since a faster metabolism would generate more heat.
Physiologic changes due to altitude
The last adaptation I will talk about is how our bodies can adapt to high altitudes and how it’s beneficial. Many human populations have adapted to the chronic hypoxia of high latitudes (Bigham and Les, 2014) which, of course, has a genetic basis. Adaptation to high altitudes also occurred due to the introgression of extinct hominin genes into modern humans.
Furthermore, people in the Andean mountains, people living in the highlands of Kenya and people living on the Tibetan plateau have shown that the three populations adapted to the same stress through different manners. Andeans, for instance, breathe the same way as people in lower latitudes but their red blood cells carry more oxygen per cell, which protects them from the effects of hypoxia. They also have higher amounts of hemoglobin in their blood in comparison to people who live at sea level, which also aids in counterbalancing hypoxia.
Tibetans, on the other hand, instead of having hematological adaptations, they have respiratory adaptations. Tibetans also have another adaptation which expands their blood vessels, allowing the whole body to deliver oxygen more efficiently to different parts. Further, Ethiopians don’t have higher hemoglobin counts than people who live at sea level, so “Right now we have no clue how they do it [live in high altitudes without hematologic differences in comparison to people who live at sea level]”.
Though Kenyans do have genetic adaptations to live in the highlands (Scheinfeldt et al, 2012). These genetic adaptations have arisen independently in Kenyan highlanders. The selective force, of course, is hypoxia—the same selective force that caused these physiologic changes in Andeans and Tibetans.
The human body is amazing. It can adapt both physiologically and physically to the environment and in turn heighten prospects for survival in most any environment on earth. These physiologic changes, of course, have followed us into the modern day and have health implications for the populations that possess these changes. Inuits, for instance, are cold-adapted while the climate is changing (which it constantly does). So, over time, when the ice caps do melt the Arctic peoples will be facing a crisis since they are adapted to a certain climate and diet.
People in colder climates need shorter bodies, higher body fat, lower limb ratio, larger brains etc to better survive in the cold. A whole slew of physiologic processes aids in peoples’ survival in the Arctic, but our ability to make clothes, houses, and fire, in conjunction with our physiological dynamicness, is why we have survived in colder climates. Tropical people need long, slender bodies to better dissipate heat, sweat and run. People who evolved in higher altitudes also have hematologic and respiratory adaptations to better deal with hypoxia and less oxygen due to living at higher elevations.
These adaptations have affected us physiologically, and genetically, which leads to changes to our phenotype and are, therefore, the cause of how and why we look different today. Human biological diversity is grand, and there are a wide variety of adaptations to differing climates. The study of these differences is what makes the study of Man and the genotypic/phenotypic diversity we have is one of the most interesting sciences we have today, in my opinion. We are learning what shaped each population through their evolutionary history and how and why certain physical and physiologic adaptations occurred.