Equipped with central heating, air-conditioning, fridges, clothes and home insulation, we’ve grown used to the fact that technology makes us independent of outside conditions, enabling us to live in the least welcoming corners of the globe throughout the year. The ongoing technological advancement expands the natural tolerance ranges of the Homo sapiens, making our innate limitations increasingly irrelevant and giving us the confidence to brave even the bitter cold of Antarctica. Thanks to technology, the only continent where no indigenous civilization has ever emerged is now home for scientists who study the region from the comfort of Antarctic research stations.
We too evolve
Before the development of human brain let us consciously shape our surroundings to our advantage, evolution had already started to adjust human physiology to the existing natural conditions.
It was no accident that humankind developed near the Equator. Our distant ancestors had no choice – without fire, shelter or clothing, they could only live in temperatures close to the thermic neutral point (TNP), which is a point in which human body requires no energy to maintain its internal temperature. It is assumed that the TNP equals between 23 and 27°C, so – in this respect – equatorial regions were perfect for the early man, especially that near the Equator the differences between the warmest and the coldest season are very slight.
Still, around 40–60 thousand years ago something drove early humans out of Africa. It might have been their sense of adventure but, much more probably, it was climate change and recurring droughts. The price they paid for abandoning warm regions was the necessity to face seasonal or permanent cold. Fortunately, evolution never sleeps and it soon responded with useful physiological adaptations.
Who sleeps well in the cold?
Conditions that go beyond our thermal comfort zone invariably cause stress to our bodies. It is true for both too warm and too hot temperatures, but – in the long run – due to our ability to sweat, we do better in excessive heat.
In the 1950s, the issue of human adaptations and responses to extreme temperatures generated a lot of interest. As a result, an experiment was devised in which a group of volunteers comprising, among others, Norwegians (of Caucasian origin), the Inuit (back then referred to as the Eskimo) and the Sámi (historically known as Laplanders) were tested for their reactions when sleeping in relatively thin sleeping bags in 0–6°C. Not used to such conditions, the Norwegian volunteers did not take the experiment too well. They were shivering, which caused an increase in their metabolic rate, and the temperature of their skin dropped. The Inuit, who are indigenous to the Arctic, reacted in a variety of ways. They did not shiver, which let them maintain a regular metabolic rate, but they did not sleep calmly either. After all, in normal conditions, they kept warm with the use of traditional igloos and exceptionally warm clothing made of polar bear fur and seal skins. The least affected were the Sámi, who live semi-nomadic lifestyles and whose traditional clothes are less “extreme” than those worn in Greenland or Northern Canada.
First come the shivers
Shivering is a natural response to the cold. It is very effective but too costly for the organism to go on indefinitely. As a result, it does not work in the Arctic, where the temperature ranges from 40°C below zero to 10°C above zero, giving no respite from the cold. Evolution, therefore, had to come up with a better, long-term alternative in the form of morphological adaptations, such as narrower nasal passages and a relatively stocky build. Narrow nasal apertures combined with longer nasal passages make it possible for the mucous membrane lining the nasal cavity to warm up the cold air and thus prevent potential damage to the lungs or the brain, and to improve the “recovery” of heat and humidity from exhaled air. This seems rather intuitive, doesn’t it? The advantage of being stocky and short, on the other hand, is a bit more complicated and it has much to do with the polar diet.
The traditional diet of the Inuit is a nightmare of a modern-day dietician: no fresh fruit or vegetables, no variety, plenty of meat and fat. It may also sound like an advert for dietary supplements: omega-3 polyunsaturated fatty acids counterbalance negative health effects of a high-fat diet. It must be kept in mind, however, that what works for the people of the Far North wouldn’t work at all for typical Europeans. It turns out that the Inuit have specific mutations in FADS1, FADS2 and FADS3 enzyme-coding genes, which determine the levels of polyunsaturated fatty acids in blood, thus helping to alleviate negative consequences of a high-fat diet of seal and whale meat. By lowering the level of bad cholesterol and insulin, they offer protection from cardiovascular diseases and diabetes. This genetic superpower is exhibited by nearly 100% of the Inuit, but extremely rare among the Europeans, occurring in a mere 2% of the population. This is why the former thrive on a diet that would be lethal for the latter. There is, however, a price to pay for this ability. Fatty acids affect the metabolism of growth hormone, which leads to a lower average height of the Inuit population.
What’s left to discuss is a stocky build (not to be mistaken for obesity), which can be explained with the help of Bergmann’s rule. Even though the principle was formulated with respect to animals, we too belong to the animal kingdom and so the rule holds for us as well. In 1847, German biologist Carl Bergman observed that warm-blooded animals living in cold regions are usually bulkier than individuals of the same species found in warmer regions. This is due to the fact that larger individuals tend to produce more heat. Why is that? The larger an animal is, the more cells it is made up of and – as body heat is a by-product of cellular metabolism – the more heat it produces. In 1877, American biologist Joel Allen went a step further than Bergmann by noticing that the rate of heat loss is affected by the length of the animal’s limbs and neck. Once again, by comparing different populations of the same species, he observed that individuals living in warmer climates have longer limbs than those living in colder conditions. This is due to the fact that animal bodies equipped with longer appendages have a greater surface area, which accelerates heat loss.
Brown fat as a gift from the Denisovians or the Neanderthals?
Apart from being a crucial ingredient of the traditional Arctic diet, fat is one of the building blocks of our bodies. There are two types of body fat: white adipose tissue (WAT) and brown adipose tissue (BAT). The former, composed mainly of fat cells known as adipocytes, stores energy and protects internal organs from injury. The latter is responsible for maintaining internal temperature and distributing it all over the body. The cells making up brown adipose tissue contain fewer lipids (only 30–50% of total cell volume), but numerous mitochondria. It is due to these organelles, which function as tiny cellular heat and power plants, that BAT is characterized by greater cellular activity and can therefore ensure effective thermoregulation.
The greatest amount of BAT can be found in newborn babies, as they need to be able to keep warm after leaving the comfort of the womb. Over time, much of the tissue disappears and in adults what’s left of it can be found at the back of the neck and along larger blood vessels. Not so in the Inuit, who keep a lot more of the “active” brown adipose tissue. This is of course genetic, dictated by WARS2 and TBX15 genes, responsible for the differentiation and distribution of adipose tissue in humans.
Figuring out this genetic adaptation to the cold made it possible to confirm that thousands of years ago Homo sapiens interbred with Neanderthals and – most probably – with Denisovians. The latter are a particularly mysterious group within genus Homo, discovered only in 2010. All we know about them is based on two molars and a single finger bone found in the Siberian Denisova Cave. What’s interesting is that the part of the genome sequence retrieved from the remains included, among others, genes corresponding with the above-mentioned mutation prevalent among the Inuit.
Vitally important are the adaptations within the cardiovascular system. The basal (resting) metabolic rate among the Inuit may be as much as 30% higher than ours, which protects them from hypothermia and ensures excellent blood circulation. As a result, the cold automatically increases blood flow to all extremities, which helps the Inuit avoid frostbite. Normally, the opposite is the case, with blood supply to more remote body parts being reduced to protect the body’s core. A high metabolic rate of the Inuit makes the contrary reaction possible, which is great news indeed, as negative consequences of severe frostbite would make life in the Far North a lot more of a challenge.
Evolution is a lengthy process. It takes many generations to increase the prevalence of desirable traits and – by means of natural selection – eliminate those, which may be a hindrance. The question remains, however, what impact does extreme cold have on an individual and is it possible to get used to it? There’s evidence to indicate that a bitterly cold environment changes not only out subjective perception of temperature, but also our very selves.
A classic example comes not from the polar regions, but from Japan. The ama are Japanese female pearl divers, who dive throughout the year, even when water temperature falls below 10°C. When studied by scientists, they turned out to possess a natural mechanism, which consistently but temporarily increases their basal metabolism (which is a sum of all metabolic processes occurring in a body at rest) by 30% during each winter season. This, of course, is not an instant adaptation. The ama begin to dive, wearing nothing but regular swimsuits, at the age of 12 and they keep diving on a daily basis for several dozen years, even when air temperature is close to 0°C.
How about the heroes of the golden age of polar exploration? Extreme times surely called for extreme toughness. Even though nobody measured the metabolic rate of Birdie Bowers, a seasoned polar explorer who lost his life during Scott’s ill-fated expedition to the South Pole, it is quite clear he must have been an absolute champion of polar acclimatization. After all, Scott himself called him “the hardest traveller that ever undertook a polar journey as well as one of the most undaunted". Bowers slept like a baby when his companions shivered uncontrollably, which was probably due to the fact that, ever since arriving in Antarctica, he started his days by stripping naked and – to his friends’ horror and fascination – dousing himself with icy water. What’s interesting is that Birdie was rather short (163 cm) and stocky, which made his surface-to-mass ratio highly favourable.
Can we really get used to everything? We probably can, but it usually comes with a price. People who regularly endure long-term exposure to the cold may, to a certain extent, grow accustomed to low temperature, but this isn’t necessarily a good thing. The weakening of natural responses to cold exposure, such as shivering and the constriction of blood vessels, means that the skin remains relatively warm, but – unless the person is Inuit – their metabolic rate is too low to counteract excessive heat loss, which may lead to hypothermia. Since the body’s reactions are highly individual, acclimatization may in some cases strengthen our natural responses to the cold. It may even lead to non-shivering thermogenesis (i.e. heat production). Still, the most effective weapon in the face of extreme cold is modern technology, especially in places like Vostok Station in Antarctica, which holds the record for the lowest reliably measured temperature on Earth (-93.2°C).
Surprising though it may seem, human evolutionary adaptations to specific climatic conditions have not yet been thoroughly studied. In fact, up until recently, it was doubted if qualities typical of particular populations and defined by their genome should be qualified as adaptations, and thus put on par with the adaptations of the reindeer lichen or the polar bear. This reluctance to acknowledge our similarity to the rest of the natural world might have been caused by our desire to be freed from the tyranny of evolution and to owe as much as possible to our own doing, rather than the natural selection. Current studies of the topic are complicated by human tendency to travel, mixing gene pools, and – most of all – by our brain, which has struggled to reduce our dependence on the environment since the moment it managed to tame that curious orange flame, which keeps the cold away.