Winter is shorter days of diminished sunlight peering at an oblique angle, never reaching zenith. The gradual light of dawn illuminates a palette devoid of color; brown trees, gray skies and an occasional patch of white snow that sometimes covers everything in monochromatic undulation. The waning light of late afternoon winks out early as a long night of quiet descends; temperatures plummet in darkness. Autotrophic plants die back in the winter senescence as the photosynthetic engine of earth slows to a crawl and in some cases ceases altogether; the heterotrophic animals either starve or eat each other and fungi bide their time in subterranean mycelial mats. Winter is testing time, when survival is by no means certain and random mutations either succeed for survival or fail in fatality. Life that is hard in summer becomes much, much harder in winter and every living thing must deal with it. Food and water are prerequisites for life and both are compromised by winter’s frozen breath. Warmth becomes a necessity against cell shattering frost as liquid water becomes solid ice. Woolliness is the insulation that traps the heat of survival.
Winter hiking is a naked ape challenge. The hair lost in the equatorial savannah of hominid origination was never restored in the subsequent diaspora ranging into northern and southern reaches where immoderate temperatures prevail. It became necessary then to compensate with insulating layers like wool to replace that which was lost with that which was domesticated. Before wool, it was skins that served the same purpose to the detriment of the animal from which they were stripped and scraped. The layers of protection extend to the hands, feet and head, extremities from the sanguinary pump that sends life sustaining oxygen along the vascular highway. While woolliness helps, it is the bodies heat engine that produces the warmth that is necessary for survival. Hiking in winter is moving, stopping only briefly and incidentally, pushed onward by the creeping ice fingers that penetrate to the core. Elevation matters, as temperature drops one degree for every three hundred feet; climbing to 3,000 feet results in a temperature drop of ten degrees (metrically 7 degrees C per kilometer). Daylight matters, as twilights last gleaming portends the loss of radiant heat; the conductive heat loss to the ground and the convective heat loss of wind chill now dominate the heat balance challenge of warm bloodedness. The Edenic warmth of fossil fuel consumption is pervasive; only hiking, hunting and camping provide a connection to the past, when winter’s struggle was existential.
The cold of winter has been an evolutionary forcing function, just as the heat of global warming may become one in the next century. Cold is the direr of the two; life teems in the torrid tropics but arctic zones are deserted by all but the most robust. Earth occupies an orbit that has been called the Goldilocks Zone primarily because it is where water, the elixir of life, is mostly liquid.  Heat is a form of energy that flows from a warmer object to a cooler object that is determined according to the temperature of each, which is quite simply a measure of the molecular vibration. Heat transfer occurs as the vibrating molecules in a hotter volume collide with the molecules of the colder volume to impart some kinetic energy and increase their movement, and therefore the temperature; over time, they equalize at the same point. Scales to measure temperature parametrically were originally calibrated according to the points at which water changed states. The Danish polymath Ole Roemer was first to establish a scale based on the melting point and boiling points of water in 1702; Daniel Fahrenheit added a third point as the temperature of the human body to devise the eponymous scale at which water boils at 212 degrees. Forty years later, the Swedish astronomer Anders Celsius proposed a simplified scale where water freezing at 0 degrees and boiling at 100 degrees, which every country except the United States and Liberia now uses. Since temperature is the measure of molecular movement, it is reasonable to consider that the real zero, or absolute zero, should be that temperature at which all motion ceases. In 1862, William Thompson, the Lord Kelvin, calculated this to be -273.15 degrees C according to thermodynamics. The accepted standard international or SI unit for temperature is accordingly 0 degrees Kelvin. Not to be outdone, the Scottish engineer William Rankine devised the Fahrenheit equivalent where absolute zero is -459.67 degrees, you guessed it, Rankine.  If you ask someone what the temperature is, be prepared for an F, a C or maybe even a K or an R, when all you want to know is whether you need a coat, a question rarely posed in Hawaii.
Periodic cycles of deep freeze glaciation have defined earth’s climate in the past. The Quaternary Period that defines the current geologic era marks the start of a series of ice ages that cycled between the cold glacial periods and the warmer interglacial periods at least four times in the last two million years. The last glacial advance reached its apogee about 18,000 years ago with 30 percent of the earth covered with the 8 percent of the water that was frozen; it melted back about 10,000 years ago marking the end of the Pleistocene and the beginning of the modern Holocene Epoch. This compares to 10 percent ice with 1.5 percent frozen water at present (and shrinking fast).  We are in an interglacial now and have been for about 10,000 years – it is not expected to end until about the year 32008, the three hundred and twenty first century. The cyclic nature of the temperature swings incident to glaciation that occur over thousands of years suggests a cosmological provenance. The eponymous Milankovitch theory postulates that changes in the Earth’s eccentricity, obliquity or tilt, and precession change the amount of sunlight reaching the earth – called insolation – sol is Latin for sun.  Three parameters account for changes in the Earth’s solar orbit:
• Eccentricity – The earth orbits the Sun in an ellipse; eccentricity is a measure of the relative roundness of the ellipse. The eccentricity varies on approximately a 100,000-year cycle from 0 percent (nearly round) to 5 percent (very elliptical). At present the difference between the maximum distance (aphelion) and the minimum distance (perihelion) is only about 3 percent – i. e. the orbital distance between the earth and the sun varies from about 91 million miles to 94.5 million miles. At maximum eccentricity, this distance can vary by up to 30 percent – clearly this would have a profound effect on insolation – and thus glaciation. 
• Obliquity – Normally, the earth’s axis has a tilt of 23.5°, which is why there are seasons. This tilt, known as obliquity, varies from 22.05° to 24.5° every 41,000 years. At a lower tilt, the warming effect of the sun would not reach as high a latitude in summer – this would contribute to the buildup of snow and ice in glaciers during the more profound winters. 
• Precession – The precession of the equinoxes (roughly March 21 and September 21 when the Sun “crosses” the equator) was first discovered in about 125 BCE by the Greek astronomer Hipparchus – as the apparent shift of the stars visible at the moment of equinox from year to year. The reason for precession is that the earth is not perfectly round, but rather is flattened at the poles so that the axis about which the earth “wobbles” like a spinning top when it slows down. This cone-shaped precession of the earth’s axis occurs about every 25,800 years in what is called the Platonic year. Over time, this causes the pole star to change – it is Polaris or the North Star now – in 12,000 years it will be Vega, a bright star in the constellation Lyra. 
The net effect of eccentricity, obliquity and precession is that the amount of insolation varies according to how the three cycles are modulated (operate in concert) to produce a net effect. The Milankovitch Theory of glaciation is that a period of minimal insolation occurs on a periodic basis. Due to this lower heat loading, the northern hemisphere becomes cold enough for ice to persist through the summer; the albedo effect of the expanded ice causes a reflection of the sun’s rays during subsequent years to create a positive cooling feedback. Ultimately, the annual accumulation of snow results in extensive glaciation and a prolonged period of cold – an ice age. For the pioneering humans who had migrated north, 11,000 BCE was the worst winter ever.
Theories for the origination of human creativity and culture abound, but unlike the DNA measures that reveal true relationships, there are no quantitative metrics to validate matters of sociology. A geographic premise is that concentrations of large seeded plants like wheat prompted agriculture to feed larger populations that domesticated mild-mannered animals for industrial enterprise. The larger, more dense societies were then able to specialize in the technology of weapons and were subject to pandemics rooted in their close animal associations. As their populations grew, they expanded outward with guns, germs, and steel to conquer and subjugate.  A more evolutionary approach would follow the same line of reasoning that explains the emergence of Homo sapiens from the various other hominid prototypes; climate change from wet to dry and habitat from arboreal to grassland required adaptation for survival. A syllogistic approach would posit that the demands of living through increasingly dire winters would require ingenuity and endurance in the quest for food, clothing and shelter. Such was the case in Europe as the last of the ices ages abated. The number of stone tools increased ten-fold starting in about 35,000 BCE with increasing specialization and sophistication, culminating in 20,000 BCE with the first bow and arrow to improve the hunt and bone needles to sew hoods, gloves, and moccasins to conserve body heat against bitter cold. As this was carried out largely at the tribal level with small numbers of family related clan groups, those that succeeded survived; those that survived retained the traits necessary and sufficient as testimony. 
The wildflower Dryas octopetala is endemic to the northernmost reaches of arctic Eurasia. Swedish researchers inspecting soil core samples drilled from beneath a southerly lake bed were therefore surprised to find extensive pollen from the Nordic flower in post ice age sediments. The hypothesis, confirmed in the 1980’s with ice cores drilled in the glaciers of northern Greenland, was that there was a refreeze that interrupted the warming from the last age. Named the Younger Dryas for the pollen of its provenance, the period of one thousand years of winter started in less than a decade and ended in a scant three years. This discovery upended the prevailing belief that climate change happened slowly and led to a search for causation. The consensus theory at present is that the thermohaline circulatory “conveyer belt” that transports the warm tropical waters north via the Gulf Stream to Europe stopped; Austria became arctic and icebergs floated off the coast of Spain.  One can only imagine the devastation wreaked on the humans who had moved north, the caveat “winter’s coming” of HBO’s Game of Thrones would be understatement absent the white walkers. While hypothetical, it is plausible that the Younger Dryas was a right of passage for all life of the northern reaches; cold adaptations were registered in the resultant genomes.
Wood frogs are able to inhabit northern climates and survive Nordic winters due to their ability to freeze about 65 percent of their bodies and subsequently thaw out without damage when warmer temperatures prevail. Up until about fifty years ago it was presumed that frozen amphibians erred in not digging deep enough to escape the cold; a researcher in Minnesota thought that it may have been intentional, which he proved by reviving one.  The process is a complex adaptation that involves the production of glucose. When the temperature drops below freezing and ice crystals begin to form on the skin of the frog, a signal is generated that stimulates the adrenal gland to release epinephrine, also called adrenaline, which triggers the conversion of glycogen to glucose in the liver. The freezing process starts outside the cells where proteins act as ice nucleation sites. Ice formation lowers the water content in this intracellular region. With a low water content outside the cells and an elevated water content inside the cells, osmosis results, drawing almost all residual water out of the cells. This would result in severe dehydration were it not for the glucose, which restores the osmotic balance and acts like antifreeze. The level of glucose released by this process is about 4500 mg/100ml of blood. By contrast, humans are diagnosed with diabetes when the level reaches about 300 mg/100ml. Therefore, the cells don’t freeze, but the surrounding areas do. The heart stops and breathing ceases and the frog becomes a comatose block of ice. The mechanism is being studied for possible use in freezing organs to extend their viability for transplantation. 
All animals adapt to cold as a matter of survival; humans are no exception. Shivering is the first line of defense; the muscles involuntarily convulse to burn energy and generate heat. Persistent cold triggers capillary constriction to ensure that the core organs are kept warm at the expense of the less critical hands and feet; numbness and eventually frostbite ensue. Among arctic dwellers, an evolved trait called the Lewis wave opens the capillaries momentarily to rewarm the hands at a rate of about ten degrees a minute to preserve viability (hands are after all not really expendable). Warm weather humans do not have cold weather traits and are therefore much more subject to permanent cold damage to extremities. Acclimatization is also a factor; my own experience in relocating from the shivering of Scotland to the warm bask of Hawaii for three years desensitized the shivering sensor to the point that it took a year to restore it on returning to the cold of the mainland. You cannot make yourself shiver to keep warm. The use of glucose as antifreeze in frogs also suggests a convergent evolutionary trait in humans that may have resulted from the refreeze of the Younger Dryas. Diabetes is a disease of insulin deficiency that results in elevated blood sugar exacerbated over time by obesity (Type II) but which can also be hereditary (Type I). That the hereditary type is more prevalent among people of northern Eurasian descent is suggestive of cause and effect, though unproven and probably unprovable. But if high glucose levels were not a human survival benefit for the great freeze, then what possible benefit would diabetes convey to humans to pass it along to future generations? 
The animals of Class Mammalia emerged as predominant in the Cenozoic Era starting about 65 million years ago. They were heirs to the reptilian dinosaurs extirpated by the climatic convulsions of the Cretaceous–Paleogene (KPg for short) extinction event attributed to either the meteor that left the Chicxulub Crater near Mexico’s Yucatan Peninsula or the volcanism that created the Deccan Traps in India. Their survival was the result of fortuitous evolutionary adaptations in both nature and nurture. The defining traits of mammals are warm blood, hair or fur and bearing live young nourished with milk from the mammary glands for which they are named. All of these characteristics enhance survival in cold climates, which was surely a forcing function. Warm bloodedness or more properly thermogenesis is freeze protection. It comes at a cost; mammals need to consume about ten times more calories than their cold-blooded counterparts to keep the heat engine’s furnace stoked. Fur is not furbelow, it is literally the winter coat of survival insulating the core against the elements. Mammal mother’s milk is nurturing sustainment, especially in a protective burrow in a desolate snowy wilderness bereft of any other nutrition. The success of the combination is evident in the diaspora of mammals to every corner of the globe. There are 5,416 different species in 1,229 genera that range from arctic walrus to equatorial hyena and from montane snow leopard to prairie dog. They come in every size from the tiniest shrew to the most gargantuan elephant. They come in every shape imaginable from anteater to zebra and a few that seem to have been conjured up as an amusing compromise like the duckbilled platypus. Some have even gone back into the ocean to establish flotillas of battleship blue whales, destroyer seals and submarine dolphins. There are globe-trotting cats, rats, bats, dogs, hogs, bears and hares that live in homes, gardens, farms and factories. Mammals are us. 
1. The analogy is to the three bears fairy tale where everything that Goldilocks uses is “just right,” especially the porridge which is not too hot and not too cold. There are some other criteria though water is key. See https://science.nasa.gov/science-news/science-at-nasa/2003/02oct_goldilocks/
2. Whitelaw, I. The Measure of All Things, The Story of Man and Measurement. St. Martin’s Press, New York, 2007, pp 79-87.
3. Cazeau, C., Hatcher, R., and Siemankowski, F. Physical Geology, Principles, Processes and Problems, Harper and Row, New York, 1976. pp 242-266. College geology text.
4. Bryson, B. A Short History of Nearly Everything, Broadway Books, New York 2003, pp 419-433.
6. Ward, P, and Brownlee D. Rare Earth, Copernicus, New York, 2000, pp 222 – 226
7. Rey, H. The Stars, Houghton Mifflin, New York 1976 pp 108-129.
8. Diamond, J. Guns, Germs, and Steel. The Fates of Human Societies, Norton and Company, New York, 1997. The whole book is devoted to this question.
9. Ponting, C. A Green History of the World, Penguin Books, London, 1991, pp 26-30.
10. Carlson, A. E. “The Younger Dryas Climate Event”. Encyclopedia of Quaternary Science. 2013. pp. 126–34. Changes in isotopic ratios of atmospheric elements. http://people.oregonstate.edu/~carlsand/carlson_encyclopedia_Quat_2013_YD.pdf
11. Schmid, W. “Survival of Frogs at Low Temperature” Science Vol. 215 No. 4533 1982.
12. Fahrenthold, D. “Trying to Crack an Icy Mystery” Washington Post !2 December 2004.
13. Moalem, S. Survival of the Sickest, Harper Collins, New York, 2007 pp 23-48. This is an intriguing and controversial book about the environment and its effects on human evolution. The notion of ‘infectious design’ is intriguing.
14. Wilson, D. and Reeder, D. eds, Mammal Species of the World, 3rd edition, Johns Hopkins University Press, Baltimore, 2005