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sugar levels can lead to rapid dehydration, coma, and death. Even when diabetes is tightly managed, its long-term complications include blindness, heart disease, stroke, and vascular disease that often leads to gangrene and amputation.

      There are two major types of diabetes, Type 1 and Type 2, commonly called juvenile diabetes and adult-onset diabetes, respectively, because of the age at which each type is usually diagnosed. (Increasingly, adult-onset diabetes is becoming a misnomer: skyrocketing rates of childhood obesity are leading to increasing numbers of children who have Type 2 diabetes.)

      Some researchers believe that Type 1 diabetes is an autoimmune disease – the body’s natural defense system incorrectly identifies certain cells as outside invaders and sets out to destroy them. In the case of Type 1 diabetes, the cells that fall victim to this biological friendly fire are the precise cells in the pancreas responsible for insulin production. No insulin means the body’s blood sugar refinery is effectively shut down. As of today, Type 1 diabetes can only be treated with daily doses of insulin, typically through self-administered injections, although it is also possible to have an insulin pump surgically implanted. On top of daily insulin doses, Type 1 requires vigilant attention to blood sugar levels and a superdisciplined approach to diet and exercise.

      In Type 2 diabetes, the pancreas still produces insulin – sometimes even at high levels – but the level of insulin production can eventually be too low or other tissues in the body are resistant to it, impairing the absorption and conversion of blood sugar. Because the body is still producing insulin, Type 2 diabetes can often be managed without insulin injections, through a combination of other medications, careful diet, exercise, weight loss, and blood sugar monitoring.

      There is also a third type of diabetes, called gestational diabetes because it occurs in pregnant women. Gestational diabetes can be a temporary type of diabetes that tends to resolve itself after pregnancy. In the United States, it occurs in as much as 4 percent of pregnant women – some 100,000 expectant mothers a year. It can also lead to a condition in the newborn called macrosomia – which is a fancy term for “really chubby baby” as all the extra sugar in the mother’s bloodstream makes its way across the placenta and feeds the fetus. Some researchers think this type of diabetes may be “intentionally” triggered by a hungry fetus looking for Mommy to stock the buffet table with sugary glucose.

      So what causes diabetes? The truth is, we don’t fully understand. It’s a complex combination that can involve inheritance, infections, diet, and environmental factors. At the very least, inheritance definitely causes a predisposition to diabetes that can be triggered by some other factor. In the case of Type 1 diabetes, that trigger may be a virus or even an environmental trigger. In the case of Type 2, scientists think many people pull the trigger themselves through poor eating habits, lack of exercise, and resulting obesity. But one thing is clear – genetics contributes to Type 1 and especially to Type 2 diabetes. And that’s where, for our purposes, things really start to heat up. Or, more precisely, to cool down, as you’ll see shortly.

      There’s a big difference in the prevalence of Type 1 and Type 2 diabetes that is largely based on geographic origin. Even though there seems to be a stronger genetic component to Type 2 diabetes, it is also closely related to lifestyle; 85 percent of people who have this type of diabetes are obese. That means it’s currently much more common in the developed world because easy access to high-calorie, low-nutrient junk food means so many more people are obese – but it seems clear that the predisposition to Type 2 diabetes exists across population groups. There are higher levels of incidence in certain populations, of course – but even that tends to occur hand in hand with higher levels of obesity. The Pima Indians of the southwestern United States, for example, have a staggering rate of diabetes – nearly half of all adults. It’s possible that their historic hunter-gatherer lifestyle produced metabolisms more suited for the Atkins diet than the carbohydrate- and sugar-heavy diet that European farmers survived on for centuries. Type 1 diabetes is different – it is much, much more common in people of Northern European descent. Finland has the highest rate of juvenile diabetes in the world. Sweden is second, and the United Kingdom and Norway are tied for third. As you head south, the rate drops lower and lower. It’s downright uncommon in people of purely African, Asian, and Hispanic descent.

      When a disease that is caused at least partially by genetics is significantly more likely to occur in a specific population, it’s time to raise the evolutionary eyebrows and start asking questions – because that almost certainly means that some aspect of the trait that causes the disease today helped the forebears of that population group to survive somewhere back up the evolutionary line.

      In the case of hemochromatosis, we know that the disease probably provided carriers with protection from the plague by denying the bacteria that causes it the iron it needs to survive. So what could diabetes possibly do for us? To answer that, we’re going to take another trip down memory lane – this time measured, not in centuries, but in millennia. Put your ski jackets on; we’re looking for an ice age.

      Until about fifty years ago, the conventional wisdom among scientists who studied global climate change was that large-scale climate change occurred very slowly. Today, of course, people from Al Gore to Julia Roberts are on a mission to make it clear that humanity has the power to cause cataclysmic change in just a few generations. But before the 1950s, most scientists believed that climate change took thousands, probably hundreds of thousands, of years.

      That doesn’t mean they didn’t accept the notion that glaciers and ice sheets had once covered the Northern Hemisphere. They were just happily certain that glaciers moved, well, glacially: eons to descend and epochs to recede. Humanity certainly didn’t have to worry about it – nobody was ever going to be run over by a speeding glacier. If massive climate change was going to lead us into a new ice age, we’d have a few hundred thousand years to do something about it.

      Of course, there were some contrary voices singing a different tune, but the larger scientific community paid them very little regard. Andrew Ellicott Douglass was an astronomer working in Arizona in 1895 when he first started cutting down trees to examine them for evidence of any effect from a specific solar activity, called sunspots, that occurs in cycles. He never found that – but he did ultimately invent dendrochronology, the scientific technique of studying tree rings for clues about the past. One of his first observations was that tree rings were thinner during cold or dry years and thicker during wet or warm years. And by rolling back the years, one ring at a time, he discovered what appeared to be a century-long climate change that occurred around the seventeenth century, with a significant drop in temperature. The reaction of the scientific community was a collective “Nah.” As far as the climate change community was concerned, Douglass was cutting down trees in a forest with nobody there to hear it. (According to Dr. Lloyd Burckle of Columbia University, not only was Douglass right: the hundred-year cold spell he discovered was responsible for some beautiful music. Burckle says the superior sound of the great European violin makers, including the famous Stradivari, is the result of the high-density wood from the trees that grew during this century-long freeze – denser because they grew less during the cold and had thinner rings as a result.)

      More evidence of the possibility of rapid climate change accumulated. In Sweden, scientists studying layers of mud from lake bottoms found evidence of climate change that occurred much more quickly than anyone at the time thought possible. These scientists discovered large amounts of pollen from an Arctic wildflower called Dryas octopetala in mud cores from only 12,000 years ago. Dryas’s usual home is the Arctic; it only truly flourished across Europe during periods of significant cold. Its widespread prevalence in Sweden around 12,000 years ago seemed to indicate that the warm weather that had followed the last ice age had been interrupted by a rapid shift back to much colder weather. In honor of the telltale wildflower, they named this arctic reprise the Younger Dryas. Of course, given prevailing thinking, even these scientists believed that the “rapid” onset of the Younger Dryas took 1,000 years or so.

      It’s hard to underestimate the chilling effect conventional wisdom can have on the scientific community. Geologists of the time believed the present was the key to the past – if this is the way the climate behaves today, that’s the way it behaved yesterday. That philosophy is called uniformitarianism and, as the physicist Spencer Weart points out in his 2003

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