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Insulin dependent diabetes is one of the most common chronic diseases in the world today. This is a hereditary disease that is caused by the inability of Beta cells in the pancreas to produce insulin. This is problematic because insulin is responsible for breaking down glucose present in the blood for energy and therefore keeps blood glucose levels from getting too high. While diabetes is considered a heritable disease, it is believed that genetics give you a predisposition for developing diabetes and then it is some environmental cue that actually prompts the onset. When trying to determine what evolutionary advantage could come from having diabetes, Dr. Moalem noticed there are some populations in which the incidence rate is very high and other populations where diabetes is virtually nonexistent. Insulin dependent diabetes is most common at high latitudes, predominantly in Finland, Sweden, the UK, and Norway, whereas in populations at low latitudes, like the people of African and Hispanic descent, diabetes is quite rare. Upon much research, Dr. Moalem came to the conclusion that insulin dependent diabetes must promote survival in extremely cold temperatures. While this is a hotly debated hypothesis, there are many facts to back it up. Diabetes is characterized by excessive water loss and excessive increase in blood glucose levels. This may be beneficial in very cold climates because the decrease in water volumes means there is less water in the body that could potentially freeze and the large amount of sugar in the blood significantly lowers its freezing point. It is believed that whatever gene is responsible for diabetes was selected for during the Younger Dryas, the most recent Ice Age. In support of this theory, children are most often diagnosed with diabetes in the colder months when the temperature begins to drop.

This chapter primarily focuses on hemochromatosis, a hereditary disease that results in the disruption of the negative feedback loop used to tell the body to stop absorbing iron. This results in an extreme excess of iron within the body. The increased iron levels cause pain and damage to joints, internal organs, and chemical properties of the body. If left alone, hemochromatosis can ultimately lead to death. The only way to treat this disease is to literally drain blood from the body to decrease iron levels. The gene that causes hemochromatosis is most common in people of Western European descent. Why has this gene not been selected against throughout the years? Dr. Sharon Moalem makes a strong argument towards the notion that this gene is responsible for an increased immunity to the bubonic plague, which predominantly affected Western Europe in the fourteenth century. Dr. Moalem indicates that all life forms need iron to live and proliferate and that many illnesses are able to overtake the immune system by absorbing the iron present in the macrophages sent to kill them. What makes people with hemochromatosis unique, is that interestingly, their macrophages have been found to lack the presence of iron, making them ideal for combating illness. So, it is believed that hemochromatosis originally came about via a genetic mutation that occurred before the bubonic plague and then was selected for during the bubonic plague, leading to the prevalence of the gene that causes hemochromatosis in the population today.