Long Burn

Evolutionary Medicine

The goal of evolutionary medicine is to understand medical conditions through the eyes of evolution. One of the best examples is sickle cell anemia. About 1 in 12 blacks worldwide carry the sickle cell trait (but only about 1 in 400 develop the disease), and it is also found in middle eastern and indian populations in areas where malaria is found. When the red blood cells of a person with the sickle cell trait are invaded by the malaria parasite they stick to the blood vessel walls, loose oxygen, change into the sickle shape and are removed from the body, removing the parasite at the same time.

Another example involves 'vitamin D' which refers to a group of alcohols that help with calcium metabolism. They are formed when ultraviolet light from the sun is absorbed by sterols found in the skin. The problem is how to generate vitamin D in the north-- not only are there the long dark months, but also the sun is always lower on the horizon, so light must pass through more atmosphere to get to the surface, meaning that there is less UV left even at midsummer. The evolutionary adaptation involves skin pigment. Near the equator humans have developed darker skin to protect themselves from UV light, which can cause mutations, skin cancer and other problems. In the north there is the opposite problem. Light skin and hair have developed to make it easier for the body to synthesize vitamin D from the photochemical modification of fat-soluble alcohols into sterols-- grouped together as vitamin D. (Of course nowadays we add vitamin D to milk so it is no longer a problem.)

I read an interesting article in the New York Times yesterday regarding type 1 diabetes which occurs almost entirely in people of northern european descent. The author had the theory that the disease occurs as a consequence of a glaciation that occurred 12,600 years ago that hit Europe and expecially northern europe quite hard. Populations were forced to adapt or become extinct. When humans get cold, the first response is to shiver-- the activity of the muscles generates heat. But as it gets colder a different mechanism takes over-- the body starts to burn brown ('adipose') fat, and high levels of glucose are needed for this biochemical process to be efficient. In addition elevated blood sugar prevents the water in cells from forming ice crystals when they freeze, preventing frostbite and its complications. Thus people with type 1 diabetes have an evolutionary advantage for surviving in cold climates. The theory is that this adaptation developed at a time when the average lifespan was about 25 years, which is not long enough for the disadvantages of the disease to show up. Doctors have noticed a seasonality in blood glucose levels in type 1 diabetes patients-- they are typically elevated in the winter relative to the summer. Most type 1 diabetese patients are diagnosed in the winter. The other northern humans-- inuits/eskimos, didn't face this sudden evolutionary pressure to adapt, and don't develop the disease. They came to northern areas earlier and have had more time to change. One adaptation is in the shape of the body. They tend to have barrel chests, flat noses and smaller hands and feet. The larger lung volume helps to survive while breathing cold air. I just learned that most of the world's inuit live in Denmark/Greenland, followed by Alaska, Canada and Siberia.

posted by Matt_J @ 9:37 AM