I already did my little write-up/reading response for Survival of the Sickest, but I am now going to talk about some specifics. I wrote these two paragraphs after I slowly read the first two chapters, but then my parents came and I read the rest on the beach, so the summaries didn't happen. I still thought I would post them though, since they were already written. But let's be honest, if you are really interested, you should probably read the book. I am summarizing a lot of data and research that has been well explained and just giving you my exclamatory remarks in reaction to what I read. I am excited to share, but I think you should follow up by reading all about it for yourself.
First of all, hemachromatosis is a hereditary disease, the most common genetic disease for people of European descent, in which the body can't register that it has enough iron. So it continuously absorbs as much of it as possible, and this can have very, very serious side effects (including death). Iron is very important for bacteria, cancer, and other things to grow. The way this disease is most easily treated? Blood letting. Looks like all those crazy blood-letting, leech-sticking doctors weren't mistreating everyone. What is the author's argument for why this disease stuck around? To really oversimplify things: that during the black plague in Europe, people with more iron in their system were more likely to die because bacteria feeds on iron. Women, children, and the elderly were significantly less targeted than men. But people with hemachromatosis also happen to have white immune system blood cells with considerably less iron than the normal person, and this counteracted the precise way that the bubonic plague killed its victims - through their own immune system. Therefore, their immune system was actually able to fight off the bubonic plague, allowing them to live while 1/3 - 1/4 of the population died off. Even though hemachromatosis will eventually overload your system with iron, unabated, and cause you to die, it will save your life against normal infections. On the other hand, anemia has evolved because not having enough iron in your system means that it is hard for bacteria to live. While we do need iron, anemia has helped many populations avoid things like malaria.
Diabetes is much more common in people of Northern European descent and very uncommon in people of purely African, Asian, and Hispanic descent. The Younger Dryas was an ice age that occurred 13,000 years ago. After many rounds of scientific research and revision, it was discovered that the Younger Dryas took only a decade to develop and then lasted only three years. That means that one population went through a very significant (30 degree) temperature drop during their lifetime. Many Europeans died out. How does this relate to diabetes? Well, one thing that sugar does is lower the freezing temperature of water. Pure water freezes at 32 degrees, but water with other substances in it, like sugar, freeze at much colder temperatures. Our blood, being largely composed of water, then, would also freeze at a lower temperature if it had higher levels of sugar. Brown fat is a type of fat that the body produces in extremely cold temperatures that quickly burns sugar into heat. So a diabetic in Northern Europe during the Younger Dryas would have lived because their higher levels of blood sugar would have kept their blood liquid and let their brown fat burn that sugar into heat.