Health: Why We Get Sick PDF

Health Why We Get Sick The Mystery of Disease Why in a body of such exquisite design, are there a thousand flaws and frailties that make us vulnerable to disease? If evolution by natural selection can shape sophisticated mechanisms such as the eye, heart, and brain, why hasn’t it shaped ways to prev...

Health Why We Get Sick The Mystery of Disease Why in a body of such exquisite design, are there a thousand flaws and frailties that make us vulnerable to disease? If evolution by natural selection can shape sophisticated mechanisms such as the eye, heart, and brain, why hasn’t it shaped ways to prevent nearsightedness, heart attacks, and Alzheimer’s disease? If our immune system can recognize and attack a million foreign proteins, why do we still get pneumonia? If a coil of DNA can reliably encode plans for an adult organism with ten trillion specialized cells, each in its proper place, why can’t we grow a replacement for a damaged finger? If we can live 100 years, why not 200? The Mystery of Disease The common answer “that natural selection just isn’t powerful enough” is usually wrong – It’s a bundle of crap The body is a bundle of careful compromises The Causes of Disease Nesse and Williams (1995) – Why We Get Sick – Propose six categories of evolutionary explanations of disease – Excellent book!!! The Causes of Disease … and an excellent follow-up released recently!!! 1. Defenses Not actually explanations of disease, but often confused with disease A fair-skinned person with severe pneumonia may take on a dusky hue and have a deep cough These are 2 signs. One a defect, one a defense – Skin is blue because hemoglobin is darker in color when it lacks oxygen. A defect. Like a clank in a car A happenstance result with no particular utility 1. Defenses Cough is a defense – Like the light on the dashboard that turns on when the gas tank is low – Results from a complex mechanisms designed specifically to expel foreign material in the respiratory tract When we cough, a coordinated pattern of movements involving the diaphragm, chest muscles, and voice box propels mucus and foreign matter up the trachea and into the back of the throat, where it can be expelled or swallowed to the stomach where acid destroys most bacteria – Cough is not the problem Cough is a protective response to a problem – A coordinated defense shaped by natural selection 1. Defenses Defenses vs. Defects – Distinction is crucial for someone who is sick – Correcting a defect is almost always a good thing Stopping the clank in the car or doing something to turn the pneumonia patient’s skin warm pink is almost always beneficial – Eliminating a defense by blocking it can be catastrophic Cut the wire to the light that indicates a low fuel supply and you are more likely to run out of gas Block your cough excessively, and you may die of pneumonia 2. Infection A never-ending arms race – Bacteria and viruses treat us mainly as meals We have evolved defenses to counter these threats They have evolved ways to overcome our defenses and even use them to their own benefit – This endless arms race explains Why we cannot eradicate all infections Some autoimmune diseases 3. Novel Environments Similar to the Mismatch hypothesis – Our bodies were designed for lives spent in small groups hunting and gathering on the plains of Africa – Natural selection has not had time to revise our bodies to cope with novel environments Fatty diets, automobiles, drugs, artificial lights, central heating, etc. – From this mismatch arises most preventable modern disease e.g., heart disease and breast cancer 3. Novel Environments Examples: – Population migration and skin cancer – Caloric availability and obesity as a cancer risk factor – Higher availability of fats that promote tumor growth – Tobacco availability and smoking as a cancer risk – Differences in number of reproductive cycles and breast cancer – Exposure to light at night may increase cancer risk 4. Genes Some of our genes are perpetuated despite the fact that they cause disease – Some are genetic “quirks” that were harmless in our more natural environment Genes that predispose to heart disease were harmless until we began overindulging in fatty diets Genes that cause nearsightedness cause problems only in cultures where children do close work early in life Some of the genes that cause aging were subject to little selection when life spans were shorter 4. Genes Many genes that cause disease have been selected for because they provide benefits – Either to the bearer or to other individuals with the gene in other combination For example, the gene that causes sickle-cell prevents malaria There are also sexually antagonistic genes that benefit fathers at the expense of mothers (or vice versa) 5. Design Compromises Costs and benefits – There are costs associated with every major structural change preserved by natural selection Walking upright – Gives us the ability to carry food and babies – Predisposes us to back problems – Many of the body’s apparent design flaws aren’t mistakes, but compromises – To better understand disease, we must understand the hidden benefits of apparent mistakes in design 6. Evolutionary Legacies Evolution can only make small changes which must be immediately beneficial – Fires occurred when a popular line of pickup truck was struck from the side because the gasoline tanks were located outside the frame To relocate the tanks within the frame would require a major redesign of everything now there – This would cause new problems and require new compromises – Our food passes through a tube in front of the windpipe, exposing the danger of choking It would be more sensible to relocate the nostrils to somewhere on the neck That will never happen… Signs and Symptoms of Infectious Disease Say you are on the side of the mouse in a cat and mouse conflict… – The mice say they hate the smell of a cat Makes them jittery and unable to concentrate on important matters such as food, courtship, and babies – You know of a drug that will dull this sense so the mice won’t be bothered by the smell as much – Do you prescribe the drug? Signs and Symptoms of Infectious Disease The ability to detect cat odor, however unpleasant is a valuable asset for mice The presence of the cats smell may signal the imminent arrival of its claws and teeth Avoiding the cat is far more important than the stress of an unpleasant odor Signs and Symptoms of Infectious Disease Suppose you are a pediatrician treating children with colds Colds bring symptoms that children may dislike – Runny nose, headache, fever, malaise, etc. Tylenol can reduce or eliminate some of these symptoms Do you prescribe Tylenol? Fever As Defense Against Infection Matt Kluger – “There is overwhelming evidence in favor of fever being an adaptive host response to infection that has persisted throughout the animal kingdom for hundreds of millions of years.” – Kluger believes that using drugs to suppress fever can make people sicker – and even die Fever As Defense Against Infection Kluger showed that cold-blooded lizards benefit from fever – When infected, they seek out a place warm enough to raise their body temperature about two degrees Celsius – If they cannot move to a warm place, they are more likely to die Body temperature is carefully regulated – When the lizard is infected, it just has a higher set point Fever is an adaptation to combat infection