Why we get sick: Darwinian medicine

Proximate explanations

-"What" & "How" questions about the structure & mechanisms of things

-Ex: explaining how the body works and why some people get sick while others don't

Ultimate explanations

-"Why" questions about the origins & functions of things

-Ex: why humans in general are susceptible to some diseases and not others

4 Darwinian explanations for why humans get sick:

1. Genetic trade-offs

2. Evolutionary legacies

3. Co-evolution of pathogens and hosts

4. Fitness-oriented behavior

Genetic trade-offs

Sickle Cell Anemia is an example of this

Evolutionary legacies

malfunctional design flaws and life in the Stone Age

malfunctional design flaws

-Respiratory & digestive paths cross

-Trachea & esophagus cross

-All vertebrates, from fish to mammals possess these criss-crossed structures

-During evolution, vertebrates co-opted food passage for respiration

-mutation causing inability to synthesize vitamin C was lost in primate ancestors

-Resulted in scurvy when fruit wasn't available

Life in the Stone Age

-Modern humans today may not be adapted to high population densities, low levels of physical activity, and other novel aspects of the world

-Humans in this age lived shorter lives, had smaller population densities (less spread of disease), and had higher levels of physical activity

advantages of pathogens

-reproduce and evolve rapidly

-reproduced in larger numbers

Co-evolution of pathogens and its hosts

Pathogens and their hosts (humans) have evolved pretty rapidly and kept up with each other. Pathogens reproduce and evolve rapidly and are reproduced in larger numbers, which could be a reason why they can infect humans at a fast rate.

Fitness-oriented behavior

Certain behaviors may also promote sickness. It is known that men have higher death rates than women in every age group. This may be due to the fact that males are more likely to engage in risky behaviors and are prone to violence. These behaviors may have evolved for success in male-male mating competition. However, nowadays these behaviors can lead to injury and death.

Evolution is about ______, not health!

fitness

Evolutionary insights into disease & health

-modern day nutritional excesses

-antibiotic resistance

-fever as a defense mechanism

-female hormones & breast cancer

modern day nutritional excesses

-It has only been recently that humans have access to excess calories

-Genes that promoted cravings for fat, and fat deposition, were formerly adaptive, but may not be so today

female hormones & breast cancer

Modern female humans may not be adapted to:

-Few pregnancies over the lifetime

-High lifetime exposure to estrogen & progesterone (150 vs. 500 menstrual cycles) increases risk of breast cancer

Antibiotic Resistance

-Toxins produced by fungi kill the bacteria that cause human disease

-Chemical warfare agents that evolved in fungi & bacteria to protect them from pathogens

-Have been shaped by millions of years of natural selection to exploit the vulnerabilities of bacteria while being non-toxic to hosts

Penicillin Resistance

-Discovered in 1928 by Alexander Fleming

-By 1941, all staph bacteria were vulnerable to this

-By 1944, some strains of staph had already evolved resistance to this antibiotic

-Today, nearly all staph bacteria are resistant to this drug

Tuberculosis

-Common cause of death in developing countries, with nearly 2 million people succumbing each year

-About .5 million cases of multi-drug resistant strains occur every year

The antibiotic crisis

Antibiotic resistant germs cause at least 23,000 deaths and 2 million illnesses in the U.S. each year, requiring treatments costing $20 billion in direct medical expenses

Variation in antibiotic resistance

Bacterial resistance to antibiotics arises through rare mutations

Competition & Differential reproduction in antibiotic resistance

The presence of an antibiotic causes the initially rare mutant strain to increase and gradually replace the ancestral type

Fever

An adaptive host response to infection that has persisted throughout the animal kingdom for hundreds of millions of years

Fever as a body defense in Syphilis

-Patients with this illness improve after being infected w/malaria

-This disease is rare where malaria is common

-Patients infected with this illness who are infected with malaria show 30% remission rates

fever as a body defense in chicken pox

Children with this illness who were given acetaminophen took on average a day longer to recover than those who took a sugar placebo

Fever as a body defense

-Most bacteria & viruses that cause infections in people thrive best at 98.6 F

-Untreated ________ caused by infection will seldom go over 105 F unless the child is overdressed or trapped in a hot place

-Brain damage from this will generally not occur unless it's over 107.6 F

-Certain proteins that stimulate the immune system are induced in a cell when it is exposed to heat ("heat shock proteins")