Medicine Needs Evolution
The citation of "Evolution in Action" as Science's 2005
breakthrough of the year confirms that evolution is the vibrant
foundation for all biology. Its contributions to understanding
infectious disease and genetics are widely recognized, but its full
potential for use in medicine has yet to be realized. Some insights
have immediate clinical applications, but most are fundamental, as is
the case in other basic sciences. Simply put, training in evolutionary
thinking can help both biomedical researchers and clinicians ask useful
questions that they might not otherwise pose.
Although anatomy, physiology, biochemistry, and embryology are recognized as basic sciences
for medicine, evolutionary biology is not. Future clinicians are generally not taught evolutionary
explanations for why our bodies are vulnerable to certain kinds of failure. The narrowness
of the birth canal, the existence of wisdom teeth, and the persistence of genes that cause
bipolar disease and senescence all have their origins in our evolutionary history. In a whole
array of clinical and basic science challenges, evolutionary biology is turning out to be
crucial. For example, the evolution of antibiotic resistance is widely recognized, but few
appreciate how competition among bacteria has shaped chemical weapons and resistance
factors in an arms race that has been going on for hundreds of millions of years. The
incorrect idea that selection reliably shapes a happy coexistence of hosts and pathogens
persists, despite evidence for the evolution of increased virulence when disease transmission
occurs through vectors such as insects, needles, or clinicians’hands. There is growing
recognition that cough, fever, and diarrhea are useful responses shaped by natural selection,
but knowing when is it safe to block them will require studies grounded in an understanding
of how selection shaped the systems that regulate such defenses and the compromises
that had to be struck.
Evolution is also the origin of apparent anatomical anomalies such as the vulnerabilities
of the lower back. Biochemistry courses cover bilirubin metabolism, but an evolutionary
explanation for why bilirubin is synthesized at all is new: It is an efficient free-radical
scavenger. Pharmacology emphasizes individual variation in genes encoding cytochrome
P450s, but their evolutionary origins in processing dietary toxins are just being fully appreciated. In
physiology, fetal nutritional stress appears to flip an evolved switch that sets the body into a state that
protects against starvation. When these individuals encounter modern diets, they respond with the
deadly metabolic syndrome of obesity, hypertension, and diabetes.
The triumphs of molecular biology call attention to evolutionary factors responsible for certain
genetic diseases. The textbook example is sickle-cell disease, whose carriers are resistant to malaria.
Similar protection against infection has been hypothesized for other disorders. Which aspects of the
modern environment are pathogenic? We need to find out. Increases in breast cancer have been
attributed to hormone exposure in modern women who have four times as many menstrual cycles as
women in cultures without birth control. Other studies suggest that nighttime exposure to light increases
the risk of breast cancer by inhibiting the normal nighttime surge of melatonin, which may decrease
tumor growth. Evolution has also provided some explanations for conditions such as infertility. The
process that eliminates 99.99% of oocytes may have evolved to protect against common genetic defects.
And some recurrent spontaneous miscarriages may arise from a system evolved to protect against
investing in offspring with combinations of specific genes that predispose to early death from infection.
These and other examples make a strong case for recognizing evolution as a basic science for
medicine. What actions would bring the full power of evolutionary biology to bear on human disease?
We suggest three. First, include questions about evolution in medical licensing examinations; this will
motivate curriculum committees to incorporate relevant basic science education. Second, ensure
evolutionary expertise in agencies that fund biomedical research. Third, incorporate evolution into every
relevant high school, undergraduate, and graduate course. These three changes will help clinicians and
biomedical researchers understand that both the human body and its pathogens are not perfectly
designed machines but evolving biological systems shaped by selection under the constraints of
tradeoffs that produce specific compromises and vulnerabilities. Powerful insights from evolutionary
biology generate new questions whose answers will help improve human health.
Randolph M. Nesse, Stephen C. Stearns, Gilbert S. Omenn
