OXFORD UNIVERSITY PRESS, JULY 2013Shakespeare memorably described the human life course, from “the infant,/ Mewling and puking in the nurse’s arms” to the “mere oblivion” of the aged, “Sans teeth, sans eyes, sans taste, sans everything.” Scientists now appreciate that human life histories have been shaped by natural selection. Evolutionary life history theory provides a valuable, if less poetic, framework for understanding our life cycle and the diseases that accompany aging.
Natural selection adjusted how humans use energy and other resources throughout our life cycles in ways that optimized the reproductive fitness of our evolutionary ancestors. Optimizing fitness has meant devoting energy to growth and development and to reproduction, at the expense of maintaining and repairing our bodies. Our evolved mechanisms of bodily maintenance and repair are sufficient to keep us alive and healthy long enough to have and raise our children, and perhaps contribute to the development of our grandchildren. But these mechanisms are not perfect. Over time, we accumulate unrepaired damage that leads to the diseases of aging and, ultimately, to death.
In my new book, Evolution and Medicine, I discuss the emerging field of evolutionary medicine. I show how integrating life history theory and other evolutionary concepts into medicine has the potential to improve our understanding of disease and, most importantly, clinical practice.
Perhaps the most dramatic contribution of evolutionary medicine to patient care has been the development of highly active antiretroviral therapy (HAART) for HIV-infected patients. HAART typically includes two reverse transcriptase inhibitors to block the activity of the enzyme that retroviruses use to replicate. The development of HAART was based on the recognition that mutant virus strains that were resistant to both drugs would have two or more mutations in the reverse transcriptase gene, and so were likely to have decreased fitness. HAART has revolutionized the treatment of HIV. The use of combination therapy for patients with hepatitis C infections and other diseases—and more broadly, our increased concern with resistance management to prolong the useful lives of new antibiotics—shows the reach of evolutionary medicine into the clinic.
Until the 20th century, most if not all humans were chronically infected by parasitic worms, or helminths. Helminths used to be so common in the environment that our ancestors evolved traits that optimized fitness in their wormy world. The effect of living in relatively worm-free environments is thought to underlie the increasing incidence of allergic and autoimmune diseases in economically developed, modern countries. These considerations have led to novel clinical trials using helminth extracts or eggs to treat patients with multiple sclerosis or inflammatory bowel disease. Although it is too early to know if these trials will be successful or will lead to new therapies for patients, they illustrate another way in which an evolutionary perspective can inform clinical research.
Evolutionary life history theory has great but as yet largely untapped potential to improve medical practice. We now know that although aging is inevitable, its time course is not fixed. Life expectancy in the United States has increased dramatically over the last century, from about 47 years in 1900 to almost 80 years today. Because of better nutrition and a decline in infectious diseases, we are born with greater amounts of physiological reserves, we experience lower rates of bodily damage, and we live longer than our grandparents and great-grandparents.
Many hormones regulate energy utilization and so play important roles in our life histories. Physical and psychosocial stresses, acting through neuroendocrine regulatory mechanisms, appear to accelerate the aging process. Better understanding these mechanisms may help us modulate the rate of aging and extend life. The integration of evolutionary medicine with biomedical research offers untold and exciting new opportunities for improving human health and well-being.
Robert Perlman is Professor Emeritus in the Departments of Pediatrics and Pharmacological and Physiological Sciences at the University of Chicago. Read an excerpt of Evolution and Medicine.