The Price of Evolution

Alumnus sees physiological self-preservation responses of the past as hazards for today's humans.

by Eric P. Widmaier

We live in an age when certain evolutionary adaptations, once needed for survival, can now work against us. Take, for example, the primitive mechanisms that govern body weight.

When hominids first evolved, they were opportunistic scavengers and gatherers, picking what berries they could find and gnawing on leftovers of carnivore kills. Undernourishment and periodic starvation would have been the norm.

Our brains evolved the capacity to monitor nutritional status and to compensate for changes above or below a programmed, desirable "set point." Thus, when food was scarce, signals were relayed to the brain to trigger a drop in metabolic rate and an increase in appetite. The motivation to find food would override other concerns, and metabolism would slow down to minimize depletion of body fat stores. Individuals especially suited for this had higher body-weight set points and strong appetites, and they deposited fat more readily. Thus, they were better adapted to long periods without food.

This primitive mechanism was designed to prevent early humans from starving to death and is retained in us today. In developed countries, where food is plentiful, we satisfy the incessant signals from our brains that compel us to eat. We also tend to eat more than needed, a holdover from earlier times when the availability of another meal was uncertain. As a result, many people find it difficult to avoid gaining weight, especially as we age and reduce our daily exercise. If we try to lose weight, the brain senses the resultant drop in fat stores and reprograms itself to increase appetite and slow down metabolism. It's often easy to lose those first few pounds, but then evolution takes over. Losing more weight -- even in the face of heroic willpower -- is very difficult because we begin burning fuel (read: fat) at a much slower rate than normal.

Unlike certain animals, such as hibernators, for whom gaining weight is a normal aspect of their physiology, we no longer need to deposit so much fat. In us, obesity is pathological, not physiological, and it contributes to several other diseases of great consequence, such as atherosclerosis, hypertension and diabetes.

Another good example of how evolution can oppose our efforts to live a healthy life is the so-called "stress response." This complex response evolved to enable animals to cope with changes in their environment, particularly life-threatening ones. When an antelope spots a lion, its stress response is stimulated; it is preparing for the possible consequences of an attack (wounds, blood loss, infection, pain). If an attack occurs, natural painkillers like endorphins dull the pain long enough for the antelope to attempt an escape, while other stress hormones such as cortisol and adrenaline power the heart and leg muscles, dilate the airways, pump sugar into the blood and activate centers in the brain that increase alertness. If the attack doesn't occur, the hormonal response is greatly diminished and quickly dissipates.

Most people are no longer in danger of being eaten by carnivores, of course. Yet all of the minor daily annoyances and anxieties that we regularly encounter trigger the same stress responses as those of an antelope spotting a lion. Our primitive, programmed stress response is frequently invoked in us all. Ending a relationship is not life threatening, nor is rushing to get the kids out of the house to day care or school. But these and countless other worries and aggravations cause our brain and adrenal glands to repeatedly pump stress hormones into our bloodstream.

It's a built-in mechanism that must run its course once initiated. Over the years, those bursts of stress hormones into the blood take their toll -- on blood pressure, resistance to disease and brain function, to name a few. Some investigators have identified brain regions in laboratory animals that are selectively damaged by chronically high levels of stress hormones. Here too, then, is another example of an important evolutionary adaptation that helped ensure survival of the species but can now work against us.

Being aware of the dangers of stress, and how we worsen things for ourselves by fretting over events that are not life threatening, has helped many people focus on controlling their anxieties.

Despite the problems discussed above, our bodies are astonishingly well suited to survival, as shown by our ability to inhabit nearly every terrestrial environment on earth. Nonetheless, one of the goals of modern medicine remains, in essence, to overcome the grip of evolution on our physiologies.

Eric P. Widmaier (WCAS, G79) is an associate professor of biology at Boston University. His book, Why Geese Don't Get Obese (And We Do), was published in 1998 by W.H. Freeman and Co.