Friday, October 24, 2008

Evolution, Explanations and Medicine

Over the past few months I have been reading chapters from this excellent book, and this has motivated me to write this post on evolution.

Consider the following events from a typical Monday morning.

(1) I wake up.
(2) My stomach grumbles with hunger.
(3) I walk downstairs to the kitchen.

Now if someone asks me to explain events (1), (2) and (3), I might say something like the following:

"I awoke because my alarm went off. I was hungry because I had not eaten for 8 hours. And I went downstairs to the kitchen because that is where my food is".

Now this kind of explanation makes perfect sense. It focuses on the immediate causes of (1), (2) and (3). And we structure, and make sense of, our world by piecing together information concerning different immediate or proximate causes.

But to return to (1), (2) and (3), we might ask: why do I sleep? Why do I feel hunger pains? And why do I have legs? These are more profound questions, ones that require us to adopt a "big picture" perspective that goes beyond the modes of query typical of making sense of our immediate day-to-day lives.

So when we frame the questions of (1) - (3) in this way we are not looking for a proximate explanation, but rather for "ultimate" or evolutionary explanations.

In Evolution in Health and Disease, the editors describe natural selection as a principle that must hold when certain conditions are present: variation in traits, variation in reproductive success, correlation of trait variation with reproductive success, and inheritance of trait variation.

They provide the useful example of water glasses in an inexpensive furnished apartment that has been repeatedly rented:

They can be explained by natural selection. Some collection of glasses came into the apartment. The fragile ones broke. The attractive ones left when renters departed. The nonfunctional ones with odd shapes were thrown out. What is left is what you find- a collection of sturdy, ugly, functional glasses. (8-9)

And the principle of natural selection provides us with an explanation of the events that unfold every morning of our lives. The events of my mundane Monday morning are in fact the result of endless trade-offs and adaptations that have taken place over billions of years.

We sleep because it aids the recovery of many different system levels (e.g. cellular and network), helps conserve energy, and helps with learning, etc. There are many theories concerning the functions of sleep, but the fact that sleeping is universal among animals suggests that it is very important to survival. Go 24 hours without sleep and you will soon realize how vital sleep is to your ability to function. The amount of sleep needed varies among species. A fruit fly, for example, can sleep up to 12 hours a day.

Why do we feel hunger pains? Well, it helps remind us that we need food, and when severe enough it will fixate our attention solely on the goal of "GETTING FOOD!". This is advantageous because it increases the likelihood that we receive a steady supply of the energy needed to survive and reproduce. A species that requires the diet needed to sustain human life would not survive long if it were not hardwired to satisfy our basic material needs.

And we have legs because they are beneficial for a creature (like us) that lives on land. If we lived in the ocean like clown fish then we wouldn't have legs (or be humans!). And if we shared the evolutionary history of birds we would have wings and feathers and be able to fly (though thanks the the magnificent human brain, we have created machines that now permit us to fly).

And so the trivial events of a regular Monday morning in fact reveal the incredible journey of evolution, and the development of the human species.

Why, you might ask, tell us this tale? Well, the emphasis we place on proximate explanations, well often very useful and appropriate, can also limit our understanding and perceptions of the challenges we face. For if my Monday morning was simply explained by the immediate causes of awakening from my sleep, being hungry and getting my breakfast, we would miss the incredible insights that evolutionary biology can contribute to our understanding of the world and our species. And these include important insights for medicine. In particular, to the aspiration to retard human aging.

To make the link from evolution to medicine, let me tell another story. The story is about Billy and his grandfather William. Billy is your typical 16 year-old boy. He is "girl crazy" and is trying to work up the courage to ask a girl from his class out on a date. Billy spends most of his free time chatting to his friends about girls, lifting weights so he can have more success with his efforts at romance, and he also spends a lot of time worrying about his clothes and hair.

Billy's grandfather, William, is 76 years-old. He has a variety of hobbies- he likes to paint and play chest, he volunteers and spends time with his grandchildren. But recently William has become increasingly more concerned about his limited mobility and chronic joint and muscle pain. Furthermore, he was recently diagnosed with high blood pressure and is at risk for a variety of other ailments. And thus his doctor has recommended he take a daily dose of medications to ease his pain and reduce the risks of more serious problems.

Billy and William are your typical 16 and 76 year-old. And yet there is an important link between Billy's obsession with girls and his grandfather's failing health. Evolutionary biology explains why Billy and his grandfather are in the situations they are in. The force of natural selection is set by the age at which reproduction first occurs in a population. And thus the surge in Billy's testosterone levels and overall gonadal function, and Williams chronic health problems, reflect the trade offs that have been made between reproduction and survival. Historically, very few humans lived to the age of William, and thus investing in repairing damage late in life would not improve the success of reproduction. Furthermore, investing scarce resources in long-term repair would be inefficient and potentially wasteful. This is known as Kirkwood's "disposable soma" thesis- aging occurs because natural selection favors a strategy in which organisms invest fewer resources in the maintenance of somatic cells and tissues that are necessary for indefinite survival of the individual.

Rose describes it this way:

Henry Ford sent one of his engineers to a scrapyard to find out which parts of defunct Model T’s still had some usable life left. When the engineer reported back with a list of the durable parts, Ford instructed his engineers and suppliers not to make those parts so good that they outlasted the rest of the car. This strategy culminated in “planned obsolescence”, an ugly tradition in American manufacturing.... Evolution is regrettably similar. (91)

Should we treat the trade-off between survival and reproduction that happens to exist at this stage in the evolutionary history of humans with deference? A trade-off that has been shaped by natural selection and the extrinsic risks that once permeated the world, and yet one that will bring disease to an unprecedented number of humans living in this century? The answer is clearly “No”. We should search for safe and effective ways of modifying the biological processes of aging, so that people can expect to live more healthy lives. Utilizing the insights of evolutionary biology, biogerontologists are now searching for ways to modify the biological processes of aging. But many people fail to understand the importance of this research because they only understand the proximate, rather than ultimate, causes of disease. To fixate only on proximate causes limits our understanding of the things that cause disease. We only look at the immediate causes- like smoking, diet, particular genes, etc. But we must also aspire to grasp "the big picture", and Darwinian medicine helps us do this.

Those who embrace the aging "status quo" must square this with other elements on the "evolutionary status quo". Take our immune system. Do we favor the immunity "status quo"? No. For if we did, we should abandon immunizations and return to the high rate of child-mortality that existed in the past. Should we not strive to also reduce late-life morbidity and mortality? Do aged individuals not deserve protection from these harms? Indeed, we already aspire for these things, which is why we fund cancer research, AD, etc. So why not fund research that might permit us to delay all age-related afflictions?

And lastly, think about human emotions, and happiness. Should we just accept our evolved natures, with all their virtues and vices? The editors of Evolution in Health and Disease make this excellent point:

We did not evolve to be happy: rather we evolved to be happy, sad, miserable, angry, anxious, and depressed, as the mood takes us. We evolved to love and to hate, and to care and be callous. Our emotions are the carrots and sticks that our genes use to persuade us to achieve their ends. But their ends need not be our ends. Goodness and happiness may be goals attainable only by hoodwinking our genes. (13)

So evolution offers many profound insights that have important implications for medicine, as well as moral and political philosophy. Once we give attention to both the proximate and ultimate causes of morbidity and mortality, our aging blinders will be discarded. Our perception of the greatest challenges facing humanity this century will change. And the things we perceive to be obstacles to promoting public health will also change. Ignorance and irrationality are among the greatest threats to public health this century. And so we should think of strategies for reducing their prevalence.