Monday, March 30, 2009

Appreciating the Impact Natural Selection Has on Our Lives

As this year marks the bicentennial of Darwin's birth I felt inspired to write the following rather lengthy post.

I begin by recalling two rather vivid memories I have that are separated by approximately twenty years in time:

Memory #1. 1978

The last shoe lace is tied tight. The light spring jackets are all buttoned or zipped up. Then there are a few seconds of anxious waiting... wait for it....wait for it....
All of the kids in my grade 2 class, including myself, storm out of the classroom. Within two minutes the once empty and quiet playground is now filled with laughter and life. Kids playing tag, ball hockey, skipping, etc. For the next 15 minutes every child is on the go. Recess is a time for play. It is the highlight of the school day for kids everywhere. Few sights in life fascinate me as much as watching children play. The joy on their faces. The connectedness they have to "living in the moment".

Memory #2. 1998

It is 5pm and dinner time at the nursing home. This is the first time I have visited my grandmother since she moved from her apartment to a nursing home. Indeed, it is the first time I have ever set foot in a nursing home. The residents of the home line up for dinner service. A large crowd of residents (the majority are women) now consume the main floor which has the dinning area. Some of the residents can stand in the long line unaided. Most have walkers, and others are confined to a wheelchair. Witnessing this scene has remained vivid in my mind over the years. In particular I have always been struck by the disconnect that seems to exist between the world "inside nursing homes" and the "outside world". The former is a place people come to visit loved ones and family. And it might be a place they themselves spend the final years of their own lives. But it is a place people seldom discuss. It is a place rarely portrayed in the media or in works of art, etc. The elderly and their plight are very much marginalized from our culture. And this is unfortunate for many reasons (though I won't detail all the reasons here today).

I find reflecting on these two distinct memories is helpful to me in many ways. The stark contrast between the health and vitality of people during the different stages of the human life cycle sparks the obvious question- why are things this way? Why is it that most (though not all) school-aged children have the energy and vitality I described above; and yet most (though not all) adults over the age of 85 suffer from one or more illness or disease?

According to the CDC stats on death rates, approximately 14.5 children (ages 5-9) per 100 000 will die this year. But for adults ages 80-84 6,712.9 per 100 000 will die. This means that the death rate for seniors (in this age bracket) is 463 times higher than it is for children (in this age bracket). But why is this so?

Before jumping to that answer consider some more details of the specific causes of death for children and seniors and some more why questions.

Drawing on these older stats (which conveniently contain the 10 leading causes of death by age) we see that for children ages 5-9 the top cause of death is accidents. So things "out there" in the external world (speeding cars, swimming pools, etc.) are the greatest threat to the survival of children in the United States. But what kills most Americans when they are over the age of 85? Heart disease is #1, then cancer, then cerebrovascular diseases, then chronic lower respiratory diseases, followed by diabetes and AD.

But why is it that the greatest single threat to the life of people when they are young are "external" factors; and yet the greatest threat to the life of aged persons are "internal" to us (i.e. chronic diseases)?

When this "why?" question is asked as a comparison between these different aged groups of humans (rather than, say, just two individuals of the same age) it helps focus our attention on the evolutionary causes of morbidity and mortality. Why do humans age the way we do? A way that fills us with life and energy at age 8, but brings disease, frailty and/or (usually) death by age 88?

The answer to understanding why we age the way we do is in fact related to the answer of another why question- why we reproduce. In other words, the health and vitality we have when young is directly linked to the failing health we have at more advanced ages. To appreciate this link we must appreciate why any species would reproduce.

Why should a species, any species, reproduce? I was reading this interesting paper last week. And the imperative to reproduce is linked to the inevitability of death. Given the extrinsic risks that permeate our environment (e.g. predation, starvation, etc.) the solution to escaping death is reproduction. Species that can successfully reproduce will continue on, while those that can't eventually become extinct. So prioritizing reproduction (rather than maintenance of the soma) is beneficial in the kind of world we have tended to inhabit. Humans are here today because we have been pretty successful (so far, in our relatively short evolutionary history) at successfully reproducing under the external threats we have faced.

The importance of reproduction thus leads to a tradeoff between investing physiological resources into reproduction, resources that could otherwise have been used to maintain the health and vitality of a parent. Because of the pervasive risk of morbidity and mortality, humans typically died either during their pre-reproductive or reproductive periods. So life expectancy before the 19th century was below the age of 30 and that had always been the case for humans.

Because most humans that have ever lived died well before age 65, the force of natural selection only applied to our pre-reproductive and reproductive periods. This explains why most serious childhood genetic disorders, like infantile Tay-Sachs disease, are very rare. A disease that kills humans early in life has little chance of being passed on to future generations. But sadly the same is not true for late- onset diseases, like AD and most cancers.

When it comes to alleles that protect humans from late-life morbidity there is no pressure to select those genes. Unlike our pre-reproductive and reproductive periods, our post-reproductive period was not selected for. Our susceptibility to frailty and disease in late life, unlike our vitality and robustness in early life, is not the outcome of natural selection. As Barnes puts it, "senescence arose from evolutionary neglect rather than evolutionary intent".

This leads me back to my two memories. In the year 2048 the children from my grade 2class will be the age of many of those in nursing homes today. Globally there will be 2 billion humans alive over the age of 60. And this will bring unprecedented levels of chronic disease (cancer, heart disease, stroke, AD, etc.).

If there was something we could do to alter this possible future of unprecedented human suffering and disease from becoming a reality, shouldn't we try to avoid it? To meet these challenges we must foster a Darwinian-based approach to medicine. Instead of feeding the next generation of inquisitive thinkers useless platitudes about the importance of switching off lights to save the world we should encourage them to harness the great potential of evolutionary biology.

Humanity must awaken from its current intellectual slumber (and its accompanying guilt complex about being alive in the first place) if we are to meet the enormous challenges we face this century. Some of the greatest dangers facing our health and economic prospects are in fact the legacies of evolutionary history. To appreciate this we must make the future vivid and have a sense of proportionality.

One doesn't need to be a longevity scientist to know the high probability (indeed certainty) that aging populations will suffer frailty and disease. We all have experiences like memory #2, even though our culture tries not to think about it (out of sight, out of mind).

Given the certainty and severity of the harm of aging you might expect that vast amounts of public funding are being invested in aging research. You might think that the brightest and most talented scientists who long to make the world a better place are being lured into the field. Unfortunately it is very hard to get people to rally behind aging research. This must change. A deceleration of the aging process might make nursing homes a thing of the past. And that would be an enormous achievement that all future generations of humans could enjoy.

To end on a positive note I leave you with yet another sage insight from John Dewey:

Society exists through a process of transmission quite as much as biological life. This transmission occurs by means of communication of habits of doing, thinking, and feeling from the older to the younger. Without this communication of ideals, hopes, expectations, standards, opinions, from those members of society who are passing out of the group life to those who are coming into it, social life could not survive.

Dewey's passage is a nice complement to the spirit of my last post.