“You may think of your body as a fairly permanent structure, but most of it is in a state of constant flux as old cells are discarded and new ones are generated.”
Even your bones, which you may think of as static and non-living, are full of cells called osteoclasts and osteoblasts that are forever (respectively) breaking down and rebuilding your bones as they adapt to the stresses they’re subjected to. (This is why bed-rest or being in weightlessness results in bone loss).
A fascinating article from the New York Times highlights the work of Doctor Jonas Frisén, of the Karolinska Institute in Stockholm, who hit on a method to estimate the average age of tissues. The method involves measuring the amount of Carbon-14 in the DNA of various tissues.
A pulse of artificially-generated Carbon-14 was created in the atmosphere in the years that above-ground nuclear testing was allowed, and since then the level (which was double the normal background level) has been slowly returning to normal. This Carbon-14 entered the food chain as plants absorbed it and incorporated it into their bodies, and from there it percolated into all living things, including us. The amount of Carbon-14 in the DNA of our tissues reveals the average age of the cells making up those tissues, since the DNA is fabricated at the birth of the cell.
Frisén’s work, and that by other researchers, tells us the following:
- “The epithelial cells that line the surface of the gut have a rough life and are known to last only five days. Ignoring these surface cells, the average age of those in the main body of the gut is 15.9 years.”
- “Red blood cells, bruised and battered after traveling nearly 1,000 miles through the maze of the body’s circulatory system, last only 120 days or so on average before being dispatched to their graveyard in the spleen.”
- “The epidermis, or surface layer of the skin, is recycled every two weeks or so.”
- “An adult human liver has a turnover time of 300 to 500 days.” This means that you have a new liver just about every year!
- “The entire human skeleton is thought to be replaced every 10 years or so in adults.”
- “The average age of all the cells in an adult’s body may turn out to be as young as 7 to 10 years.”
About the only pieces of the body that last a lifetime, on present evidence, seem to be the neurons of the cerebral cortex, the inner lens cells of the eye and perhaps the muscle cells of the heart. But even there, it’s the age of the DNA that Frisén’s method measures. There are ongoing processes that replace individual components of the cells around that DNA, meaning that even in the cerebral cortex and other “static” tissues, there may be considerable change taking place. And other work has shown that new neurons are in fact generated in the cerebral cortex, although presumably not in numbers sufficient to show up in Frisén’s work. Additionally, although cells in the brain may be long-lived, they still change; brain cells have a life-long ability to develop new connections with each other. This is how learning takes place.
The upshot is that the body is continually changing.
And yet we are attached to our bodies. This brings up the question, how can we be attached to something that is constantly changing? How can we cling to something that doesn’t remain the same from one moment to the next? Well, we can’t. We identify with our bodies, thinking that if the body fails, we fail. We think that when others judge the body, they judge us. And so clinging (or trying to cling) to something impermanent leads to suffering.
The Buddhist tradition encourages us to regard all things as being like mist, or flowing water, or a mirage — what the mind takes to be solid, substantial, and graspable is actually ever-changing and characterized by impermanence. The Six Element Practice — the subject of my new book, Living as a River, is a way of developing an experiential appreciation of the transience of the body. This helps us to let go, stop identifying the body with the self, to suffer less, and to experience a profound sense of freedom.
Extract from Chapter 8: The Earth Element
Where did this body begin, and was there anything at the beginning that I could call me or mine?
There’s no mention of conception in the traditional outlines of the practice, but since the body begins there, that’s where I tend to start my reflections. In my meditation practice I recollect that my life began with the fusion of an egg from my mother with a sperm from my father. I share with most people the reluctance to consider in any detail the conjugal activities of my parents, and so in this particular exercise of the imagination I find it useful to indulge in a little vague generality. So there is an egg and a sperm, floating, as it were, in the mid-air of my imagination. Sperm and eggs are of course delivery vehicles for your parents’ DNA. Not that it’s your parents’ DNA, not yours. In fact your parents’ DNA was not theirs either. They both got their DNA from their parents, and so on and so on back in time until the first common ancestor of all life. DNA perforce flows through time’s river, or it ceases to exist. Like everything else in the body, our DNA is not owned, but only borrowed. We can look at the provenance of DNA from another angle as well. The atoms that constitute your DNA are borrowed from the outside world. Your DNA is doubly other, coming from your parental lineage and from food ingested by your progenitors.
I reflect that neither the sperm nor the egg was me. One was clearly part of my mother’s body, while the other was likewise just one of the roughly 100 trillion cells constituting my father’s body at that moment in his life. Now, I can regard the product of the fusion of these two “not-me” cells as being, in some way, “me,” but the knowledge that the parts were not-me rather undermines the notion of their sum being “me.” Everything that made up that first “me” cell was borrowed. I have the sense, reflecting on this, of a disconcerting absence of “me” at the moment of my creation. I’d like to think that there was a “me” created, but considering that “I” was made from stuff that was entirely “not me” I experience the same dizzying sense of insubstantiality that arose in Chapter 1, when we considered the possibility of another sperm having met the mother’s egg.
Next I witness, in the mind’s eye, the growth and development of this single-celled, half-mother/half-father/not-me entity, in a series of mental snapshots and time-lapse movies taking me right up to the present day. In my reflections on the Earth element, I’m not concerned with visualizing the specifics of the entire process of embryogenesis—one doesn’t need a degree in embryology to reflect on it—but in a general sense I call to mind that the following process took place in my life: conception was followed by cell-division and the development of an embryo, which becomes a fetus, and then a baby. This growth was possible because of the borrowing of the Earth element from the outside world. The embryo at first got nutrition by absorbing secretions from the uterus, and dumped waste products into my mother’s uterine cavity, but later it grew by absorbing nutrition through the placenta. And what I absorbed from my mother was in turn what she had borrowed. The flesh and other products of animals, crops that have grown in fields, and fruits that have been cultivated in orchards were all funneled through my mother’s body to feed “me.” That which is other (food) flowed through that which is other (my mother) into the body, which—remembering that the embryo began as a cell from each of my parents—is also other. When we look closely, it seems that there is no “me” to be found in the Earth element constituting the developing embryo: “Now the internal Earth element and the external element are simply Earth element. And that should be seen as it actually is…”
We can visualize ourselves being born and growing to the point where we ingest food ourselves. Rather than being channeled through the mother’s body, food now flows directly into the body, being chewed, digested, and assimilated. We can imagine this process of growth, with its attendant flow and absorption of the Earth element, taking place right up to the present moment, right up to the form that is reading this book. In my meditation I picture this flow of the Earth element. I call to mind fields and animals, seeing the Earth element in the form of food flow from fields, along roads, into factories and stores, into my home, and into my body. The Earth element flows like a river, and I am just one eddy amongst its countless currents. This is another opportunity to experience humility and gratitude. How many countless people have been involved in growing, transporting, processing, and selling the food we have eaten over an entire lifetime? How many lives of plants and animals have helped sustain our small eddy? We can think of this and give thanks.