The Count

The lining of your gut replaces itself every three to five days. The outer layer of your skin turns over every few weeks. Red blood cells last about four months. These are the fast parts.

Fat cells turn over at roughly ten percent per year — so the ones you have now are, on average, about ten years old. Heart muscle cells renew more slowly: approximately one percent of cardiomyocytes are replaced each year at age twenty-five, declining to less than half a percent by age seventy-five. Less than half of the heart cells present at your death will be original — but more than half will be, which means your heart, in some literal sense, contains material that was there at the beginning.

Cortical neurons don't turn over at all, or nearly not. The cells doing the reading right now were present before you could form memories. The substrate of thought is older than thought.

All of this was established by measuring C-14 in cell DNA. The atmospheric spike from Cold War nuclear testing settled into every cell being born during the 1950s and '60s, then declined after the 1963 test ban. The isotope ratio in a cell's DNA indicates when that cell was formed. Kirsty Spalding and Jonas Frisén at the Karolinska Institute applied this systematically across tissue types and found that a human body is not one thing located at one time. It is a mosaic of layers — gut lining from earlier this week, fat cells from a decade ago, cardiac tissue from birth.

The fat cells are the strange case. Not their age — ten years is just a number. The strange thing is the count. The number of fat cells is set during childhood and adolescence, and then it stays fixed. Gaining weight inflates the cells; losing weight deflates them; the number doesn't change. Even after surgical weight loss, the count returns to the set-point established in development. The specific cells that constituted the childhood count are gone — they have turned over many times. What persists is the number itself, maintained by regulatory mechanisms that have not been fully characterized. Whatever established the count in a developing body encodes it as a target, and the adult body maintains it by replacing each dying cell with exactly one new cell.

This is a different kind of persistence from the neurons, which are literally the same cells from birth. The neurons persist as objects. The fat architecture persists as a count — as a property of the population that no individual cell holds. The cells are new. The pattern they instantiate is decades old. What continues across time is not material but a form being continuously re-instantiated. A musical score persists through performances without requiring that any performance share physical material with any other.

What the bomb pulse can measure is individual cells — it dates them one by one. The count has no isotope. It doesn't live in any single cell; it's a systemic property, a relationship between cells and whatever is monitoring and replacing them. You can date every fat cell in a body and find that none of them is from childhood, and what you won't have found is where the childhood architecture is stored. It isn't in the cells. It's in something that coordinates the cells — and that thing, whatever it is, isn't directly accessible to this method or probably any other.

The record of when each cell was born is in its DNA, and the bomb pulse makes it readable. The record of the original count — what set the target, why this number — is somewhere else, and it's not clear it's the kind of thing that can be isolated and measured at all. Some information about the body is in the body's material. Some is in the pattern the material enacts. These are not the same place.