You have heard this one. Every seven years, every cell in your body has been replaced, so you are a physically new person, and the coffee stain you feel guilty about was technically spilled by somebody else.

It is a lovely thought and it is not true. Some of the cells reading this sentence have been with you since before you were born.

We know that with unusual confidence, and the reason is grim and wonderful. Between 1955 and 1963 the world tested nuclear weapons in the atmosphere, and in doing so accidentally wrote a date stamp into every living thing on the planet.

The bomb wrote a clock

Atmospheric nuclear tests threw enormous quantities of neutrons into the air, which struck nitrogen and produced carbon-14. The level of carbon-14 in the atmosphere roughly doubled. Then in 1963 the Partial Test Ban Treaty pushed the testing underground, and the spike began to fall away as the oceans and the biosphere absorbed it.

That rise and fall is called the bomb pulse, and it is the single most useful accident in modern biology.

Plants take up atmospheric carbon. You eat the plants, or you eat something that ate the plants. So the carbon in your body carries whatever ratio of carbon-14 was in the air at the time you acquired it. Crucially, when a cell divides and copies its DNA, it builds that new DNA out of whatever carbon is available right then, and then it never rebuilds it again. The DNA in a cell is laid down once, at the cell’s birth, and left alone.

So the carbon-14 in a cell’s DNA is a timestamp. Measure it, compare against the known atmospheric curve, and you can read the year the cell was born.

Jonas Frisén’s group at the Karolinska Institute worked this out and published it in Cell in 2005. Their description is admirably flat about it: “Testing of nuclear weapons resulted in a dramatic global increase in the levels of the isotope 14C in the atmosphere, followed by an exponential decrease after 1963. We show that the level of 14C in genomic DNA closely parallels atmospheric levels and can be used to establish the time point when the DNA was synthesized and cells were born.”

Then they pointed the technique at a human brain.

What the clock says

The 2005 paper looked at the occipital cortex, the visual processing region at the back of the head. The non-neuronal cells there were being exchanged, as expected. The neurons were not. In the paper’s words, “occipital neurons are as old as the individual.”

Not seven years old. As old as you.

It gets better, in the sense of worse for the myth. Your eye contains lens crystallins, the proteins that make the lens transparent. The lens is sealed inside a capsule with no blood supply and no way to shed old cells, so nothing can get in to replace them. A Danish group radiocarbon dated them in 2008 and found that lens crystallin formation “almost entirely takes place around the time of birth,” with only a very small and decreasing trickle afterwards. You are looking at this screen through proteins you were issued in the womb.

Tooth enamel is the same story, and forensic scientists have made it useful. Because enamel is laid down in childhood and never remodelled, its carbon-14 content records the year it formed. Frisén’s group showed in Nature that enamel dates a person’s year of birth “to within 1.6 years,” against the 5 to 10 year error of the skeletal and tooth-wear methods pathologists had been using. If you are found dead and unidentified, your molars will testify to when you were born.

Elsewhere in the body the picture is more mixed, and more interesting than a single number.

Your heart replaces its muscle cells slowly. Bergmann and colleagues measured about 1 percent turning over annually at age 25, falling to 0.45 percent by 75. Do the arithmetic and fewer than half of the cardiomyocytes you were born with get exchanged across an entire life, which is exactly what they concluded.

Your fat cells turn over at about 10 percent per year, and here is the cruel part. Adipocyte number is fixed during childhood and adolescence and then stays constant in adults, “even after marked weight loss.” Lose thirty kilos and you still have all the fat cells. They are simply emptier.

Your hippocampus, unlike your occipital cortex, does make new neurons. About 700 per day, per hippocampus, which works out to an annual turnover of 1.75 percent of the neurons in the renewing population. That is a real and surprising finding, and it is also a rounding error against the tens of millions of cortical neurons that will never be replaced at all.

Where the seven comes from, and why it cannot work

The seven-year figure is usually defended as an average. Add up the lifespans of the various cell types, weight them somehow, and you land somewhere in the range of seven to ten years.

The average is real. The conclusion drawn from it is nonsense, and the reason is worth being precise about.

An average tells you nothing about the extremes of a distribution. This distribution has a spike sitting at “never.” No matter how many years you wait, the cells that are never replaced are still not replaced. There is no waiting period after which you can say every cell is new, because the set of cells that never turn over does not shrink with time. Seven years, seventy years, it makes no difference. Your occipital neurons will be there at the end.

Saying you get a new body every seven years because the mean cell lifespan is seven years is like saying that because the average car on the road is nine years old, there are no vintage cars.

The number that is actually enormous

Here is what makes the myth so seductive: the turnover really is staggering. It is just distributed with wild unfairness.

Ron Sender and Ron Milo went and counted properly in 2021. The human body replaces about 0.33 × 10¹² cells every day, which is 330 billion, and close to 90 percent of them are blood cells. By weight, that is 80 grams of new cells daily, dominated by blood and the lining of your gut.

Run that over the mythical seven years. You will produce roughly 8.4 × 10¹⁴ cells, which is about 28 times the total number of human cells in your body. By mass you will have built around 204 kilograms of new cells, close to three times the weight of the person doing the building.

And your visual cortex will contain the same neurons it contained when you were three.

That is the fact worth carrying around. Not that everything changes, but that a body can churn through three times its own weight in fresh cells while quietly declining to touch the parts that hold your eyesight, your memories, and your teeth.

Sender and Milo have form here. This is the same Ron Milo whose lab demolished the claim that bacteria outnumber your cells ten to one. Twice now, a widely repeated number about the cells in your body has turned out to be somewhere between unsourced and wrong, and the same quiet Israeli lab has been the one to go and count.

The clock is being erased

There is an ending to this that I did not see coming.

The bomb pulse works because atmospheric carbon-14 changed sharply and then decayed in a known way, giving each year a distinguishable signature. That signature is now disappearing, and not because of anything nuclear.

Fossil fuels are old enough that all their carbon-14 has decayed away. Burning them pumps carbon into the atmosphere with no carbon-14 in it at all, diluting what remains. Heather Graven modelled this in 2015 and the conclusion is startling. Under business-as-usual emissions, atmospheric carbon-14 falls to a level “equivalent to the depletion expected from over 2,000 y of radioactive decay,” and, she writes, “radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.”

A cell born today already carries a fossil-diluted signature that looks like a cell born centuries ago. The window in which a human being could read the birthdate of their own neurons opened when we started testing nuclear weapons and is closing because we are burning coal.

We got about seventy years of it. Long enough to find out that we do not, in fact, get a new body every seven years.

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