cell turnover

turnover · persistence · identity

You've probably heard it: every cell in your body is replaced every seven years, so you're a completely different person than you were a decade ago. The real number, from a 2005 carbon-dating study, is a whole-body average — not a shared clock. Turnover is wildly uneven: gut lining renews in about five days, while cerebellar neurons, formed in early childhood, last a lifetime and never turn over at all. The tissue most tied to your sense of self is exactly the tissue that persists. That's a real problem for the idea that "you" are just your current atoms — but it's not evidence for anything beyond materialism either. Even the replacement atoms aren't new: physicist James Jeans calculated that a molecule from Julius Caesar's last breath is, by now, in almost every breath you take — matter on loan from a stock every living thing has already used. Whether identity depends on matter at all is a question philosophers were already asking long before carbon dating existed, all the way back to Lucretius arguing that a dispersing, borrowed body is exactly why death isn't worth fearing.