h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Given that most (but not all, so I understand) cells in the human body are constantly dying and being replaced, how much of me is original?

Probably not much at all I would imagine.

Your cells, even if not replaced yet, will in themselves constantly be remaking the proteins, lipids and other essential molecules within them from the food you eat and oxygen you breathe - so that almost none of the atoms and molecules themselves are original from you as a baby when born. The old degraded proteins, lipids etc are reused or excreted in your (ahem) effluent as time goes by.

Not sure about the DNA itself though - that is very tightly bound within a cell's nucleus so may not actually get remade except when the cell divides...

The DNA though represents a very small proportion of your body.

So my entirely handwaving answer is - probably not much

I have read, completely unverified, that nearly every cell in your body replaces itself every month or so.

The only exception is your neurons. Your nervous system is the same age as you and will remain so.

B

Oh, didn't know that, Mu, it may explain a few things!

Now I'm curious!

If 'nearly every cell in your body replaces itself every month or so', how come tattoos remain visible (if a little blurry) for decades?

Because your body is not just composed of cells. Bones aren't cells for example. Your skin is composed not just of skin cells (many of them dead incidentally) but also interconnecting tissue. A tattoo is also made of material that your body's degradation systems cannot destroy.

Bone's aren't cellular I should say.

And before someone jumps in, yes there are bone cells, but the matrix itself is extracellular.