Where do those damn atoms go?

[04/23/10 Edit: If you came across this post by Googling to find out how many of our bodies’ atoms are replaced each year, would you drop me a comment and tell me WHY? I’m intrigued by the fact that so many people hit my blog by searching on this specific question! Thanks.]

Mark Jones left the following interesting question in another thread and I thought it was worth starting a whole new post in reply, because I’m often asked this question and I’m sure people would like to discuss it or at least hear me expand on it:

“Richard Dawkins intriguingly quoted you in his recent book, The God Delusion, “not a single atom that is in your body today was there when that event took place…” meaning, that every atom in the human body is replaced within our lifetime. I have read this assertion many times, but it’s source is never attributed. It is a facinating if true but I don’t want to quote it unless I’m sure of its scientic origins. Who can be cited?”

I can’t cite any single definitive authority, although see the last paragraph below. For my part I draw the conclusion from a whole bunch of evidence.

The first thing to say, though, is that it ISN’T ABSOLUTELY TRUE. Not quite. Not as an irrefutable and precise fact. I didn’t say it in the first place to declare a free-standing scientific fact but to add emphasis to an important but otherwise hard to believe point, and that point is still true even though I may have exaggerated by an atom or two.

The reason it can’t be true is actually supportive of my main point, so I might as well admit to it. It’s often pointed out (and again I can’t quote a higher authority on this) that statistically speaking, any glass of water you drink is likely to contain one or more water molecules that were previously drunk and later excreted by, say, Isaac Newton. Since water makes up a large percentage of our tissues, your morning coffee probably contained a molecule or two that was once an active part of Newton’s brain.

Logically speaking, therefore, you probably also have some atoms/molecules that were in your own body on the day you were born but then were excreted, recycled through rivers and seas, the sap of trees and the bodies of other creatures, only to turn up a second time in your food.

But this doesn’t alter the fact that all these molecules are transient parts of you. You are a system in flux; a pattern and not the sand in which the pattern is drawn; you are not the stuff of which you are made, and this was the point I was trying to make in my book.

Nevertheless, people still don’t believe me, so I need to counter some potential objections. The thing that makes it so hard to believe in the first place is the intuition that we absorb food, excrete most of it but retain a little, growing as we go. I’ll deal with some trickier examples in a moment, but in general this just isn’t true. Biological molecules don’t tend to last very long and need to be replaced. That’s why every cell’s DNA is kept so incredibly busy making new proteins. We grow when the input outweighs the output, that’s all.

Not only do whole cells die in their millions and need to be replaced, but individual molecules in those cells are constantly being recycled and repaired. Even the DNA itself. We have no problem recognising that our hair and fingernails look the same from day to day yet are constantly being replaced, but it’s harder to see this going on inside the cell membrane so we find it hard to believe. Yet every cell is engaged in a constant struggle to maintain itself against decay, not just day to day but second to second. It consumes a lot of our energy.

“What about brain cells?” people say. “I’ve heard that neurons aren’t replaced once we become an adult.” Well, until recently we didn’t think that adult brains produce new neurons but we now know this isn’t true. Maybe the new neurons go on to add new matter and not “replace existing memories”, but our brains don’t get heavier so it seems to me most likely that the new cells take on connection patterns similar to existing ones, which later die. The memories and mental functions remain but the neurons that represent them change over time.

Either way, just because a given neuron stays around for many years, this doesn’t mean the cell is made from the same molecules all that time. Just like any other cell (and neurons are especially metabolically active) they consume energy to maintain themselves against decay and repair themselves from damage. Keep taking those antioxidants, because oxidation due to free radicals is one way in which the components of cells are repeatedly being broken down.

“Alright then, so soft tissue is constantly being replaced, but what about bone? People exposed to radioisotopes like Strontium absorb it into their bones and the radiation from these can still be detected many years later, so bone must be a permanent fixture, surely?”

Ok, you kinda got me on the Strontium, maybe. Same with lead. But not the bone. Bone is constantly being broken down, not just by the ravages of time but by cells called osteoclasts. And new bone is constructed by osteoblasts to replace it. This deliberate destruction of body tissues is more the rule than the exception: the body is maintained in a dynamic equlibrium, between in-built destructive and constructive processes. If it weren’t then it couldn’t adapt, and adaptation is the name of the game, even for bone. You only (!) have to go into space for a few days, or lie in a hospital bed for a few weeks, to see your bone mass drop substantially as the constructive forces are slowed (because the body thinks they’re now an unnecessary waste of energy), leaving the destructive forces to set a new balance.

And about that Strontium. I imagine it’s probably true that you can pick up a little radiation for years after exposure (although I don’t know for sure that it is retained for a lifetime, and the retention still has a half-life [a chemical half-life, independent of the radioactive half-life], so all of it would be removed if you lived long enough). But that doesn’t mean the Strontium atoms are in the same bits of bone from moment to moment. Calcium, magnesium and elements that mimic them get removed from bone all the time by osteoclasts, and much of it gets re-used to make new bone. Some of the minerals in bone do take a long time to leave the body but they’re still being recycled (resorbed) all the time, and that’s all I was trying to say.

Things get a bit more philosophical when we ask the question: “what does it mean for a molecule to be the same molecule anyway?” One of the things I was trying to get across in my book was that, not only are bodies processes or patterns, not “stuff”, but even stuff is a process. A subatomic particle is (I believe) a resonant state, not a little lump floating in space. An electron is conceptually more like a hurricane or the whirlpool that forms around your bath plughole – it’s a persistent disturbance in something (in this case a “field”). It’s more like the ringing of a bell than the bell itself. And is the sound you hear from a bell now the same sound you heard a moment ago, or is it a different ring that simply sounds the same as the previous one? When you start to look at particles, and therefore atoms and molecules, as self-preserving dynamic states, the whole concept of static “sameness” ceases to be meaningful.

What I was trying to do in Creation was to dismantle our intuition that the universe is made of (a) “real stuff” and (b) mere patterns in that stuff, by showing that even the real stuff is patterns too. There’s therefore no fundamental distinction between, say, mind and matter. It’s not that matter is somehow real and physical, while mind is not, they’re just different levels of self-maintaining pattern. Nor do we need to invent a special kind of (dualistic) reality for mind. And I then went on to assert that, when the processes that we think of as “real things” in the so-called physical universe arise inside a virtual universe in the same fashion, then we have to consider them just as real. Artificial life can therefore (under the right circumstances) be real life. But that’s a long argument and one book wasn’t nearly enough to do it justice.

So that’s the reason I made the assertion about the total replacement of our bodily atoms. I may have overstated it – there may be a few atoms that still ring like the same bell all our lives, but it’s more of a statistical accident than anything else. The central point is that our bodies are in constant flux. We are not the stuff of which we are made; we are a self-maintaining pattern in a constantly changing substrate.

I hope you’ll agree, because I think that once we rid ourselves of our innate dualism, the whole universe takes on a very different and much more creative look.

So I can’t cite a single definitive authority, but the evidence for the principle of constant renewal is out there in spadefuls. Someone once sent me a link to an NPR story that discusses some of this, and that does quote some specific experiments that show 98% of our bodily mass being replaced each year. Now, assuming this doesn’t mean that everything that could be replaced is totally replaced inside a year but 2% remains forever indelible, then if 2% remains after one year, only 2% of that 2% will remain the second year. When you get to be as old as I am, that brings it down to 1E-83 of a percent, and yet our bodies only contain something of the order of 1E29 atoms. So that suggests we are totally replaced many, many, many times over, and any atoms we have now that WERE in our body on the day we were born are almost certainly back for their second or third appearance!