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DEBBIE ELLIOTT, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Debbie Elliott.

Time now for our look at some of the elemental questions of life in our segment, Science Out of the Box.

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ELLIOTT: Many have heard someone say, I'm not the person I was. Well, that may be more true than you know.

NPR science correspondent David Kestenbaum has this tale.

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DAVID KESTENBAUM: Okay, we all get haircuts. But have you ever thought that when we do we lose atoms. That hair that was once part of us goes its own way. So at the end of the day, what happens to my hair on the floor here?

Unidentified Woman: Well, they get swept up and put in the trash.

KESTENBAUM: Those are my atoms.

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KESTENBAUM: How much of me is really with me throughout my life? I mean, my skin rubs off, I trim my fingernails, part of me is eroding and presumably being rebuilt. But how much is it, 1 percent? Is it 10 percent?

I called around to biologists and chemists, and no one seemed to know the answer.

Mr. LOGAN McCARTHY (Chemistry, Harvard University): I'm Logan McCarthy. I teach chemistry at Harvard University.

KESTENBAUM: Had you ever thought about to what extent we're just spare parts being replaced all the time?

Mr. McCARTHY: I guess it never really occurred to me. You know, who I was is not - the atoms that I have now are not the same atoms that I had, you know, yesterday or the week before.

KESTENBAUM: McCarthy did some research and he found this article from a Smithsonian Institution publication from 1953. So this is the beginning of the Atomic Age. And the article described these experiments where researchers fed to people radioactive atoms. Or they injected them with radioactive atoms. And then using radiation detectors, they could watch the atoms as they moved around. So they'd watch them go up one arm, into the heart and down the other arm.

Mr. McCARTHY: You can follow it through their body. Does it get excreted through urine, or is it excreted through their sweat or through feces or, you know, what happens to it? Does it end up in their fingers or in their eyeballs, or you know? So you can follow where these atoms go.

KESTENBAUM: Where do they by doing that?

Mr. McCARTHY: Well, they end up in all of your tissues.

KESTENBAUM: A lot of the atoms get incorporated into our bodies. The article says the atom turnover is quite rapid and quite complete. In a year, 98 percent of the atoms in us now will be replaced by other atoms that we take in, in our air, food and drink. So that means 98 percent of me is new - every year.

So this is the very profound rule of nature: You are what you eat.

Mr. McCARTHY: Yes, absolutely. If you eat a hamburger one day, then the atoms and molecules in that hamburger will end up making up your cell walls and different organs and tissues.

KESTENBAUM: So my friend - I have a friend who eats soup every day for lunch so he's mostly made of soup, or largely.

Mr. McCARTHY: He is mostly made of soup, yes.

KESTENBAUM: But don't be saddened. McCarthy says this constant replacement of parts is actually what makes life so robust and so adaptable. It's so lively.

Mr. McCARTHY: Life is remarkably in that it requires always a flow of energy and matter through the system. So if the system isn't constantly bringing in new energy and bringing in new matter, then it's basically dead.

KESTENBAUM: Still, this means that in a very real sense, we are not the people that we were a year ago. We're this collection of atoms that hang out together for a while and then they go on to do other things - sort of a momentary cloud of organization.

So what is me? Am I still me if my parts have been replaced?

Professor DANIEL DENNETT (Director, Center for Cognitive Studies, Tufts University): Well, of course, the question goes way back to ancient philosophy.

KESTENBAUM: This is Daniel Dennett. He's a philosopher at Tufts University. Remember, he says, the old joke about Abe Lincoln's axe?

Prof. DENNETT: There it is in the glass case and it says, this is Abe Lincoln's axe. So I say, that's really his axe? And he says, oh, yes, but, of course, the head has been replaced twice and the handle three times.

KESTENBAUM: There's also a modern atomic version of this puzzle that really gets to the heart of things.

Prof. DENNETT: We imagine that your rocket ship has landed on Mars and you have to get back from Mars to Earth by teleporter.

KESTENBAUM: Here's how the teleporter works. It dismantles you, atom by atom, (unintelligible), you know, records the precise location of every carbon, every hydrogen, every phosphorus, and it sends that information to Earth, (unintelligible), where a receiver transporter reconstructs you, (unintelligible) out of new atoms.

Prof. DENNETT: And you step out of the teleporter receiver on Earth, is that really you? I say, of course, it's you.

KESTENBAUM: Okay, that's clear enough.

But now imagine, he says, instead, the teleporter on Mars doesn't take you apart - it doesn't disassemble you - it just scans your atoms, do-dot-dot-do-do(ph), leaving you intact.

Prof. DENNETT: So now you're - there's a you that's stranded on Mars and there's a you that's back on Earth. Which is the real you?

KESTENBAUM: Well, it's pretty clear to me. That there's - that I have David 1 and David 2.

Prof. DENNETT: Yeah. And does one of them have some sort of special priority? Is one of them sort of realer, more you than the other?

KESTENBAUM: Yeah. What does my wife do?

Prof DENNETT: Exactly. Yes.

KESTENBAUM: I could tell you what my wife would do. My wife would groan if two of me showed up. It turns out there are some atoms that are with us fosr our entire life. This comes from a researcher in Sweden and the atoms are actually in some interesting places. They are deep in the DNA of some cells in our brain and in our heart, and also some atoms in our teeth. So brain, heart and teeth. Don't forget to brush.

ELLIOTT: That is the temporary collection of atoms currently known as our science correspondent David Kestenbaum.

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