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Is it true that we’re breathing the same molecules once breathed by the dinosaurs, Julius Caesar or Jesus Christ?

The expert: Dr. Martin St. Maurice, assistant professor of biological sciences

“There is some truth to this possibility. The air we breathe is composed primarily of nitrogen gas and oxygen gas with a small amount of other gases, including carbon dioxide. All of these individual molecules are constantly rearranged and recycled through biochemical and geochemical processes, so you aren’t breathing in the exact same gas molecules that dinosaurs and Julius Caesar once breathed.

"The individual atoms making up those molecules, however, have been on earth for a long time very little carbon, oxygen or nitrogen is lost to outer space, and only the occasional meteor brings a small extraterrestrial source of new carbon or oxygen to this planet. So, every breath you take and every bite you swallow is composed of atoms that have been here for a long time.

"It’s certainly possible to imagine a scenario where you breathe in a molecule of oxygen gas and, in one of your billions of cells, it gets combined with carbon from last night’s cupcake to make carbon dioxide. Your exhaled molecule of carbon dioxide is taken up by a young oak tree and, with the help of sunlight, the carbon gets converted into a molecule of cellulose that gets locked into that tree’s biomass for years. Eventually, over hundreds of years, that tree will grow, die and decompose. As it decomposes, that atom of carbon is released back to the atmosphere as carbon dioxide and is used to generate oxygen through photosynthesis.

"That newly formed molecule of oxygen might be available to your great-great grandchild. It’s not the same molecule of oxygen that you breathed in years before, but it could be traced back to atoms that once passed through your body. In fact, this concept is also true of the food we eat. There may have been a carbon atom in last night’s cupcake that was once integral to the structure of Julius Caesar’s left toenail.

"All that said, what are the chances that an atom of oxygen you just breathed was once a small part of Julius Caesar? We can estimate approximately 67,500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 atoms of oxygen on earth and 6,350,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000 atoms of carbon (that’s a lot of zeros). A quick calculation, loaded with assumptions, reveals that even if we sample completely different atoms of oxygen with every breath of life, we sample at most 0.0000000001 percent of all the oxygen atoms on earth over an 80-year lifespan.

"You don’t have to be a stats whiz to see that the chances of you and Julius Caesar sharing an identical atom of oxygen are extremely slim. There’s much less carbon than oxygen on earth and it’s contained over a much smaller volume, so I think you have a slightly better chance of eating a snack that was once a part of Caesar’s toenail (though these chances are still extremely slim).

"Approximately 3.5 billion years ago, there was no oxygen gas in the atmosphere; it developed in our atmosphere thanks to ancient photosynthetic microorganisms. So, while you aren’t likely to ever share exactly the same atom of oxygen as Brad Pitt or eat a cupcake that was once a part of Caesar’s toenail, every breath you take has, at one time or another, been associated with another living organism."

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Comment by Jeffrey Scott Nuttall at Nov 04 2011 04:28 pm
Your conclusion about the slimness of the chances of sharing an atom of oxygen with Caesar is mistaken. It may seem an intuitively obvious result given some of the assumptions and intermediate calculations, but this is a case in which intuition is misleading. For a detailed calculation of the problem, see <I>Atom: An Odyssey From the Big Bang to Life on Earth</I>, by Lawrence Maxwell Krauss -- you can read the relevant portion on Google Books (I tried to include a link to the relevant page, but apparently links aren't allowed on the comment forms here--but you can find it easily by looking up the book on Google Books and searching for "Caesar"). Counterintuitively, and contrary to your conclusion, it turns out that your sharing oxygen atoms with Caesar is not only likely, but almost inevitable--indeed, there's better than a 99% chance that <I>any given breath you take</I> shares an atom with Caesar's last breath.
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