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[–] Dereliction [S] ago 

Enjoyable answer, though I'm still left wondering how much of us is made up of stuff that was, at one time or another, part of another critter on this planet? Or is most of our atomic make up "freshly" used, essentially untouched by other life on this planet during its billions of years spinning around the sun?

I realize that inspecting a newborn, which has been laying about in the soup of its mother for nine months, will undoubtedly result in a different answer than that of a 100 year old who has been eating and doing who knows what during its existence. But by how much?

More importantly, and to iterate my original question, how much of me (or you) is T-Rex atoms? Are we 10% T-Rex in terms of the atoms we've shared? 1%? 0.00001%?

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[–] 0x5f3759df 0 points 1 point (+1|-0) ago 

Well, on that link I gave you can also see there were an estimated 1050 bacteria in the history of ever. Since the majority of organisms are, and have always been bacteria, this could just as easily be taken as the total number of critters ever (and certainly it is a lower bound). Compare this to the only 1027 atoms in a body, there have been 1022 more creatures than atoms in a creature. Given these numbers, I would guess the majority of the atoms in a body would have been part of many others before.

However, I doubt it will be all of your atoms. The total amount of atoms in the earth, even only counting the major components of organic materials, is far larges still than the number in the entire biomass. So, despite the huge number of atoms in those 1050 organisms, there are even more that have not been used yet. Question is, though, how likely it is that an atom from the center of the earth gets to the surface to be absorbed into the biomass. I have no idea about the answer to this; I guess volcanoes probably bring up some not yet used material. There is also a constant stream of matter coming of from the sun, called the solar wind, which deposits new atoms on the earth. Since they come down from the atmosphere, they are more easily absorbed in a body than the ones from deep inside the earth. Chances are, at least a few of those will end up being first used in your body.

So, in summary, probably most atoms have been used before, but most likely not a hundred percent.