Why is it possible that you share some of the same matter as a dinosaur that lived millions of years ago?

Answer 1

Elements and compounds are recycled in nature, so that carbon and oxygen and nitrogen in our bodies may have also existed in the bodies of dinosaurs who have decomposed back into the biosphere.

The physical integrity of atoms can only be affected by nuclear reactions such as fission and fusion, and such reactions are very uncommon on Earth. These days (as in, the last few billion years) the only atomic nuclei on Earth to be affected naturally are those that are subject to radioactive decay (various heavy elements and isotopes etc.). Therefore, the vast majority of the atoms that exist today also existed during the Mesozoic, when dinosaurs ruled the Earth, considering that living beings are for the most part composed of stable elements. Those atoms may have taken part in countless multitudes of chemical reactions, and been part of many organisms and inert material since then, but the atoms themselves remain unchanged. Therefore we can safely say that at least some of the atoms that are now in your body existed in the bodies of dinosaurs millions of years ago (and in the bodies of many other creatures that lived even earlier). Living beings constantly exchange matter and energy, through excretion and the like, and millions of years in the future many atoms that are now in your body will be present in the organisms that will exist at that time.