Many have asserted that the water we drink is exactly the same water that has been eliminated from the bodies of humans and animals from the past. The argument goes that water excreted from our bodies by exhalation or urine ultimately ends up back in the water cycle. That is, water eliminated from biological organisms finds its way into rivers, lakes, ponds, and oceans. This same water evaporates back into the atmosphere, and rains back down onto the Earth in an endless cycle. Because the water we consume comes from water that participates in this timeless process, it stands to reason that we drink water molecules that have been excreted from Muhammad Ali, Gandhi, Marie Curie, and even the dinosaurs!

I think chemistry informs this argument in an interesting way. Consider that water on Earth participates in a variety of chemical reactions such as photosynthesis (Equation 1):¹

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂                Equation 1

Notice that water molecules are consumed – in a sense, destroyed – during photosynthesis. In fact, during the first chemical step of photosynthesis, light energy is used to oxidize water into oxygen gas, removing hydrogen from the water in the process (Equation 2):¹

2 H₂O → O₂ + 4 H⁺ + 4e^-                                          Equation 2

Consider a single water molecule involved in this process. The two specific hydrogen atoms and specific oxygen atom contained in this water molecule become separated, with the oxygen atom combining with another oxygen atom to make O₂. As a result, the two water molecules have entirely lost their chemical identities. Certainly, the O₂, protons, and electrons formed become extremely well mixed as they participate in a variety of other physical and chemical processes after separation.

The reverse of these processes happens in respiration, during which brand new water molecules are created (Equations 3 – 4):

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O                 Equation 3

O₂ + 4 H⁺ + 4e^- → 2 H₂O                               Equation 4

Furthermore, water molecules are created or destroyed in a whole host of geochemical process,² one of which is displayed below (Equation 5):

CaAl₂Si₂O₈  + H₂O + CO₂  + ½ O₂ → Al₂Si₂O₅(OH)₄ +   CaCO₃                  Equation 5

Finally, consider that water molecules undergo hydrogen atom exchange reactions during which they do exactly that: trade hydrogen atoms with neighboring water molecules.^3,4 Such reactions can be monitored using deuterated water, D₂O (Equation 6):⁵

D₂O + H₂O → 2 HDO                        Equation 6

These exchange reactions happen very quickly, on the timescale of picoseconds,⁴ and can also occur between water and amines, alcohols, and acids.⁵

I made a short video that succinctly describes how chemistry informs the assertion that we drink dinosaur pee (Video 1).⁶

Video 1: Drinking dinosaur pee?!, Tommy Technetium YouTube Channel, November 22, 2023

After viewing this video, one person commented that my argument brings to mind The Ship of Theseus,⁷ a philosophical thought experiment. The Ship of Theseus poses the question: “If the component parts of an object are completely replaced, does the identity of the object remain the same?”. What do you think? Is it appropriate to say that we drink the same water molecules that the dinosaurs drank and peed out? Regardless, I hope you consider the chemistry involved when you think about how you might answer this question.

References:

1. https://pubs.acs.org/doi/epdf/10.1021/acs.chemrev.5b00340
2. Drever, A. I. The Geochemistry of Natural Waters: surface and groundwater environments, 3rd ed., Prentice Hall, Inc. 1997.
3. https://www.pnas.org/doi/10.1073/pnas.1306642110
4. https://pubs.acs.org/doi/epdf/10.1021/acscentsci.9b00603
5. https://pubs.acs.org/doi/abs/10.1021/ja01652a030
6. Drinking Dinosaur Pee?!, Tommy Technetium YouTube Channel
7. https://en.wikipedia.org/wiki/Ship_of_Theseus