After many calculations, the answer is more complicated than it seems.
Many of us have asked ourselves: has the water that comes out of the tap ever passed through the body of a dinosaur, a mammoth or another prehistoric animal? The question may seem strange, but it helps explain the size and complexity of the Earth’s water cycle.
Over millions of years, water molecules evaporate, condense, fall as precipitation, freeze, flow through rivers, accumulate in the oceans and can even end up inside living organisms — before returning to the environment again.
Neil Donahue, professor of chemistry and chemical engineering at Carnegie Mellon University, says that it is likely that, in a very general estimate, much of the planet’s water has already passed through the body of some animal.
The researcher starts from an approximate calculation: a person weighing around 50 kilos urinates, on average, one liter per day. Assuming, with great reservations, that animals urinate a similar proportion of their body weight, and considering the total mass of chordates — a group that includes mammals, birds and fish —, it would be possible to estimate a daily urine production of around 0.02 gigatons.
Earth, on the other hand, contains about 1.4 billion gigatons of water, including oceans, glaciers, lakes, rivers, groundwater and atmospheric vapor. Dividing this volume by the estimated amount of urine produced daily, Donahue calculates that it would take about 191 million years so that all the water on the planet had passed, at least once, through the organism of animals. Since chordates have existed for more than 500 million years, there would have been enough time for this to have happened.
But the answer is not that simple. Says David Kreamer, professor of hydrology at the University of Nevada, remembering that these calculations depend on many generalizations and have a high margin of error. Furthermore, not all water circulates at the same speed — some is trapped in glacial ice for hundreds of thousands of years; another remains in deep aquifers for tens of thousands of years.
There is still the call juvenile watertrapped deep within the Earth and never entering the surface water cycle. This water can be released through volcanic activity, in the form of steam or associated with magma, then entering the hydrological cycle for the first time.
In other words: a lot of Earth’s water has probably passed through living organisms, but not all of it. With each volcanic eruption, “new” water can reach the surface, which was never urine — at least, not yet.
