Most of Earth’s water has likely passed through an animal’s body as urine at some point in history. According to researchers Neil Donahue of Carnegie Mellon University and David Kreamer of the University of Nevada, the 500-million-year history of chordates provides enough time to cycle the planet’s 1.4 billion gigatonnes of water through biological systems.
How much of Earth’s water has been urine?
The mathematical probability suggests that the water you drink today may have once been part of a dinosaur or a prehistoric fish. Neil Donahue arrived at this conclusion by analyzing the relationship between the total volume of Earth’s water and the daily biological output of animals.
Using data from the United States Geological Survey (USGS), Donahue identified that Earth holds approximately 1.4 billion gigatonnes of water. To estimate the biological turnover, he applied a hypothetical model where animals produce urine equal to 1% of their body weight. He calculated that chordates—a group including mammals, birds, reptiles, amphibians, and fish—produce roughly 0.02 gigatonnes of urine every day.
“It would take approximately 191 million years to empty the entire ocean with urine,” Donahue told Live Science.
Since chordates have existed for at least 500 million years, the timeframe is more than sufficient for the planet’s entire water supply to have cycled through living organisms multiple times. While Donahue admits his 1% weight assumption is an “absurd” simplification, he notes it provides a useful baseline for understanding how much water interacts with life.
The daily urine output of the world’s chordates is equivalent to roughly 8,000 Olympic-sized swimming pools.
Why hasn’t every drop of water been through a living organism?
Despite the massive scale of the biological cycle, not all water on Earth has been “used” by animals. Hydrologist David Kreamer notes that significant portions of the planet’s water remain isolated from the surface cycle.
Large reservoirs of water are locked away in locations that prevent them from circulating through rivers, oceans, or the atmosphere. Kreamer points to three primary “untouched” sources:
- Glaciers: Massive ice sheets hold water in frozen states for millennia.
- Aquifers: Deep underground layers of rock and sediment trap water far from surface life.
- Juvenile Water: Water trapped deep within the Earth’s mantle that has never reached the surface.
This isolation means that while the majority of the water cycle is biologically active, a subset of the Earth’s hydrosphere remains chemically “fresh” from a biological standpoint.
What is juvenile water and how does it enter the cycle?
Juvenile water represents a frontier in hydrological science. This water is stored at extreme depths within the Earth and has no prior history of interacting with the atmosphere or biological life. It remains sequestered until geological forces bring it to the surface.
Volcanic activity serves as the primary delivery mechanism for this “new” water. During eruptions, lava and rising magma release water in the form of steam, vapor, and ash. According to Kreamer, these events introduce water to the surface that has effectively “never been urinated” by an animal. This process ensures that the Earth’s water supply is not a closed loop, but a system constantly being replenished by deep-earth reservoirs.
Understanding the release of juvenile water through volcanic activity helps scientists predict how much “new” water enters the global cycle during periods of high tectonic activity.
The future of water discovery and planetary science
The distinction between “cycled” water and “juvenile” water has major implications for the future of resource management and space exploration. As humans look toward more efficient ways to manage dwindling freshwater supplies, understanding the deep-earth reservoirs could become vital.
Furthermore, this research provides a framework for astrobiologists. When searching for life on moons like Europa or planets like Mars, scientists look for evidence of a water cycle. Determining whether water has been biologically processed or is merely “juvenile” could help distinguish between a dead world and one that has supported life.
Frequently Asked Questions
Has all the water on Earth been inside an animal?
No. While researchers suggest most water has likely cycled through animals, much of it remains trapped in glaciers, deep aquifers, and deep within the Earth as juvenile water.
How do scientists know water comes from inside the Earth?
Volcanic eruptions release water in the form of steam and vapor from the Earth’s interior, introducing “new” water to the surface cycle.
What are chordates?
Chordates are a biological group that includes humans, mammals, birds, reptiles, amphibians, and fish.
What do you think about the scale of Earth’s water cycle? Leave a comment below or subscribe to our newsletter for more deep dives into scientific discoveries.
