Water is essential for our survival, yet more than 96% of the planet’s liquid water is ocean water, and it contains so much salt that it is not safe for humans to drink.
Salty seawater does not quench thirst, and drinking too much of it can even lead to death from dehydration.
But if saltwater is still water, why can’t we drink it?
The answer to this question is quite simple: Salt water is too salty for our kidneys.
Approximately 3.5% of the weight of seawater comes from dissolved salt, according to the National Oceanic and Atmospheric Administration). If all the salt were extracted from the oceans and spread over all the Earth’s land surfaces, the salty layer would rise more than 166 meters high, about as high as a 40-story office building, NOAA says
The salinity of seawater is too high for humans to process safely, since our cells need water “in a relatively pure form.”
“For most animals, the kidneys filter impurities out of the water. What happens when you drink salt water is that you ingest a large amount of salt that the body now needs to wash out.
It does this in the form of urine, which the kidneys produce by dissolving the impurities in the excess water, which is then sent to the bladder to be eliminated. But the kidneys can only produce urine that is less salty than our blood, according to the National Oceanic and Atmospheric Administration, and salt water contains more than three times the amount of salt normally present in human blood. This means that for every cup of salt water you drink, you’ll need to drink at least the same volume of water for your kidneys to get rid of all that salt.
You might ask yourself, “Why not drink more salt water? But then you’re left with more salt that you’ll have to flush out with even more water. So salt water can never quench your thirst, it can only make you thirstier.”
Some animals can drink salt water, why can’t we?
However, some animals in ocean ecosystems have adaptations that allow them to drink saltwater safely
Seabirds, such as albatross seabirds, gulls and penguins, which can spend weeks in the open ocean with no fresh water in sight, have specialized salt glands and grooves in their beaks to filter and purge excess salt from ingested water before it reaches their stomachs and is absorbed into their blood
Marine mammals, such as whales, dolphins and seals, have also evolved adaptations to live in an environment where freshwater is scarce or non-existent.
Marine mammals have adapted special enzymes and cellular structures that allow them to purge excess salt from their systems.
So why didn’t we, and why did humans – and almost all other land animals – evolve to drink fresh water when salt water is much more abundant? When animals moved out of the ancient seas hundreds of millions of years ago and began adapting to life on land, species moved away from coastal habitats where there was plenty of salt water.
Many terrestrial species-including our primate ancestors-ended up inhabiting inland ecosystems with abundant freshwater in lakes and rivers, but with little or no saltwater sources. This probably resulted in biological adaptations for drinking water that was not salty.
Most of our ancestors were not exposed to salt water, whether general animals, primates or insectivores, so natural selection focused on processing non-salt water, and our physiology became so fine-tuned that disrupting it with salt water later on becomes very dangerous and detrimental.”