[patates]

I always thought that if separating water and salt were easy, our bodies would have evolved to do it so that we'd be able to drink sea water and be fine. It must have been so expensive that searching for fresh water was worth it or there were plenty of fresh water that it was never a evolutionary pressure. Evolving kidneys capable of concentrating urine beyond 3 something percent concentration (sea water) perhaps required a massive restructuring of our internal organs and a huge constant energy expenditure, so we kept seeking fresh water.

ps. I have no clue what I'm talking about

[Tagbert]

It’s mostly that it takes energy. If fresh water is we drink that. There aren’t a lot of places where only salt water is available so, for most animals, it isn’t worth it to have evolved a way to extract water from salt water.

Animals in the ocean of course do live without fresh water. Some of them just live off of water extracted directly from their food or from metabolizing that food, which produces water. Some animals have specialized cells that excrete salt so that can take in salt water and separate out the salt.

[blackoil]

Salt water fish can process sea water, no point in evolving for saltier brine if you have oceans of 3% water.

[scythe]

>I always thought that if separating water and salt were easy, our bodies would have evolved to do it so that we'd be able to drink sea water and be fine.

Unfortunately for terrestrial animals, it's just not that simple. Seawater contains a lot of microbial life, some of which can be infectious or toxic. Going to the coastline to drink is potentially hazardous, because it usually means descending a hill on a predictable route which will be attractive to predators. And you need to get pretty far into the water, usually, because of nasty stagnant runoff, which can come from decaying matter that washes ashore, and sand in the surf. That means you risk drowning. Plus, you don't just need the energy for desalination, but the infrastructure (similar problem to real life!), which means more and larger juxtamedullary nephrons in the kidney, which is already a major weak point on the back due to the high blood flow in the kidney. Meanwhile, most of your food contains a lot of water, especially if you're one of the 99.99999% of animal species that doesn't cook it.