Why didn't we evolve to live on saltwater?

Question

With 97% of the earth's water being saltwater, and survival of the fittest, why didn't humans evolve to survive on saltwater instead of freshwater? If we evolved from animals that crawled out of the sea, didn't they survive on saltwater?

Answer 1

Keep in mind that "survival of the fittest" was a phrase Darwin introduced in the 6th edition of _On the Origin of Species_ on the recommendation of a colleague and then removed immediately after that and spent most of the rest of his career trying to correct people from using that phrase incorrectly.

Animals that are adapted for saltwater are not somehow "stronger" than other animals simply because there is a great deal of saltwater on the planet. Certainly a saltwater animal is going to have a hard time surviving if it travels upstream into freshwater areas or moves far inland where only freshwater is available. Thus, in these upstream or inland areas, the "fittest" animal is the one that is adapted for freshwater.

So really you should be asking why certain animals at one time had an advantage as they moved into freshwater areas. Perhaps those animal that could adapt to the lower salinity could isolate themselves from saltwater predators and survive in their new environment, become prolific, and generate offspring that could move into more freshwater areas. Certainly some of those animals may have found that they could leave the water, travel on land, and easily find freshwater far inland after (for example) heavy rains.

Natural selection doesn't move to produce animals that can conquer the entire planet. Natural selection is about diversity. It explains diversity in biology by looking at the diversity of the biotic and abiotic environments around that biology as it develops. (and, of course, development changes those environments, so this is a dynamic process)

So the real reason for freshwater animals is the presence of freshwater. If 3% of the earth's water is freshwater, maybe 3% of the earth's animals will be freshwater animals (and plants, of course). Is that so hard to believe?

Answer 2

You know that is a good question. The earth is a funny place. But, ever since the first rains, fresh water organisms began to form. The foods that humans began eating on land is simply not digestible with salt water, further, the constant exposure to the sunlight makes it impossible to stay hydrated if you drink salt water. Our bodies had to adjust or cease to exist! Every cell in our body had to change metabolically! That's an amazing change really!

I have studied dolphins almost all my life. There was a time where dolphins walked the earth before entering the ocean. There are some ancient fossils of dolphin's skeletal structure, which suggest that dolphins could have been a hoofed animal before going back to the water. A hoofed animal suggest it was a vegetarian! Go figure! Going from a veggie eater to a fish eater in the ocean!

Take Care

Answer 3

Life probably evolved in the oceans initially so when animals evolved to survive on land, they would have needed to evolve to survive on freshwater. If they hadn't evolved to live on freshwater, then they would have been limited to within a close distance of costal areas in order to survive on saltwater. But as most land is far inland from costal areas, inevitably some animals would have evolved to live on freshwater in order to be able to exploit the benefits of the huge inland areas. So really we didnt evolve to live on saltwater, because we have evolved from life which relied on saltwater.

Answer 4

Freshwater and saltwater have distinct oxygen tiers and fish have tailored wisely to income the main quantity of oxygen from their ecosystem. A saltwater fish won't be able to get sufficient oxygen from sparkling water and vice versa. aside from bull sharks which look an exception, yet no longer a stable puppy selection.

Answer 5

Most living cells can't function with a salt concentration as high as that of seawater. Besides, we loose water by sweating and urinating and our kidneys and sweat glands haven't been able to evolve to excrete salt in the same concentration as that of seawater. Some organisms (such as sharks) have evolved to tolerate salt concentration in their bodies almost as high as seawater but at the price of constrains to various cellular functions.

Actually, I think it's a good question. I just get a little sick of all those fundamentalist science-bashers who say that something can't be explained scientifically just because they don't know the scientific explanation themselves, and then jump to the conclusion that science is flawed and some religious "explanation" is called for. Science can't explain gravity, either. Does that necessarily mean that when Newton was hit by a falling apple it was because some extra-terrestrial monster threw it in his neck? Please take your superstitious nonsense to a superstitious forum, and stick to science here.

Answer 6

That's a good question there, man.

I think it's because of our ancestors moved on to dry land and they wanted to go there.
Since that they're on dry land, their survivability in saltwater probably devolved and they evolved the ability to live on land without saturated oxygen inside water. That's why we've lungs instead of gills.
But we could be aquatic lifeforms if we adapt ourselves more into water than on land. Say, in about 300 thousand years or so, who knows? Maybe we could be living in saltwaters - without breathing apparatus!

Answer 7

My guess is that the vast amount of primordial ooze that crawled out of the water stayed nearby. Thus most of it is associated with it today.
However, the more aggressive ooze, bent on becoming the most obnoxious beings on the planet, travelled farther with reckless abandon. It makes sense, therefore, that mankind should have developed without salt water; we have a tendency to forget our roots.

Answer 8

As life evolves, it begins to fill in different ecological niches. As the oceans became crowded, some organisms, including our ancestors, began moving onto land, where there was less competition for resources. As they took over the land, where fresh water was more common, some of them adapted to using fresh water.

You can see that history reflected in the distribution of life today. There are species that live only in salt water, species that require fresh water, and species that live in between (euryhaline species).

Here's something else to think about:

"Indeed, it seems likely that vertebrate life evolved when the oceans were approximately one quarter as salty as they are today. As the oceans became saltier and vertebrates evolved further, several groups of vertebrates (birds, mammals, reptiles and amphibians) left the oceans to inhabit the land masses, carrying the seawater with them as their blood. They maintained their blood salt concentrations by drinking freshwater and absorbing salts from food."
http://www.ca.uky.edu/wkrec/VertebrateFishEvolution.htm

JMB

Answer 9

The amount of lifeforms that live on land, as opposed to those that live on water, are surely proportionate to one another. (The number and variety of lifeforms in the oceans are quite vast. I would assume there are far more species, and much larger populations of creatures than on land)

But in essence, if land was there... it makes sense that some creatures would eventually make use of it. (Our ancestors were probably not the best sea predators, in fact. Our very survival may have depended upon us gaining the safety of land...)

Answer 10

The vast majority of the life forms on this planet still live in the ocean. Kind of makes you wonder if other factors are involved in our evolution. God's will for example.