Why didn't humans evolve to better tolerate dirty water?

I was just reading something about a fancy filtration thing that lets people safely drink water from rivers, lakes, etc., and it made me wonder why we didn’t evolve to better tolerate all the protozoa and bacteria found in the fresh water supplies on our planet. Other animals drink dirty water 100% of the time and don’t get sick from doing so. Fish live in that water their whole lives. Other than those in captivity I doubt there’s a single hippo that’s ever had a sip of clean, pure, filtered water. How come other animals don’t have to boil water to drink it safely?

We evolved drinking from these same water supplies with the same bacteria for millions of years, so why are we so sensitive to it? I’d think, by now, after millions of years we’d have a natural tolerance and would be able to dunk our heads in the Ganges and gulp up as much of the dark brown water as we want without getting sick. Yes I do realize someone living in the Ganges river will have more of a tolerance to the pathogens than someone from the US who has had nothing but bottled water his entire life. Still, they get sick too, and providing them clean water is a big problem in need of a solution (hence the fancy filtering devices that can be made inexpensively for 3rd world countries). But why? I’d think evolution would have given us a pretty strong immunity to anything living in the fresh water we need to survive. Humans didn’t even have truly clean water until pretty recently.

Note: i’m not questioning evolution nor am I seeking to debate the merits of evolution.

Why do you assume animals never get sick from bad water? Wild animals are loaded with various parasites and bacteria. They do get sick. They don’t die from bad water too often because a sick, weakened animal usually becomes lunch for some other animal first.

Oh i’m sure some do get sick in extreme cases but it doesn’t seem that they’re anywhere near as sensitive to polluted water as humans are.

Also, if it wasn’t clear I’m only talking about bacteria/protozoa and the like, not chemical and heavy metal contamination that is almost certain to be caused by (modern) humans. I have no doubt humans dumping lead and chromium and benzene and the like does affect animals every bit as much as it affects us.

There are so many different types of pathogens, though. Many of which have a physical structure that is worlds away from others. Look at the hundreds of varieties of antimicrobial, antibiotic, antifungal, etc. that have been developed. Lots of these are derivatives of toxins produced by other pathogens. Our white blood cells and other immune responders are versatile, true, but we’d really have to carry even more bacteria ourselves to fight off others. And many (probably all) of the toxins produced that can harm pathogenic organisms are also detrimental to our own cells.

IIRC, the main way that these nasties are killed off is by either disrupting their cell membranes so they “dissolve,” or by blocking the mechanisms by which the organisms reproduce. So, something that attacks the proteins and sugars that are on the outside of the cells, or prevents others from going in properly. (Forgive my terminology, been out of school a while ;)) However, human cells and our beneficial/symbiotic yeasts and bacteria are also made up of the same types of protein etc. So it’s difficult for our own immune system to be selective enough, just as it is to take drugs that are selective to kill the germs before it damages the host too much.

I just found this, kind of explains what I mean:


Ever hear things like anaerobic, gram-negative, or acid-fast microorganisms? These are often used in the context of how the critters are chemically treated in order to identify them in a microbiology lab. However, the same mechanisms apply in giving us a clue to what the germs are susceptible to. Again, some of this is not dissimilar to what our own cells contain. Furthermore, if our own immune response gets to be too “broad-spectrum,” that can lead to certain autoimmune disorders, arthritis, etc.

Heck, the female body has to alter some of its immune responsiveness in order to deliver a healthy child!

Some more links that you might find interesting:


We created a semi-artificial environment in the 1st world, we have evolutionary pressure to that new world we have created, so things like water nasties are pretty much a non-issue, so such immunity is not so much a beneficial trait, perhaps tolerance to tap water chemical and additives is now more beneficial.

Also perhaps with the above we have lost a connection to our old natural world, no longer taught the ways of the people who lived in a more natural setting, and lacking the ability to observe the signs of what water should be good to drink and which ones not to be drunk from.

Oh, and you might find it interesting that sperm itself is practically a virus. The female immune response has to be modulated, both internally and via mechanisms carried in through semen so that initial insemination can occur at all.

In other words, too good of an immune response=no more people.


But the first world clean environment is a pretty recent advance. As little as 200 years ago nobody drank clean water as a matter of course. It made them sick so they drank beer and wine. Didn’t water become an actual beverage in the 20th century? So for all that time you’d think we’d have developed a stronger immunity to fecal matter and all the other gunk that’s living in fresh water sources.

Humans, and our ancestors, have been around a long time, and for all that time we’ve needed to drink fresh water. We’ve lived near fresh water, used it to wash ourselves, etc. I’d think over those millions of years we’d have developed immunities to amoebas and the typical bacteria in fecal matter.

I’m not saying they never get sick, but they never drink fresh water either - how do alligators and hippos and elephants and mice and birds drink filthy protozoa, feces-filled water and seemingly survive just fine?

Typically drinking untreated water in the wild does not make a human sick. We also seem to have a preference for cool water and seeing it sparkle and dance, which makes us tend to prefer rapidly moving water over stagnant water, and maybe as animals go we are relatively more dependent on water purity, but there would be a range in what all animals like. Trout are not going to do well in the water where carp usually live.

Think about swimming in lakes, streams, rivers and the ocean. The swimmer always ingests a little of that water, and rarely gets sick from it.

Consider it also a byproduct of civilization.
When there were a few hundred thousand people wandering the forests hunting the odd deer and turkey, water quality was not a big deal. Build a village every few miles, keep massive herds of animals penned in small fields, water quality suffers.

(Ever watch the movie “The Man Who Would Be King”? The chief complaint in each village is that the people upriver piss in their drinking water… :slight_smile: Movie is banned in Afghanistan).

BY removing the grass cover, fields allow the water to run into the nearest river quickly, carrying contaminants from the cow manure, pig pens, and human latrines - if we don’t just go in the river already. Civilization and agriculture concentrated the volume and ruined the water quality around human settlement.

Out in the wild - of course there are occasional diseases. We just haven’t allowed our immune systems to develop to fight them off, for a very simple reason… we’d rather not be among the 10% who die while the rest develop better immune systems. We’d prefer our children not to be among the 50% who died before the age of 5 so the rest would have good immune systems.

Plus, as pointed out above, the wild is full of bacteria and parasites. You should read the list of interesting creatures that could find you a cozy home if you venture into the water in Africa. Malaria is one of the more benign. Yet, while travelling in Egypt, we saw plenty of kids swimming in the Nile (well upriver of Cairo, of course!!) despite the tourist warning brochures. Possibly some got sick for the rest of their lives. You can probably drink from mountain streams in America just fine, but you just might catch beaver fever. (Seriously).

Let’s say the odds are only 1% that you get some exotic disease? How desperate do you have to be to drink? Some may adopt a macho hairy-chested disregard for their safety and do so, but for sure society would not let a business sell a product with those odds (except tobacco, I guess); and from a business or municipal water supply is where we get a lot of our water today.

Animals in the wild that are drinking dubious water, among other health issues do not tend to live anything like as long as those in human managed environments. So they are affected by unclean water to a significant extent.

Humans can live plenty long enough even when clean water is not as readily available, but infant mortality will be high. The mortality of young and infant animals will also be high due to poor water quality.

It will depend partly upon the evolutionary strategy, some animals have high birth rates, short lifespans along with higher body temperatures and much shorter digestive tracts. All these have an effect on resistance to disease and longer term species survival.

For humans, we have relatively low birth rates, relatively longer lives, relatively long digestive systems. The things that tend to compromise water quality for humans seem frequently to be human activities, especially over-population but also water pollution. We evolved as a hunter gatherer species so it shouldn’t be any surprise that high densities of human populations is likely to be less than ideal, unless some other control measure is adopted - such as treating water by heat, or chemicals, or by fermentation.

Surely there are naturally dirty lakes, rivers, etc. Especially where the water is stagnant. It may be full of mosquito larvae etc… i doubt “gross dirty water” is entirely byproduct of human civilization. Though surely we do contribute to much of it.

Was the Ganges (my prototypical disgustingly filthy source of water) sparkling clear and clean 200 years ago? 1000 years ago? 10,000 years ago? Surely small stagnant ponds with dirty, bacteria filled water have always existed, not just bc of humans.

I’m guessing before the Ganges basin was wall-to-wall people, before it was to goto destination for toilets and funerals, it was probably pretty clean. However, it was the ultimate drain for the nearby Himalayas, and like the Mississippi and the Nile, likely a source of relatively bacteria-free deep brown coloured water.

What about those large communal puddles (something far smaller than a lake and not really a pond) that you see in Africa that all the animals flock to and drink from? There’s a name for them I just can’t recall what it is. But the water in them is dark mud, all the animals stomp around in it, i’m sure they all piss and defecate in it. Elephants roll around in it to cool off and cover themselves with mud.

And they flock from miles around because it’s the only source of drinking water. I’m pretty sure that sickness would result for most humans if they were to drink any significant amount of water from such a source.

Water holes.

People have evolved just exactly the level of tolerance of bad water needed in a natural state. Unnatural states of living have not been around long enough to affect our evolution in any meaningful way.

In fact, humans have just exactly the the level of tolerance for bad water needed in a “civilized” state, because the only thing evolution “cares” about is that enough individuals survive long enough to reproduce. Whether those individuals live a full, comfortable and satisfactory life is irrelevant.

We do have a natural tolerance and I’m puzzled as to why you think humans don’t?

Clean treated water is a very recent invention and only available to about 1/3 of the planet. The other 2/3rds nevertheless multiply and thrive albeit it with higher levels of disease.

My teenage children learned to their horror this very evening that the tap water at our holiday home comes as rainwater from the roof completely unfiltered and untreated. They are seriously shocked - and disturbingly healthy. :smiley:

First of all, we didn’t evolve for millions of years in Asia-- we’re recent arrivals. But secondly, and you touch on this, people in a given area are less prone to getting sick by drinking the local water. Our evolutionary past didn’t prepare us for hopping on a plane, flying 10,000 miles away, and drinking the local water. Which shouldn’t be at all surprising. And thirdly, as has already been noted, rivers like the Ganges are nothing like they would have been 10,000 years ago (prior to civilization).

I’m not a microbiologist, epidemiologist or animal pathologist, but I’d say that:

  1. Animals in nature do die of bacterial and protozoal infections, all the time.

  2. People drinking untreated water would have an unacceptably high mortality/disease burden by most people’s moral standards, but in most parts of the world, most people still do survive to reproductive age. We aren’t really that weak.

  3. People live at higher densities than a lot of animals, which increases the risk of infectious disease from untreated water, etc…

  4. There are probably metabolic tradeoffs to having too much disease resistance. The more energy you devote to your immune system, the less you have to devote to thinks like growth, developing higher cognitive capacities, etc…

Even today, there’s plenty of ‘clean’ water around that you can drink without treating it. When I was a Peace Corps volunteer in Africa I didn’t need to treat my water (it came from far enough underground, and naturally filtered through sand, so it was clean enough to drink). Water from the river, by contrast would not have been clean.

Re: Unnatural states of living have not been around long enough to affect our evolution in any meaningful way.

Evolution can happen rapidly, if the selective pressures are strong enough. Artificial selection has led to very rapid changes in our domesticated plant and animal species over just a few thousand years (in some cases much less). Natural selection could in theory work the same way, if the selection pressures were extreme enough.

In what sense? I mean, they have nuclei, cell membranes, mitochondria etc. Biology is not a particular strength for me, but if I heard someone say that anywhere but here (where I can be almost certain there’s someone who knows more than me about any subject) I’d wonder what the hell they’re talking about.

I’m not claiming humans have no immunity or that animals are totally immune. It just seems animals drink filthy water all the time and for the most part thrive, while humans - even those native to the 3rd world - are more prone to sickness from water-based contaminants that animals tolerate much better.

I’ve been given the impression that dysentery and such conditions are a big problem in the 3rd world. Not enough of a problem to wipe out humans there, mind you, but a big problem nonetheless.

Maybe i’m wrong about that. Do people in india play around in and drink from the Ganges with no serious ill effects? I’m pretty sure I’d drop dead from coming within smelling distance to the Ganges River. It’s full of feces, garbage, and dead bodies…