ELI5: What is the biological reason why feces smell bad?

r/

Also from an evolutionary perspective.

Comments

  1. -BlancheDevereaux Avatar

    They are incredibly unsanitary, have lots of bacteria and parasites, are dirty and hard to clean off, and have absolutely zero nutritional value since they’ve already been digested. So those of our ancestors that kept their distance from feces because they didn’t like the smell had less of a chance to develop infections compared to those that rolled around in feces because they loved the smell. So they bred more.

  2. whiteb8917 Avatar

    Well, trying to pad out the response instead of one word, short answer, Bacteria.

    Or specifically, Gases produced by bacteria in the gut as they break down the food you eat. Like Bakery, when bread rises, the yeast produced gases in the bread so kneeding the dough helps reduce the gases from forming pockets inside the bread.

    But in humans, once gases are produced, you either fart it out, or it comes out within the waste products, causing the smell.

    Like when you eat garlic, and it is broken down in the gut, the Bacteria produces gases with some of the compounds from Garlic, hence Garlic smelling farts.

  3. EUmoriotorio Avatar

    Poop is mostly a product of bacteria. Our body doesn’t like things that smell like bacteria because it makes use sick. Examples include rotting meat or produce, and also the smell of standing water filled with bacteria can be unpleasant. Our body knows with our nose that those are no-no’s.

  4. Craxin Avatar

    Feces and other rotting, bacteria laden things being hazardous to survival, we evolved a repulsion to the chemicals associated with rotting to protect ourselves. Among other chemicals, sulfurous compounds are produced through microbial activity and are easily smelled. Over time, any creature that avoided things that produced such odors survived longer and passed on its genes. Having it hard wired, meaning not having to be taught such odors are produced by dangerous bacteria, means an automatic repulsion to such smells.

  5. atticdoor Avatar

    We consider it to smell bad because people in ancient times who considered it to smell bad were less likely to get diseases like E coli and thus were more likely to pass their genes on.  And so we are descended from creatures who considered it to smell bad.  Notice that dog poo smells even worse- because that is even more harmful.  

    Things that are good for us normally smell good, because we are descended from people who ate well.  

  6. Frescanation Avatar

    Fecal material has an odor. That odor comes from the gases that are put off by bacteria breaking down the residual nutrients that your body did not or could not process.

    The reason you think it smells bad is that way back in time when they started to live in close proximity to each other, some of our ancestors developed an aversion to it. That aversion came in handy because the feces often contained parasites and high amounts of potentially harmful bacteria. Our ancestors who developed this aversion were more likely to survive and have children. The aversion became a trait, and also a taught behavior.

    You probably still make a distinction between the smell of your own fecal material and that of others.