Some substances have a strong, unpleasant smell such as those in feces or dead animals, what are these substances that make them smell foul to our sense of smell? And why does our sense of smell process it as a foul smell?
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The reason they are perceived as foul is to keep us away.
Contact with feces or eating rotting animals is a good way to become sick and even die. So our ancestor that experience the smell as unpressed was more likely to stay away from it and more likely to survive. So we are the decedents of the ones that survived. So there is a selection pressure to not like the smell
In rotting flesh, it is the primary two compounds putrescine and cadaverine that is responsible for the smell. In feces it is skatole. The name is clearly from their this origin.
There might not really be anything special about them other than they are in/produced by the process and was a point that our ancestors start to dislike the smell of. It does not matter exactly what it was we started to dislike just that it is present in the air around it. It might be a chemical reason that makes the simple to detect or just that the early ones are released as a gas we can detect. But that has nothing to do with the experience of detecting them.
The experience of smelling them is subject to humans. Flys might be attracted to the exact same smell because they can lay eggs in it that can grow. Noting smells intrinsically good or bad it is only the interpretation of out brain.
A lot of time it’s sulfur compounds. As for why our sense of smell processes it as a foul smell? You said it yourself, feces or dead animals — two things which you are definitely *not* supposed to eat.
Those who found such things to smell pleasant tended to not out-compete those whose sense of smell said “stay away” so the genes that make us interpret it as a bad smell tended to spread.
Not everything that smells bad is toxic and not everything that’s toxic smells bad, but most things that smell really bad are things you don’t want to be in contact with.
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