Eli5: Why do decaying animals smell worse than decaying plants?
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Because our noses are especially sensitive to the smells released by decaying animals. Humans are particularly sensitive to, and averse to, *cadaverine* and *putrecine*, two chemicals released by decaying animal matter.
Early primates who were either not sensitive to those smells, or not repulsed by them, tended to die out, thus not becoming our ancestors.
Decaying plant matter also smells bad, but the health effects of eating spoilt meat can be much more dire than eating spoilt plants, thus possibly explaining the difference in the strength of the reactions.
Since it was not mentioned. There is an aerobic decay, and anaerobic decay of organic matter.
Aerobic: Has oxygen, This does not smell bad, regardless of animal or plant. This is what a healthy compost pile will smell like, similar to the floor of a healthy forest.
Anaerobic: Low Oxygen decomposition, This smells terrible, regardless of plant or animal.
Animals are harder to compost aerobically because their nitrogen (protein) is much higher than plant matter, and this material is so nutrient rich that it is easy for the decomposition of it to use up all the oxygen and quickly go anaerobic. (grass clipping are high in nitrogen and can smell horrendous once they go rancid, but if you compost it correctly, then it smells like a forest)
Why they smell bad? Both types od decomp release different gaseous chemicals while Aerobic decomposition microbes are most often harmless, Anaerobic microbes are often dangerous (bad infections), so them smelling different may have been a survival evolution.
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