There in the back of your fridge is an undated, unidentified leftover. Whether you decide to eat it or not may depend upon how desperately hungry you are, but even then you will probably check the smell and appearance. If it smells yucky and looks icky, you will probably not eat it. If you go so far, its taste may be unappealing too. And without question some of the subtances involved can be very toxic. Our instinct is to avoid ickylooking, yucky smelling slimy food growing fuzzy.
This all has a nice evolutionary explanation. Many microorganisms, bacteria and fungi, make a living by decomposition of organic material. When this organic material is your food, they compete with us for it. So your decision to pass on something that clearly was food (unless it was something a biologist stored in the fridge for another reason, but more on this another time) means the microorganisms have won the competition.
What if the microorganisms, which surely are present, did not make the leftovers icky or yucky looking? Well, then we would surely eat the food, microorganisms and all, and their ability to produce offspring would drop to zero. The ability of microorganisms to make food look icky and yucky in a hurry is directly related to their reproductive success. In other words, those microorganisms who are most successful at warning off their gargantuan competitors win the competitions most often and leave the most offspring.
And there is a flip side too. Those humans who responded best to these signels from microorganisms avoided potentially unsafe, toxic foods, and so both parties in this interaction "win" so to speak. This is how instinctual likes and dislikes are shaped. All of our ancestors thought toxic ickiness was to be avoided, and those who did not ended up at the shallow end of the gene pool.
Now of course not all microorganisms produce toxic metabolic byproducts, and we have learned to like some safe "spoiled" foods, like cheese and sauerkraut. Because our instincts have been shaped by toxic food spoilers, innocuous food spoilers can hide in plain sight, so long as they make our food look icky too.
Hopefully this has given you new insights on appreciating the mystery items from the deep recesses of your fridge.
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