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My thought is that they noticed you get less sick the older the berry/plant is, despite it being poisonous at the start. So, they realized the bad parts were breaking down the longer it existed and thus decided to try and keep it for a long time to get something edible.
Yes, the observation could be made on other plants and applied to berries.
They could also look at other animals. You definitely remember when your ate moldy solidified milk and survived.