When the alternative is starvation, one becomes open to trying new/dangerous things. Given a number of starving groups with only poisonous plant A available, let's say only one group stumbles on a sequence of steps that work while the others die off. The survivors pass along the knowledge that poisonous plant A can be made edible by performing a sequence of steps (even without understanding exactly why those steps worked).
So now the descendants of those survivors can transfer that knowledge (i.e. that a poisonous plant can be made edible) the next time a food shortage occurs and all that is available is poisonous plant B. Rinse and repeat over and over again through the centuries.
Over time, the substances that no viable process was discovered for get labeled as poisonous. So now your 'culture' has a list of safe foods, a spectrum of unsafe foods that can be made safe (and how), and unsafe foods.
So now the descendants of those survivors can transfer that knowledge (i.e. that a poisonous plant can be made edible) the next time a food shortage occurs and all that is available is poisonous plant B. Rinse and repeat over and over again through the centuries.
Over time, the substances that no viable process was discovered for get labeled as poisonous. So now your 'culture' has a list of safe foods, a spectrum of unsafe foods that can be made safe (and how), and unsafe foods.