Trial and Error: people just ate the cooked vegetables and hoped that they wouldn't be poisoned, particularly when there wasn't enough edible food and they were super hungry. Remember that in the past many people starved to death; and when humans get really hungry, they will eat anything with even a chance of nourishment, even rotten food.

More importantly, by the time humans discovered plants, they were already cooking meat. To them, it seemed intuitive that some normally-inedible plant would become edible after it was cooked, since the exact same thing happens to meat

Furthermore, there aren't many plants and animals which will kill you with only a tiny taste; most plants will only make your stomach hurt and you feel ill for a while. So people ate a small amount of food, and then if they didn't suffer an adverse reaction, they ate more of it.

And once someone finally ate the cooked vegetable and was not poisoned, then it was obvious to others around them that cooking the plant made it edible. And people communicate, and word-of-mouth spreads through entire continents. It only take one bold person to try, and within a few decades, the entire country would be aware that the cooked vegetable was edible.

I hope that answered your question, and you can always ask for further clarification.
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence