Question

How do you think people at that time learnt what plants to eat and what plants not to eat?

Answer 1

What makes some plants edible and others not? A few reasons: Poison and other deterrents are the first (and most obvious) reason, but there are others: Digestibility. Cellulose is a complex carbohydrate that we can't digest; so plants that are mostly cellulose are indigestible to humans.

Answer 2

When we eat spinach or lettuce, we are eating the plant's leaves. We eat the fruit of squash, cucumber and tomato plants. When we eat corn or peas we are eating seeds, and when we eat radish or carrot, we are eating roots. Cauliflower and broccoli plants produce flowers we like to eat.