How did people figure out what to eat and why?

Take tapioca for instance.  Do you know it's full of cyanide?  To make it edible you do the following;

The cassava must be cleaned and peeled, then finely grated or milled to break the cell walls and expose the starch. This mass is then washed in a large amount of water and the insoluble pulp is strained out and the starchy water reserved. The starch-water mixture, called "Starch milk", is allowed to settle for 6+ hours and the water is drained off until a thick slurry is left. This slurry is then either purified further with more water then dried to form the flour. The additional washing of the slurry is important to remove residual insoluble fiber and, more importantly, cyanide-related toxins naturally found in the cassava. Traditional cultures dried the slurry on basketwork trays, but it is more efficient to do it in an oven at low temperature with regular raking and turning. The dried starch would then be sifted and used for cooking.