All the common coles: broccoli, brussel sprouts, cabbage, kale, kohlrabi, cauliflower, etc. are the same species and all are thought to be domestications of something called sea cabbage, which grows off the coast of England.
All the grains derive from grasses of one sort or another. They were apparently first harvested wild and then selected for desirable characteristics such as grain size and not spontaneously falling off to the ground. This made them non-viable without human intervention and so cultivation became necessary. Corn too is a grain and it is now pretty universally accepted that the wild progenitor was teosinte. Genetic studies have nailed it down. The biggest mutation there was the loss of the hard outer shell so no hulling is required. It is the only grain that can be easily made edible just by dropping it into boiling water.
I think wild potatoes are small, but since you can still cultivate them by planting a tuber, I assume they are not so different from the wild type. I know that the primordial tomatos were berry sized, probably like small cherry tomatos today. I have no idea if they were sweet at all.