The answer is at an article at Vox. In brief, they didn’t look like anything you’d recognize today, thanks to selective breeding. That’s also why most of us today can’t eat a true Paleolithic diet. Regarding corn:
As maize became domesticated in Mesoamerica, it was radically altered through selective breeding. Early farmers would examine their plants and save the seeds of those that were larger or tastier, or whose kernels were easier to grind. By 4000 BC, cobs were already an inch long. Within just a few thousand years, cobs had grown to many times that size. Later on, plant hybridization became an important breeding method to further cultivate certain traits.