Let's go to the Thought Bubble. So 15,000 years ago, humans were foragers and hunters. Foraging meant gathering fruits, nuts, also wild grains and grasses. Hunting allowed for a more protein-rich diet, so long as you could find something with meat to kill. By far the best hunting gig in the prehistoric world, incidentally, was fishing. Which is one of the reasons that if you look at the history of people populating the planet, we tended to run for the shore and then stay there. Marine life was A, abundant, and B, relatively unlikely to eat you.