Humans are omnivores in the truest sense of the word, and have been so since the development of what we call human today. But the balance between herbivore, canivore and frugivore diets is what has changed. Over the period of our existance and micro evolutionary changes and adaptions, we have mostly ever ate veges and fruit and nuts and some meat.
Winters didnt bring a scarcity of plants, fruit and nuts to eat, but a scarcity of meat which is why fruit, nuts and veges were the winter diets as there was never a shortage of those and fruit could be dried and nuts just needed to be stored.
The big change came with technology, hunting tools, meat preservation techniques, animal herding.
Of course, the biggest swing toward meat eating didnt happen in some cave somewhere in southern France, but happened with the invention of the refrigerator, revolutionizing the meat industry.