Necessity is the mother of invention and prehistoric men who lived in frozen areas ate anything that they could to survive. The prehistoric man had to eat meat in order to survive. This would be the first time that they had ever eaten meat.
This changed the way people would eat and health forever.
The first meat that was eaten would have been cooked by fire that was natural started by natural forest fires. Without fire, they could have possibly eaten raw meat as well.
The digestive system most likely rebelled to eating the raw meat, but as they became adjusted meat became a part of their regular diets.
You may have heard of people who have lived vegetarian lives for a long period of time and then became violently ill afterwards. This is similar to what prehistoric men would have gone through. Biologists will tell you that we are not really designed to digest meat, but that we have adapted to it over time.