There are burnt bones dating to to 1.4 Million years ago at Swartkrans Cave Member 3 (South Africa) that most archaeologists consider probable evidence for human-controlled fire. The evidence gets better after around 1 Million years or so, suggesting that humans had figured out how to make fire and how to transport it. The big change that occurs after 40,000-60,000 (though actually closer to 20,000-30,000 BP) is that we see more evidence for humans using fire as a technological instrument, -firing clay, heat-treating flint, "storing" heat in stones (possibly to roast or boil food). I think that early humans used natural fires to cook food fairly early on, more than 1 million years ago. They probably figured out ways to transport fire and to make it (probably by friction, -the most common method) not very long thereafter, say between 1.0-0.5 Million years ago. Another reason for human use of fire may have been as an aid to woodworking. Fire really speeds up progress in woodworking/carpentry. Using fire to char-and-scrape wood can cut in half the time one needs to make a spear, a bowl, or a digging stick. Whether cooking, warmth, carpentry, or antipredator defense were more important undoubtedly varied from time to time and depending on the context.

No living person will really ever know.

Right after beef was discovered. Beef is the reason that Man needed fire. LOL