This is actually more of an anthropological/archaeological question than a historical one, as cooking pre-dates written records according to archaeological evidence. You might want to try /r/anthropology for more indepth answers.

However to give you a reference, Catching Fire written by a professor of anthropology at Harvard delves into the subject.

He elaborates on the idea (keeping in mind this idea is not without its dissenters) that humans evolved specifically to eat cooked food because it reduced the amount of digestive body mass needed to sustain ourselves, thus re-allocating body mass toward brain power.

Namely that cooked food universally results in greater food energy absorption, and is almost universally preferred by any animal that gets a hold of it, but is over time unhealthy because the act of cooking is carcinogenic.

Basically, the specific human adaptation is to have digestive systems that "reduce" (but not eliminate) the carcinogenic dangers of cooked food as a tradeoff for increased brain mass.

I don't think this is a historical question as much as it is an anthropological one. While I can't speculate on where the concept of cooking originated from, cooking food does have numerous benefits. Cooking food allows people to access more of the calories that are in food than eating it raw, as you can see here:

http://www.reddit.com/r/science/comments/n4wo4/research_shows_that_cooking_food_adds_calories/

And some, like Richard Wrangham, argue that cooking is essentially what makes us human.

http://www.nytimes.com/2009/05/27/books/27garn.html?_r=2&pagewanted=1

If you want a more detailed answer on this, I'm sure the folks at /r/AskAnthropology can help you out.