Most animals have a very limited diet. They only eat a small variety of other creatures and plants. Perhaps humans were the same, once upon a time. If we were only eating a small variety of foods, we didn't have to worry about anything that would be bad for digestion. Becoming the alpha-predators of the world turned just about everything else into food for us. As a result, I'm guessing we had to find ways to make it safe to eat. The necessity of making it safe turned to an art of making it taste good. The cooking is probably just a cultural hold over from our early days of expanding our diet and needing to find a way to make things safe to eat.

That's my 2 cents.

As far as what the healthiest diet currently is for humans...you'll probably get a different answer from every nutritional expert. Perhaps finding out what our native diet was before fire (someone else posted that it was 80% seafood with a small amount of meat and plants). Maybe that's your answer. Of course, our oceans are toxic wastelands these days, so most of the seafood you get is probably tainted.