Our diet had to be nothing but plants and raw meat very early on. At what point did humans combine meat and fire, and how did they know it would be beneficial?
This is more a pre-history question, so while some people here might be able to give you some info, I would suggest X-posting to /r/AskAnthropology too.
I actually just answered a question very similar to this a couple days ago. As it turns out, we do have some clue about when people began to manipulate fire
The first human use of fire occurred approximately 1 million years ago in Wonderwerk Cave, South Africa. The evidence is burned bones and ashed plant remains found in situ during archaeological excavation.
This is the publication of the study which discovered this. I actually performed some of the FTIR analyses on the burned bones for Dr. Berna, while I worked in the university lab.
We have convincing and undisputable evidence of the consuption of cooked foods from the Shanidar 3 skeleton as well as from the El Sidron site , which goes back about 50,000 years ago (source 1; source 2). The data comes from the observation of cooked starches in fossilised tartar deposits on teeth.
The origin of cooking food is thus buried deep into prehistory. We have no written record of what might have motivated this behavior.