Culinary research questions

I often think about combinations of food that many take for granted in every day contexts. Here are two combinations that linger at the forefront of my mind.

Cereal and milk:
Who, when, and how did someone think to combine grains with milk--especially considering that the milk comes from an entirely different species to begin with!

When you think about it, it's quite a bizarre combination, and many of us eat it first thing in the morning. I hypothesize that it started with re-hydrating grain products to make them chewable again--as we might do with oatmeal or cream of wheat by adding hot water. Perhaps later as milk became more abundant the stuff was used in place of water. My next inquiry would be directed into wondering whether milk was safer than water during that time.

Ketchup--or perhaps just tomato and eggs:
I'm more interested in correlations here, one of my friends introduced me to the Catalina style of breakfast: rub a tomato onto toast, put some egg on the toast, add salt, and eat. I enjoyed it a lot. mmm...

I later realized that this was very similar to something that I've seen done (and have myself done) for many years--adding ketchup to eggs, eating it with toast (among other sides).

Clearly, an emerging theme of flavors can be found. But why tomatoes, toast, and eggs?

Eggs (especially the yellow of yolk) and wheat toast evokes an interesting sensation of taste. Tomatoes are known to contain glutamate, are also mildly sweet, and hydrating. Juice from a tomato, or to some extent even ketchup seems to balance out the dryness of toast. [H]mmm...