I've seen articles and books over the years about "food combining" (not eating certain foods together), but I've always dismissed it as a fad. Imagine my surprise when I opened up the new Paleo Diet update and found it to be the leading topic!
The first line provides a good overview: "Because it was relatively rare for our hunter-gatherer ancestors to consume carbohydrates and fats together for millions of years, our species may have became genetically dependent upon certain nutritional combinations."
The article goes on to say that animal products (protein and fat) were usually eaten separately from plant foods (carbohydrate). Plant food was generally eaten as it was cultivated, and animal food was eaten directly after a successful hunt.
I honestly have never experimented much with this. I read somewhere (not sure where) that hunter-gatherers consumed mixed meals of meat and vegetables. I've always done this, but this evolutionary argument against mixed meals is interesting.