Because I'm still sick, and people seemed interested, and it's one of my favorite topics. This is more "The History of Fat" than "I'm such a damned idiot". Everyone in the comments had it right, or parts of the story right. When you cobble the whole thing together, it's easy to see why people who eat a Western diet are dropping like flies from 'excess diseases' - heart disease, cancer, etc.
Through most of human history, fat was pretty hard to get. Animals were wild, and only fatty at the very end of summer, if at all - anyone who has ever eaten venison, imagine a steady diet of it. Plus, the bulk of the meat came from smaller game - while we did go out and hunt mammoths and bison and other large animals, it was more easy and more common for squirrel-size critters to wind up in the pot every night. Those, too, are very low in fat. Even when we domesticated animals, until the revolution in animal husbandry that happened in the MIDDLE AGES (like barely a thousand years ago), we really didn't understand how to keep our critters fat and happy all year, so we still wound up with a big late-summer slaughter and skinny animals the rest of the year.
A few OTHER factors to keep in mind: Most meat was dried or smoked for preservation, and while I can't find a good article about it to link to, and you may not believe me, drying reduces the fat content of meat, particularly the unsaturated fats you need more of. Also, if you're out hunting a wooly mammoth, you've got to go out and HUNT IT DOWN, and haul it back to the cave - there's a significant calorie OUTPUT to get the fat in the first place, which leads to much less overall intake than you'd first think. (There is a big deal in anthropology class where you look at calories expended vs. calories brought home, and in many cases, big-game hunting is only beneficial to the small children and nursing mothers who stay home - it's actually a loss or barely break-even for the hunters themselves. It depends a lot on animal, time of year, blah blah, but that's the overall gist of it.)
Oil crops like olives and sunflowers and canola and avocado and peanuts were non-existent for most of our history (particularly in the west - many good oil crops like peanuts and sunflowers were native to the New World, which means Euros have only had them five hundred years). In the case of olives, they were toxic to begin with and had a major domestication job in front of them. (But the fact that we went to the trouble of domesticating them should tell you how desperate we were for fats.) The others were either unknown or grown in very small areas.
All this leads to what anthropologists call the human 'fat tooth'. Like a sweet tooth, but instead of sweets, we crave fats. Back when it was impossible to GET, craving fats was a good thing - it motivated us to go out after the damn mammoths and kept us from starving to death. (It's called protein starvation, and it rarely happens any more in the West, but it used to be a major problem. Without a good bit of protein and a small amout of fat every day, you will eventually drop dead.)
Dairy products were late to the game, a result of the domestication of animals in the neolithic. Before that, it was just too hard to get milk - you gonna go milk a wild cow, which was about twice the size of a modern dairy cow? But of course they're full of fat and other goodies like minerals and amino acids and protein. In a very low-fat diet, dairy products are incredibly good for you. They fill a huge niche in the diet. Nowadays, not so much. So of course when they became available we scarfed up dairy products like there was no tomorrow, and by now, for European-heritage people, it's like a heroin habit that we REALLY need to give up. Or at least cut way back on.
And now, in the West, thanks to our fat tooth (and lack of self control... we do have brains, we don't HAVE to listen to our instincts), we now eat twice to three times the amount of fat that we did in our original, stone-age diet. Pretty depressing, isn't it?
The diet we evolved on is now called the Paleolithic Diet. A short introduction is available here; scroll down to the 'eat none of the following' part. You'll find it depressing and enlightening, all at once (though he says to eat more liver and kidney and I disagree - thanks to modern pesticides and other crap, those organs are unsafe, though they used to be very good for you). And, the coolest thing for me and any other history buffs reading, The Food Timeline. Fascinating stuff.
If I were to completely overhaul my diet and change the way I eat (I don't have the self-control for it), I'd switch over to the Paleolithic Diet. As it is, I settle for cutting back on the 'bad stuff' they list. Which includes, haha, dairy products.
As for McDonald's, they use up TWENTY FIVE PERCENT of the country's potato crop, and between three and five percent of the entire beef output of the country. That obviously makes them a huge force in FARMING, which most people don't realize. They speak, and farmers listen. Occasionally they use their powers for good - they've asked beef farmers who supply them to phase out antibiotic use on their cattle by 2008, and often where McDonald's goes, the market follows.
Sometimes, it's not so good. McD's insists on Russet Burbank potatos for it's french fries, and it's a really old cloned crop that has no modern resistance to bugs or disease - it costs more in pesticides to grow Russets than you can sell them for. Without the government subsidizing the whole clusterfuck, the farmers would go under. As it is, they've laid waste to huge parts of Idaho with all the pesticides and herbicides. The soil is literally sterilized (that's not good; all those bugs play a role, yanno).
You wanna save the environment? Fuck the snowy owls. Quit eating McDonald's french fries.