I've quoted this a million times now, so apologies to those who have seen it one of the previous 999,999 times, but the phrase that brought this all into perspective for me is "We are a species that has evolved to survive starvation, not resist abundance." (Atul Gawande.)
I believe that there are biological
predispositions that vary, and the biology has a lot of components to it. This doesn't mean that person A, born with these predispositions, will definitely be fat; it means that person A, born with these predispositions, will have to work a lot harder to stay thin than person B, who does not have those predispositions. So it's easy for person B to say, "I just eat reasonably and exercise a bit and I'm fine; obviously if that person over there is fat it's because they're eating like a pig and never exercise." When it's a lot more complicated than that.
The article that the above phrase is from used to be available online in several places, now it doesn't seem to be anywhere. This one is different but contains some of the same information:
http://www.telegraph.co.uk/health/main.jhtml?xml=/health/2002/10/21/hfat.xml
At any rate, I think that our lives are inherently more sedentary and inherently more filled with abundance than has been true for most of human history. I think there are all kinds of things that conspire -- the move to an information economy (sitting at your desk typing rather than up at the crack of dawn to keep your farm going so you can survive), the move of women into the workforce (so that the at-home-mom default is broken, and kids can't just go out the door and play outside with other kids), the move towards labor-saving devices (from not needing to scrub linens against a washboard [reduced need for physical exercise] to the additives necessary to make food both storable and palatable when it is warmed up), etc., etc. It's a whole bunch of interlocking issues.
A lot of those things aren't bad things in and of themselves, they just have to be compensated for, and they haven't been. Yet.