My own anecdotal/intuitive guess is that modern HVAC technology is a major factor in the obesity epidemic.
The human body evolved with a highly sophisticated mechanism for regulating its internal temperature, that had been tightly integrated with technology such as clothing and fire up to about 1970 world wide. Then for most urban and suburban life styles HVAC technology brought about significant change. Before that happened, If it was too hot, you took off clothing, or tied a wet bandanna over your head, and sweated it out. If it was too cold-- the most common thing-- you put on clothing. In either case you burn calories, either in keeping the clothing warm or by increasing the circulation of blood through the scalp, activating perspiration systems, etc. But since 1970 the common way to adjust the internal temperature is to dial the thermostat up or down. This is now true even for farmers in their air conditioned cabs on their tractors. Farm laborers, though, who are out in the weather, do not seem to be part of the obesity epidemic.
My hypothesis is that obesity is strongly determined by the atrophy of the individual's autonomic thermal controls that we once had to use on an hourly basis, that have now been replaced by HVAC systems.
I have just come back from a 7 day / 6 night camping trip that consisted of a few short walks, a lot of sedentary photography, daytime temperatures in the high 80s and night time temperatures in the low 60s. I lost more than 6 pounds on that trip: unhappily stable at 225# before, stable at 219# after (I track my daily weights, just because). And yet I'm more active at home-- if I had not gone camping I would have done 2 or 3 bike rides of 12 to 20 miles each during that time--- and would not have lost any weight. The significant difference is that on that kind of camping trip getting out of the warm sleeping bag into the chill of the morning is a thermal challenge; sweating through several water bottles during the afternoon heat is a different kind of thermal challenge.
Perhaps now that we do not engage in thermal challenges so much as we once did, the mechanisms for controlling those have atrophied and that has led to obesity.
If anyone wants to test this hypothesis, I would be a willing experimental subject. I suggest equipping me with a full set of REI camping gear and sending me to the Hartz Mountain Antelope Preserve in the wilds of eastern Oregon for several months. My dietary needs would be covered by weekly deliveries of a couple of dozen eggs, several pounds of bacon and sausage, seasonal organic veggies from the Canby Farmers Market, assorted breads from Portland's Grand Central Bakery, and a couple of kegs of McMenamin's Terminator Stout. We can work out the other details, like the carrot cakes, cheeses, etc, later. If, with this kind of diet, I came back heavier than I left then that would suggest the hypothesis is flawed. If I weighed the same or less on return, then possibly the hypothesis is true, and someone should set me up to explore it further in a different environment, perhaps in some remote corner of the Oregon coast. But whether the hypothesis is shown to be flawed or is supported by the evidence, I would certainly benefit from the experience.