Stratified societies that involve finely partitioned division of labor require enormous amounts of energy to be directed at the production of large food surpluses in order to feed everyone not involved directly in the production of food. And the addition of a single minor domain of specialization, a trivial additional division of labor, involves a disproportionate increase in energy demand.
Foraging life-ways that involve little or no division of labor require only about 5 Kcal per person per day, of which 3 are directly consumed as food. Agriculture requires between 12 and 26 Kcal per person per day, depending on the extent to which domesticated food is augmented by hunting and wild harvesting. Once we move to industrial agriculture, the figure jumps to 77 Kcal per person per day. And in our post-industrial electrically-charged coal and nuclear civilization, the number is over 230 Kcal per person per day, of which less than 5% is (over)consumed as food.
To put this another way, life in post-industrial civilization requires 77 times more energy per person than it takes for a hunter-gatherer to feed herself.
Or to put it yet another way, the energy it takes to keep one civilized person alive for a single year is more than a hunter-gatherer can eat in a lifetime.
Source of Kcal/person/day data: O’Sullivan, P. (2008). The ‘collapse’ of civilizations: what paleoenvironmental reconstruction cannot tell us, but anthropology can. The Holocene, 18, 45 – 55.