The era prior to the fossil fuel age, and at its most simple - 10,000 years ago - when all people had was their own bodies to use for mechanical energy.
All work was achieved using the energy of bodies and limited to the number and quality of bodies one could apply to a particular place at a particular time.
For the most part, even though people could use animals, using other humans if possible was the most efficient application of meat power. The energy conversion of food in the human is around 18% while in horses it much less, around 10%. That is, of the 100KJ we eat - chemical energy)-, 18KJ can be converted into mechanical energy.
Animals and population - meat - acts as fly wheels in an energy system based on crops, for rulers somatic work could alway be obtained despite fluctuations in primary production.
The Domestication of the biological converters of chemical to mechanical energy creates a surplus and what one does with that - becomes the stuff of politics.
Notes
- measured in watts, until the age of fossil fuels, rulers had in their reach a few thousand watts, the same as a single bulldozer.
The average human in a burst of effort can produce 100watts
Source:
Something new under the sun J. R McNeil