The dawn of significant Force and Power
When primates became humans they discovered themselves being the owners of agile bodies, capable of exerting forces on the same order of magnitude as their own weights, along considerable distances.
The human machine typically needs to produce 85 W (around 73 kcal/h) of power just to keep the basic functions going. Engaged in continuous work along the active part of the day it can produce an extra output of 75 W (65 kcal/h). In exceptional cases it can deliver more than 1000 W (860kcal/h) of power overall.
With the discovery of agriculture, human work became more balanced in terms of power peaks , but the amount of daily hours of maintained effort increased, and the variety of their movements decreased. Then, the specialization of tasks.