The authors of In the Servitude of Power: Energy and Civilization Through the Ages, Jean-Claude Débeir, Jean-Paul Deléage and Daniel Hémery have pointed out that human slavery, in terms of biological efficiency of energy resources, made perfect sense in the past biobased-economy.

“Nevertheless, and this is a very important point, the efficiency of the human-machine [20%] is the highest of the animal kingdom. That of the horse, for example, which had a major role in past energy systems, scarcely rises above ten per cent and that of the ox is still lower.

From the standpoint of energy, the use of draught animals is, therefore, a luxury. It will now be understood why, for centuries, the human converter was the most desirable. In historical circumstances where exosomatic organs are underdeveloped, slavery is a more rational energy system."

This energy rationale is why human slavery, and similar forms like serfdom, was invented in every major civilization on Earth. And lasted until we started using fossil fuels…:) This begs the question of what happens when we stop using them....