[Retric]

Slavery in Europe was replaced with a more efficient system in the Middle Ages.

So calling it a constant throughout history is only true in the way that slavery still exists today, in that you could find it somewhere on the globe.

[mr_toad]

That’s partly true, but it’s also the case that medieval Europe didn’t have any convenient sources of slaves. Conquests had dried up, Europe was effectively surrounded by stronger neighbors, and instead of being able to take their neighbors as slaves many of their neighbors were taking Europeans as slaves.

That didn’t change until the Age of Sail opened up new frontiers and the wheel turned again.