It seems that the cattle were first harnessed and put to work around 4000 BC, and the castration of bulls to turn them into oxen have happened at about the same time. A little later, the yoke was invented in Mesopotamia and two oxen could be bound firmly together to pull much heavier plows. Light work required just one pair of oxes, while for heavier work, further pairs were added as necessary. A team used for a heavy load over difficult ground might exceeded nine or ten pairs.
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