He proposes a human interpretation. Nomads would once have despised the settled farmers who bred pigs, and that feeling in some way transferred to the animals themselves.

A nomad when she walked in through its fence, in the moment of settling into its impoverished community she became something else instead: part of the burgeoning class of pastoral dropouts.

Cattle and pigs came next as nomads settled down on farms, and modern chickens followed a few thousand years later; turkeys took much longer to tame, finally giving in to the Aztecs around the 1300s.