Or yawning is just a communication mechanism, formed in a time before using words became commonplace and maintained because it works.

It might also be a type of communication which goes against the typical 'pack' animal communication hierarchy. It might make sense for the pack to sleep when any of the pack is tired, rather than waiting for the alpha-dog to feel tired. 'contagious' yawning allows that.

Hell, for me seeing a video of someone yawning can make me yawn. Even reading these comments... I seriously yawned three times just thinking about people yawning.

It's quite possible that it is a social signal, I think. Something designed to facilitate a non-spoken syn-ack handshake between members in a social group. I see you, you see me, I see that you see me, end of transmit.

Just to clarify, the hypothesis in the post above was intended to be humorous.

For me, even reading the word "yawning" so many times triggers yawning.