I as well have yawned many times reading this... i do believe it is contagious and could very well be subconscious communication... the early homosapien tree theory thing seems interesting, and holds together very well. However this cannot be the sole reason for yawning. People yawn when they are nervous, starting actions, etc to communicate, but they also yawn when they are just tired! After all, may animals yawn... like cats, who arent social animals at all, but rather solemn predators.
And as for why yawning originally happened, i think its either that the brain needs more oxygen, or to equalize the pressure... i yawn alot on airplanes...
and on hiccups: i always thought it was a normal muscle spasm of the diaphragm, much like eye spasms etc
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