Fact of the Day
One of the sources I read through actually brought up a theory about yawning that stems from early humans. First off, anyone with a dog or cat knows that animals yawn just as much as humans and sometimes contagiously along with them. So it definitely doesn't come from humans alone, but the rest of the theory could be something. It stated that the yawn might have been an involuntarily sent red flag from your body to your consciousness. Basically, to tell you to go do something else already!, or some sort of alert like this. This doesn't have to be a strictly human characteristic.
Other theories suggest that it serves a group purpose, hence it's contagiousness. Since more oxygen is inhaled and more CO2 exhaled it could be a means of replenishing the oxygen in your body. In a group setting this would be needed after being suffocated by five of your closest friends spewing their toxic CO2 emissions at you during conversations in close quarters. By this rationale: it is contagious because everyone else also needs air ?...
Sources show that in the womb week-old fetuses yawn. This makes me settle on the third theory. Stretching. Your lungs/jaw/face must just need a stretch. This also explains the boredom factor: Why do we yawn out of boredom? References also explain that in a state of boredom, breathing is slowed causing less oxygen intake. A short stretch of the lungs with some extra intake of oxygen spells a bit of quick relief. Maybe it's contagious because you eventually realize how bored you are as well.
Sorry, but it is also contagious if you read about it and pandiculation is what it is called when one yawn's and stretch's at the same time. [sources: 1, 2, 3]
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