Somewhere back in the vast recesses of time, staying alive required one to get things out of your nose so fast that a mere finger just wouldn't do the job quick enough. Only total involuntary facial evacuation would keep your species struggling along.
Nowadays, we don't live in the same dangerous world that our ancestors did, but we do live in an accessibly archived world. And nowhere in that archive is there an instance when a sneeze, somewhere, somehow, saved someone's life. If anything, the sneeze seems counterproductive.
Sneezes happen when the lining of the nose becomes irritated. The nose sends signals to the trigeminal nerve. First, liquid starts to gush, air builds up behind locked vocal cords, certain muscles in our torsos brace so we don't knock ourselves over in the forthcoming spasm, and then the fluid goes flying through the air on a gust of breath when the cords release. Whatever irritated the lining, including viruses, is ejected with the fluid. This might be good for the individual, but what about the family around them? How does it benefit sick children to infect their parents, or the other way around?
There are a couple of possible answers. One is that sneezes — scientifically known as sternutation (because there's nothing science won't rename) —could be not an evolutionary advantage, but an overreaction to harmless stimulation, similar to the way nut allergies are overreactions to harmless particles in peanuts. Then there's the more sinister idea. Life is a struggle, and a lot of things are struggling to stay alive with us. Some scientists think that viruses may have taken over the sneeze-response and made it their express train to a new body. We're not the only things evolving.
We're certainly not the most efficient things evolving. The trigeminal nerve, it seems, crosses over with or near enough other nerves to be a real irritant. It can be stimulated and overwhelmed. About one out of every four people has a photic sneeze response, which triggers sneezes when sunlight hits the face. This response goes back not only in evolutionary history, but recorded history as well.
It's been a biological mystery all the way back to Aristotle, who thought it was the heat on a person's face that triggered the sneeze. Aristotle was proven wrong by Francis Bacon, who showed that heat without light triggered nothing, but then made up his own hogwash by claiming that the light caused "braine humor" to seep into the nose and trigger sneezes.
Today, scientists think that the optic nerve crosses close enough to the trigeminal nerve to occasionally overstimulate it and cause sneezing. Their guess may be no more accurate than Bacon's or Aristotle's. They do know, however, that whatever physically causes the sneezes, it's a genetic trait and not an acquired disorder.
A study in the 1960s of families with this response showed how children inherited this trait from their parents. Boys and girls were equally likely to inherit the response, and roughly one-half of the offspring of a photic sneezer had the trait themselves. This shows that only one copy of the gene needs to be present for the trait to be expressed, and that the gene is not on the X or Y chromosome.
Few sneezing studies have been done since then, but the number of recognized types abnormal sternutators has multiplied. There are people who experience snatiation; sneezing fits after a big meal. At least one person has noted that she sneezes when she eats over seventy-percent cacao chocolate. One woman sneezed when she thought about her loved ones. A more titillating response comes from a host of people who sneeze whenever they have sexual thoughts or directly after they have sex.
Modern scientists have no clue what nerve might be rubbing up against what other nerve in order to trigger sexy sneezing. To be fair to the scientific establishment, few people come forward with this problem. (Although the first one who did went to a doctor who was a friend of Freud's. That must have lead to an interesting conversation.)
Oddly, this kind of sneeze response has apparently been well-documented in teenage girls. The best idea that doctors can come up with is that the sneeze reflex is part of the parasympathetic nervous system; the system designed to bring a speeding heart rate lower, relax sphincter muscles, and generally calm the body down. Sneezes could just be another way the body thinks it will relieve stress. This doesn't necessarily explain the reaction to chocolate or love, but it's the best guess that we've got.
The longest sneezing fit ever recorded goes to Donna Griffiths, a two-year-old who made the world's record by sneezing 4,687,514 times over 978 days between 1981 and 1983, but no one has ever died from a sneeze, nor ever failed to stop sneezing. Since there are more dire conditions out there, sneezing studies are sparse. But some sneeze researchers believe that the sneeze could help people with other parasympathetic nervous system diseases or help those with epilepsy.
For now, the phenomenon is documented but largely unstudied. We know that people sneeze, and that it has something to do with the way the brain is wired up, but exactly why they sneeze is still a mystery.