The photic sneeze reflex, also called photoptarmosis or Autosomal dominant Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome is a hereditary trait which in some cases causes sneezing when a individual is suddenly exposed to large amount of bright light. This gene has been shown to exist more often in the genome of Caucasians, with more than 90% of photic sneezers observed being Caucasian, and 64% being female. People for hundreds of years have wondered exactly why we do this, what purpose does it serve? Our earliest records of this phenomenon can be tracked back to Aristotle in his book, Problems, book XXXIII, which was written somewhere between third and sixth century CE. In our modern age with advances in chemistry and biology we can finally see exactly why this occurs, and our knowledge of evolutionary biology shows us why this gene helped our ancestors.
Our ancestors had little in the way of cleaning and washing technology. Not only did we not yet understand the importance of cleanliness, but we simple did not have the means to keep ourselves clean. No antibacterial soap or running water meant that our primitive bodies had to create means of defense versus the bacteria which threatened our bodies. There are lots of examples of techniques our bodies use to help defend ourselves from harmful bacteria. An example which closely relates with our phenomenon in question is when the olfactory glands are irritated by something such as black pepper. Black pepper contains a chemical piperine, which acts to mimic the chemical makeup of many bacteria. This is a defense adapted by the pepper itself which acts to scare off potential predators who may be sniffing around for food. Since our olfactory glands are already attuned to "flush the system" when it detects bacteria, it will then detect the pepper, mistaking it for bacteria, and thus inducing the sternutation.
Many bacteria have evolved to be undetectable by our olfactory glands. This leads to problems for humans when the bacteria start to reproduce and grow unattended to. So what is the solution? If our bodies were to sneeze at random intervals, bacteria would of course never be able to settle and to reproduce. This explains the reason for sneezin', but why the photic response in some individuals? The answer is that individuals with the gene have a small amount of cone cells coating their nostrils which serve to detect UV light given off by the sun, causing the sneeze reaction. Studies have shown that before the 14th century between 10-20% of the world population had this gene.
But why are there such a significant number of Caucasian and specifically Anglo-Saxon people with this gene? Originally scientists determined that it was simply a random coincidence which led to this, however recent research has shown that the number of people of Anglo-Saxon descent with this gene is simply too great compared to people of other descent in order to be a coincidence. It turns out that this can be explained due to the massive loss of population during the Black Death. The Black Death killed around 60% of the entire European population in 14th century, and it isn't hard to understand exactly how useful the photic sneeze reflex would be for a person living in that time. Even the few extra times per day that it allowed the carriers of the gene to sneeze was enough to decrease likelihood of disease transmission by 60%! In the 15th century it is estimated that between 90-95% of all Europeans had the gene for photic sneeze reflex.
No comments:
Post a Comment