Our noses are arguably the most prominent feature on our faces; and, although some people's noses are more prominent than others, their shape and form is unique to humans.
Our noses carry out a number of important roles. Other than the obvious task of shepherding air and aromas into our head, it conditions the air we breathe, ensuring that it is warm and moist by the time it reaches our lungs.
The nasal cavity also helps to prevent infections. Hairs trap larger particles and pathogens that we inhale and sticky mucus traps other unwanted visitors and microbes. The captured debris is then cleared away by tiny hair-like structures called cilia.
The shape of your nose is, of course, dependent on the shape of your parents' noses and their parents' noses, and so on. Is there something else driving the variation seen in nose shapes across the world?
Nose shape investigated
As far back as the 1800s, Arthur Thomson - a British anatomist and anthropologist - pointed out that long, thin noses tended to occur in colder, drier regions, whereas shorter, wider noses more often appeared in hotter, more humid areas. This rule is now referred to as Thomson's nose rule.
Although the rule is well-established, it was not clear whether the differences arose as a response to selection pressures, or whether it was simply due to genetic drift - chance mutations, conserved without any particular survival benefits.
Thomson's nose rule has previously been examined by studying skulls but, for the first time, the theory has been tested using live humans and 3-D facial imaging. Researchers set out to uncover whether nose size evolved as an adaptation to climate.
The team, led by Arslan Zaidi and Mark Shriver, examined people of West African, East Asian, South Asian, and Northern European ancestry. Their results are published today in PLOS Genetics.
The team measured each individual's nostril width, the distance between nostrils, the height of the nose, the length of the ridge, nose protrusion, external nose area, and the area of the nostrils.
Once the 3-D data had been analyzed, the team concluded that the width of the nostrils and the base of nose measurements across these regions could not be explained by chance alone. They also found that wider nostrils correlated with populations whose ancestors evolved in warmer, more humid regions. This suggests that climate was a driving factor in the evolution of nose shape.
Why does nose size matter in colder environments?
The theory is that narrower nostrils alter airflow in such a way that the mucus-covered nasal membrane can warm and humidify incoming air more efficiently. This would help individuals with narrower noses to survive better in colder climes and, therefore, be more likely to reproduce. Over time, this would have slowly driven nose size down.
Of course, climate is not the only factor that has had a large impact on the evolution of the nose. For instance, cultural preferences for nose shape are thought to play a part. If a certain culture finds larger noses attractive, then individuals with genes that prescribe a larger nose are more likely to breed and spread their large nose genes to the next generation.
"Even though there are substantial differences in nose shape among human populations, much of this variation can be explained by random genetic drift alone. This finding is in line with the consensus that most human variation is shared among populations and primarily due to genetic drift. Traits like skin pigmentation and nostril width are exceptions rather than the rule. "
The current findings are fascinating in their own right, but they could also have implications for medicine. Knowing how humans adapt is essential for understanding certain diseases and conditions. For instance, lactose intolerance, sickle cell anemia, and skin cancer are more prevalent in some populations.
Future research may investigate whether the size of an individual's nasal cavity has an impact on respiratory conditions for those who now live in climates different from their ancestors'. As Zaidi says, "these traits are important to study because they are likely tied to our health, especially as we become more of a global community and migrate to new climes."