What Your Nose Knows About Human Evolution

They can be bulbous, pert or pointy, but why do noses look so different from one another? It could have something to do with how humans evolved to live in certain climates, a new study suggests.

In the study, the researchers found that wider noses are more commonly found among people living in warm and humid climates, and narrower noses are more commonly found among people in cold and dry climates. 

One possible explanation for why nose shapes vary around the world is genetic drift, which is a mechanism of evolution through which the frequency of certain genes "drifts" upward or downward at random, leading to measurable differences between populations that don't often mingle. Genetic drift has a played a predominant role in human evolution, said the study, published today (March 16) in the journal PLOS Genetics.

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But for the evolution of some human traits, it's likely that another mechanism, natural selection, also played a role, the researchers wrote. In other words, the evolution of some traits occurred not solely due to a random drift of genes, but also in response to outside factors. For example, human skin color is thought to have evolved in different human populations in response to the amounts of ultraviolet radiation they were exposed to, the study authors wrote. [7 Biggest Mysteries of the Human Body]

To see what mechanism likely influenced nose shape, the researchers used 3D facial imaging to measure the noses of more than 2,600 participants from West Africa, South Asia, East Asia and Northern Europe. The researchers scrutinized the noses, measuring the nostril width, distance between nostrils, nose height, nose-ridge length, nose protrusion and nostril area. In addition, the researchers estimated each participant's ancestry using genetic testing.

The researchers found that two nose measurements — nostril width and the width of the nose at its base — appeared to be linked to climate. People with wider nostrils were more likely to live in hot, humid climates, and people with narrower nostrils were more likely to live in cold and dry climates, the study said.

Why does shape matter?

The nose's purpose goes beyond smelling and breathing. It also helps warm and moisten the air before it reaches the lungs. The right temperature and humidity levels are important throughout the respiratory tract, because they help the tiny, hair-like cells that line the tract to keep out germs and allergens.

In fact, the nose is so good at regulating air temperature and humidity levels that the air is already 90 of the way to its ideal temperature and moisture level by the time the air reaches the back of the throat, the researchers wrote. [Gasp! 11 Surprising Facts About the Respiratory System]

Air that is already hot and humid doesn't need to change much as it flows through the nostrils. Cool and dry air, on the other hand, needs to be warmed, and moisture must be added. Narrower nostrils could help facilitate this, as they make the air flow in more turbulently and come into greater contact with the warm, moist mucus in the nose, the researchers wrote. Indeed, it was probably more helpful for humans in cold and dry climates to have a narrower nose, senior study Mark Shriver, a professor of anthropology at Pennsylvania State University, said in statement.

The new study's findings appear to support "Thomson's Rule," an idea put forth by the British anatomist Arthur Thomson in the late 1800s, Shriver said. Thomson "said that long and thin noses occurred in dry, cold areas, while short and wide noses occurred in hot, humid areas," Shriver said. People have tested this rule by measuring skulls; however, no one had done the measurements on living people, Shriver added.

He noted that natural selection isn't the only possible explanation for nose differences. Another explanation could be sexual dimorphism, in other words, differences between males and females, the study said. The researchers did note that there were differences between men's noses and women's in their findings, for example, men's noses were larger, on average, than women's noses.

The findings could also have medical implications, particularly as people travel more around the world, the study said. For example, the researchers asked if someone with a narrow nose could have an increased risk for respiratory problems if he or she lived in a hot and humid climate.

In future studies, the researchers hope to also look at people who live at high altitudes, such as people in the Andes, Tibet and Ethiopia, to learn if low atmospheric-oxygen levels also play a role in nose shape, the researchers said. 

Originally published on Live Science.