Nearly all noses accomplish the tasks of sniffing, breathing and providing a first line of defense against bacterial invaders. But distinct differences among noses abound, from the length of your noggin to the width of your nostrils. For more than a century, anthropologists have speculated and debated about which of these differences are due to the effects of our environments. Now, scientists have proof that the climates our ancestors evolved in helped determine how wide or narrow our noses are today.
"I've always been fascinated by things that are different between human populations," says Mark Shriver, a geneticist and anthropologist at Pennsylvania State University. Shriver has spent his career looking at the variations within our species that make us unique—skin pigmentation, voice pitch and even preterm birth risk—and trying to connect those variations with specific genes. A big part of that research has involved 3D-scanning the faces of more than 10,000 people to analyze the myriad minute ways a face can differ—and why.
For this study Shriver and his team decided to focus specifically on the nose, since it’s a structure easily measured and compared with images. Using their 3D scans, Shriver and his team could precisely measure the noses of different people, and using data about their ancestry, map out how nose shape varies based on differing backgrounds among more than 2,500 people from four regions of the world with different climates.
In a study published today in the journal PLOS Genetics, Shriver shows how the temperature and humidity of the air we breathe has impacted the shape of what we breathe with.
The idea that climate impacts nose shape is not new. By 1905, U.S. Army physician Charles E. Woodruff was writing in his scientific treatise The Effects of Tropical Light on White Men that "the shape and size of the nose and the position of the nostrils are now fairly well proved to be a matter of selection of the fittest varieties." He went to describe how, in his opinion, a nose's shape could help one adapt to a climate over time:
"In the tropics where the air is hot and therefore rarefied, more of it is necessary and it is essential that there should be no impediment to the air currents so the nostrils are open and wide and the nose very flat. Such a nose is unsuited for cold countries as it permits masses of cold air to flood the air passages and irritate the lining membrane, so that the nose must be large and have much warming surface, and the nostrils therefore are slender slits to admit air in thin ribbons easily warmed. [...] Hence there must have been a natural selection in cold countries of one kind of variations—large contracted noses, and a selection in hot countries of the other extreme, so that the various types gradually arose."
Yet more than a century later, scientists were unable to definitively prove whether these nasals variations were more than just random background noise in the messy process of evolution. Now, with his thousands of scanned noses, Shriver and his team have mapped not only the geographic variations in nasal width, but also calculated whether these changes developed faster than the rate of normal "genetic drift."
"We clearly show that parts of the nose have evolved quicker than you would expect if it was just neutral evolution," Shriver says of his results. Along with his map of geographic variations, this rate of evolution proves that climate is driving at least some of the changes in nasal shape.
The reasons Shriver suspects this occurs are similar to those of Woodruff's: In the warm, humid climates where humans first evolved, a wide nose would allow more air to be inhaled with less effort. But as anyone who gets frequent nosebleeds and coughs in the winter could attest, colder, drier air is much more irritating to the membranes of the nose and throat. A more narrow nose will cause more "turbulence" as air is inhaled, mixing the air together inside the nostrils to help warm it like a convection oven, Shriver says.
These impacts may seem minor compared to other factors that could drive evolution, but Shriver points out that any factor that can contribute to a person's fitness can be selected for, no matter how small. "They can be really small and still have a really definitive effect," Shriver says.
Still, there is likely a far more dramatic pressure responsible for your nose shape: sex. "If anything has been shaped by sexual selection, it's the face," Shriver says. Humans use faces to evaluate a wide range of factors about their potential mates, and nose shape certainly plays into that. Consider the fact that nearly 250,000 Americans underwent rhinoplasty—commonly known as a nose job—in 2011, most of those for cosmetic reasons.
Now that most humans in the developed world spend their lives in human-mediated climates with artificial air-conditioning and heating, Shriver says, the "primary force moving forward" in natural selection of noses will be sexual selection. Sexual selection's impact on noses could also help explain why male and female noses vary so much, though Shriver says that particular area has not been studied yet. (Research also suggests that men have bigger noses so that they can consume the higher levels of oxygen their bodies require.)
"I think the differences between the sexes is bigger than any of the population differences we saw," Shriver says, indicating that nose shape has become a gendered trait that men and women can use to evaluate each other with.
Tübingen University paleoanthropologist Katerina Harvati, who was not involved this study, says that Shriver's research does a good job of building on this long-held theory. Harvati has conducted research looking at how the size of the skull's nasal cavity differs among people from different climate regions. She says Shriver's analysis of the soft nose tissues "better accounts for the underlying genetics and conducts a more thorough assessment of the potential influence of natural selection in shaping the nose."
However, Harvati notes that only nasal width appeared to correlate with climate, while other factors of the nose such as height or overall size measured by Shriver's team showed no relation. This suggests "that the overall shape of the nose is not related to climate, and is influenced by many other factors which are not necessarily under selection."
So far, Shriver’s previous work on facial feature variations and the genes behind them has been used to help build mugshots of potential suspects with DNA evidence. He hopes that, eventually, it will also help people use DNA to reconstruct faces of long dead humans and human ancestors. But his next step for the nose research is to look for the specific genes that cause these differences in nasal width between different human populations.
Until then, breathe deep and enjoy the centuries of evolutionary work that has gone into making your nose the way it is.