Washington, March 17
The shape of our noses was formed by a long process of adaptation to our local climate, according to a new study published today.
Researchers found that wider noses are more common in warm-humid climates, while narrower noses are more common in cold-dry climates.
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"We are interested in recent human evolution and what explains the evident variation in things like skin color, hair color and the face itself," said Mark D Shriver, professor at Pennsylvania State University in the US.
"We focused on nose traits that differ across populations and looked at geographical variation with respect to temperature and humidity," said Shriver.
Researchers considered a variety of nose measurements, looking at the width of the nostrils, the distance between nostrils, the height of the nose, nose ridge length, nose protrusion, external area of the nose and the area of the nostrils. The measurements were made using 3D facial imaging.
Differences in the human nose may have accumulated among populations through time as a result of a random process called genetic drift.
However, divergent selection - variation in natural selection across populations - may also be the reason that different populations have differing noses. Teasing the two apart is difficult, especially in humans.
The researchers found that the width of the nostrils and the base of the nose measurements differed across populations more than could be accounted for by genetic drift, indicating a role for natural selection in the evolution of nose shape.
To show that local climate contributed to this difference, researchers looked at the spatial distribution of these traits and correlated them with local temperatures and humidity.
They showed that the width of the nostrils is strongly correlated with temperature and absolute humidity.
One purpose of the nose is to condition inhaled air so that it is warm and moist. The narrower nostrils seem to alter the airflow so that the mucous-covered inside of the nose can humidify and warm the air more efficiently.
It was probably more essential to have this trait in cold and dry climates, said Shriver. People with narrower nostrils probably fared better and had more offspring than people with wider nostrils, in colder climates.
This lead to a gradual decrease in nose width in populations living far away from the equator.
This is not the only explanation for nose-shape variation in humans. Researchers also found differences between men and women in nose features across the board.
Another way that the cross-population differences in nose size may occur is through sexual selection. People may choose mates simply because they find a smaller nose more attractive.
If an entire group thinks small is better, then those with large noses will have less success in reproducing.
Over time, the nose size in the group will shrink relative to other groups. These notions of beauty may be linked to how well-adapted the nose is to the local climate.
The study was published in the journal PLOS Genetics. —PTI