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Why does it hurt to breathe cold air? Who “nose”?

I am always astounded that we humans have made it as far as we have. Being large, nearly-hairless apes, it’s amazing that our ancestors managed to survive as they migrated out of Africa and into Eurasia, facing much harsher and colder climates than they had been accustomed to. But I assume that human success has a lot to do with the evolution of our brain. Larger brains first meant success at outsmarting predators. They later meant learning how to manipulate items for food, shelter, and clothing, and we’ve been so successful at it that I’m here now, using a mass of minerals that have been manipulated into a computer that is transmitting a message written in shapes arranged to represent the various shrill and guttural sounds that we, as humans, have all agreed represent a common language, through invisible wavelengths in the air—all so that I can tell you this very important message: Winter is coming.

Game of Thrones jokes aside, winter did come. It came in the Pleistocene, and with it came very, very bitter conditions in which our human ancestors had to survive. And that wasn’t easy, considering some disadvantages our ancestors faced because of their noses.

I’ve never really given any thought about human noses. They’re there on our faces and seem harmless enough. Some people have small ones, some people have big ones, some people had big ones but had them surgically altered to resemble small ones (and whether or not those people did so realizing that they were following a basic biological principle of possessing desirable traits so as to potentially improve biological fitness is, as Maz Kanata in Star Wars: The Force Awakens said, “a story for another day”).

Did you know that having big brains impacted our noses? The size increase with the brain in Homo reconfigured the shapes of our skulls and faces, giving us a “flat-faced” appearance when compared to our primate relatives. And a flat face means changes to the morphological structure of the nose.

So, does this change in shape have implications on our breathing? And what could this have meant for the success of our ancestors as the migrated “Out of Africa”? A new study published today in PLOS Computational Biology decided to take a look.

The study, conducted by a team of Japanese researchers, led by Takeshi Nishimura from the Primate Research Institute at Kyoto University, examined the differences in air inhalation, in terms of temperature and humidity, between a human, a chimpanzee, and a macaque. If the temperature or the humidity of air inhaled is too different from our internal conditions, it can disrupt our respiratory performance and damage mucosal tissues, which negatively impacts health. And so noses, besides smelling, also act as air conditioners, altering the condition of ambient air to more closely match internal parameters.

The team behind this study used computational fluid dynamics (CFD) and 3-D models of the nasal passages of each organism to simulate conditions and test the performance of air-conditioning by each. Tests were done under multiple conditions we see in different climates: hot and dry, cold and dry, and warm and wet.

Airflow and flow velocity in the nasal passage. (A) Human volunteer, (B) chimpanzee, and (C) macaque. The streamlines (upper) and the contours (bottom) indicate the airflow direction and velocity distributions through the nasal passage, respectively. The number of streamlines reflects the relative airflow volume for a given subject. From Nishimura et al. (2016).

What they found was that airflow in humans is upward and curved, as opposed to horizontal flow of straight air in chimpanzees and macaques, and that we, Homo, condition air rather poorly when compared to non-human primates. In fact, simulations showed that chimpanzees and macaques conditioned inhaled air so well, even in more extreme conditions, that the air was completely adjusted to preferable conditions before even reaching halfway through the nasal cavity. Humans, well, not so much.

 Distribution of temperature in the nasal passage. (A) Warm and wet; (B) cold and dry; and (C) hot and dry conditions. The contours represent the distribution of temperature at each level from the nares to the nasopharynx.
Distribution of temperature in the nasal passage. (A) Warm and wet; (B) cold and dry; and (C) hot and dry conditions. The contours represent the distribution of temperature at each level from the nares to the nasopharynx. From Nishimura et al. (2016).

Some of this performance is due to morphology. Earlier hominins other than the genus Homo had nasal passages more similar to chimpanzees, suggesting that they might have more effectively conditioned air than Homo. But, large brains became the evolutionary priority, and so as the human skull evolved to accommodate brain size, the facial structure became more “flat-faced,” which impaired nasal passages when it came to air-conditioning.

So you may ask then, why were early humans so successful at establishing themselves in the more inhospitable climates of Plio-Pleistocene Eurasia, if they were so impaired by their poor ability to condition harsh air? Shouldn’t they be damaging their lungs in the cold climates in which they were living? Well, as this study by Nishimura et al. points out, not all morphological changes to the skull were negative regarding the ability to breathe ambient air. When human skulls flattened and elongated vertically, not only did it change the dimensions of the nasal cavities, but also the oral and pharyngeal cavities. The morphological changes to the skull caused the tongue to be pushed down toward the pharynx, and thus lengthening the pharyngeal cavity. So what air is not sufficiently conditioned in the nasal passages could be further conditioned in the pharynx before reaching the lungs. And not to mention, these changes to the pharyngeal cavity are what allows us to have such a complex vocal system.

So there you have it, we humans may not be the best when it comes to breathing nasty cold air, but we sure can talk about it well.

Read the paper out today in PLOS Computational Biology:

Nishimura T, Mori F, Hanida S, Kumahata K, Ishikawa S, Samarat K, et al. (2016) Impaired Air Conditioning within the Nasal Cavity in Flat-Faced Homo. PLoS Comput Biol 12(3): e1004807. doi:10.1371/journal.pcbi.1004807

Featured photo: Human skeleton on display at the Museum of Ancient Life, Lehi, Utah. Photo by Sarah Gibson.

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