A new study on nasal air conditioning in humans highlights the need to consider not just adaptive evolution but also compensating evolution.
The Pleistocene epoch, which lasted from 2.6 million years ago to 11,700 years ago, is considered to be the period when the most significant events of human evolution took place. It was during this time that Earth began cooling and there were dramatic swings in climate marked by stretches of warming periods with present day-like temperatures – interspersed between ice ages. Fossils show that this is when humans migrated outside Africa and evolved to their present forms.
In a study published on March 24 in the journal PLOS Computational Biology, a team of Japanese scientists have found out that despite their need to adapt to the changes in climate, human nasal cavities actually became less efficient at conditioning inhaled air than that of their fellow primates, chimpanzees and macaques. However, to make up for this, humans evolved longer pharyngeal cavities than their ancestors.
When we inhale, the space inside our noses adjust the temperature and humidity of the air to levels that the lungs require. “Insufficient conditioning can damage the mucosal tissues in the respiratory system and impair respiratory performance, thereby undermining health and increasing the likelihood of death,” explained Kyoto University’s Takeshi Nishimura, the lead author of the paper.
Modern humans and their ancestors from the Homo genus are marked by their flat faces and protruding external noses. Contrastingly, earlier hominins like the australopithecines had long faces that were oriented away from the brain similar to chimpanzees. This means that while the nasal cavities of Homo are high and quadrangular with a vertically-oriented nasal vestibule (the tunnel enclosed by the nostril), that of non-human primates are longer and triangular with a horizontal nasal vestibule.
Almost like the real thing
In their quest to uncover the evolutionary process of the inner structure of the face responsible for air-conditioning, Nishimura and team compared air flow in the nasal passage of humans, chimpanzees and macaques. Since anaesthesia potentially changes the property in air-conditioning, they used the next best thing: computational fluid dynamics (CFD).
CFD uses applied mathematics and physics to design three-dimensional models based on MRI and CT scans of real subjects. These models simulated the flow of air in the three genera. The same simulation conditions of 34º C and 100% relative humidity were used across the three models so that the observed air-conditioning performances could be compared.
The scientists wanted to study how anatomical differences contributed to air conditioning performances. The CFD model showed them a number of things. First, in humans the air flows upward into the nostrils, then into the nasal cavity, and then downward into the middle throat – whereas in chimps and macaques, the air flows straight through the nostrils on their flat noses into the nasal cavity and then enters the throat.
Second: while chimps and macaques were able to efficiently condition the inhaled air even in severe humidity and temperature, humans were not. In dry conditions, the human nasal cavity failed to adjust the inhaled air to the required 100% relative humidity, and in cold conditions, air in the nasal cavity remained below 30º C, several degrees lower than the ideal 34º C.
In addition to these three models, the scientists also created two virtual CFD models of humans with variations that were similar to features on chimpanzees: one with no nasal valve and the other with horizontal nasal vestibules. They found that neither case helped improve air conditioning performance much. “This showed that the human external nose has little effect on air-conditioning,” said Nishimura.
A side effect of facial reorganisation
It is believed that the anatomy of the nasal cavity evolves in response to atmospheric conditions of a given habitat. However, the observation that humans evolved noses that are less capable of conditioning air than those of their chimp-like predecessors suggests this may not always be. “These findings mean that the morphology of the nasal cavity can accept some morphological evolutionary modifications that might impair air-conditioning in nonhuman primates,” wrote the authors. To compensate for the poorly performing nasal cavity, inhaled air can be adjusted through their longer pharyngeal cavities to be fully conditioned in humans.
The scientists also inferred that, unlike the eyes, jaws and other parts of humans that have maintained their functions despite evolving in structure, the nasal region is just a “buffering module” for the facial re-organisation that occurred during the Pleistocene. But the authors concede that our unique external nose is not useless. Previous studies have demonstrated its ability to retain water vapour from expired air and to generate a vortex airflow with inhaled air to improve air conditioning.
According to Nishimura, this study is the first investigation of nasal air conditioning in nonhuman hominoids based on computational fluid dynamics. “Human evolution is often regarded as the evolution of adaptive traits, but our study highlights the importance of compensating evolution as well as adaptive evolution.”
“These linked changes in the nasal and pharyngeal regions would in part have contributed to how flat-faced hominins,” such as Homo members, “must have survived such fluctuations in climate, before they moved ‘Out of Africa’ in the early Pleistocene to explore the more severe climates and ecological environments of Eurasia,” the team wrote.