One of the jobs of the nose and nasal passage is to “condition” the air we breathe: Matching the temperature and humidity of inhaled air to that of the lungs prevents damage to tissues in the respiratory system. Compared to the flat nasal features of chimpanzees and macaques, our protruding nose – a legacy of earlier Homo – conditions the air poorly, according to new findings published in PLOS Computational Biology. Despite these impaired nasal faculties, though, our Homo ancestors survived ancient climate changes and still managed to move out of Africa to explore the more severe climates of Eurasia.
To study air-conditioning performance, a team led by Kyoto University’s Takeshi Nishimura scanned six humans, four chimps, four Japanese macaques, and two Rhesus macaques using either a CT or MRI scanner to create 3D models of the nasal passages. Then they conducted what’s called computational fluid dynamics with heat and airflow to compare air conditioning in the different primates.
The streamlines (upper) and the contours (bottom) indicate the airflow direction and velocity distributions through the nasal passage. T. Nishimura et al.
Their simulations revealed that airflow direction in the nasal cavities of chimpanzees and macaques differs from ours in a few key ways: Inhaled air had a horizontal and straight flow in chimps and macaques but an upward and curved flow in humans – which impairs air conditioning.
However, even though inhaled air isn’t well adjusted within our nasal cavity, it can be fully conditioned subsequently in the pharyngeal cavity – where the nose connects to the mouth and throat. This region is lengthened in the flat-faced Homo.
Earlier human ancestors, such as Australopithecus, had long faces that protruded away from the brain case. Our flat, short faces with a nose that juts out are derived from more recent Homo species. According to the researchers, our high nasal cavity was formed during the evolutionary facial reorganization that occurred as our genus diverged, although the lengths of the pharyngeal cavity are unknown for previous Homo.
1
