Impaired Air Conditioning within the Nasal Cavity in Flat-Faced Homo

We are flat-faced hominins with an external nose that protrudes from the face. This feature was derived in the genus Homo, along with facial flattening and reorientation to form a high nasal cavity. The nasal passage conditions the inhaled air in terms of temperature and humidity to match the conditions required in the lung, and its anatomical variation is believed to be evolutionarily sensitive to the ambient atmospheric conditions of a given habitat. In this study, we used computational fluid dynamics (CFD) with three-dimensional topology models of the nasal passage under the same simulation conditions, to investigate air-conditioning performance in humans, chimpanzees, and macaques. The CFD simulation showed a horizontal straight flow of inhaled air in chimpanzees and macaques, contrasting with the upward and curved flow in humans. The inhaled air is conditioned poorly in humans compared with nonhuman primates. Virtual modifications to the human external nose topology, in which the nasal vestibule and valve are modified to resemble those of chimpanzees, change the airflow to be horizontal, but have little influence on the air-conditioning performance in humans. These findings suggest that morphological variation of the nasal passage topology was only weakly sensitive to the ambient atmosphere conditions; rather, the high nasal cavity in humans was formed simply by evolutionary facial reorganization in the divergence of Homo from the other hominin lineages, impairing the air-conditioning performance. Even though the inhaled air is not adjusted well within the nasal cavity in humans, it can be fully conditioned subsequently in the pharyngeal cavity, which is lengthened in the flat-faced Homo. Thus, the air-conditioning faculty in the nasal passages was probably impaired in early Homo members, although they have survived successfully under the fluctuating climate of the Plio-Pleistocene, and then they moved “Out of Africa” to explore the more severe climates of Eurasia.

This is the first investigation of nasal air conditioning in nonhuman hominoids based on computational fluid dynamics with digital topological models of the nasal passage made using medical imaging. Our comparative results of humans, chimpanzees, and macaques show that the inhaled air is conditioned poorly in humans compared with nonhuman primates. We also show that our protruding external nose has little effect on improving air conditioning. The nasal anatomy in Homo was weakly sensitive to the ambient atmosphere conditions in evolution, but was formed passively by facial reorganization in this genus. Even though the inhaled air is not adjusted well within the nasal cavity in humans, it can be fully conditioned subsequently in the pharyngeal cavity, which is lengthened in flat-faced Homo. Thus, despite an impaired air-conditioning conformation in the nasal passages, Homo members must have survived successfully under the fluctuating climate of the Plio-Pleistocene, and then they moved “Out of Africa” in the Early Pleistocene to explore the more severe climates and ecological environments of Eurasia.