title: Impaired Air Conditioning within the Nasal Cavity in Flat-Faced Homo.
abstract: 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.
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nishimura
authors.affiliation: Institute for Biomedical Sciences, Iwate Medical University, Yahaba, Iwate, Japan.
authors.forename: Futoshi
authors.initials: F
authors.lastname: Mori
authors.affiliation: Kanazawa Institute of Technology, Nonoichi, Ishikawa, Japan.
authors.forename: Sho
authors.initials: S
authors.lastname: Hanida
authors.affiliation: RIKEN Advanced Institute for Computational Science, Kobe, Hyogo, Japan.
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Kumahata
authors.affiliation: Kanazawa Municipal Hospital, Kanazawa, Ishikawa, Japan.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Ishikawa
authors.affiliation: Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan.
authors.forename: Kaouthar
authors.initials: K
authors.lastname: Samarat
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Takako
authors.initials: T
authors.lastname: Miyabe-Nishiwaki
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Misato
authors.initials: M
authors.lastname: Hayashi
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Tomonaga
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Juri
authors.initials: J
authors.lastname: Suzuki
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi, Japan.
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Matsuzawa
authors.affiliation: Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan.
authors.forename: Teruo
authors.initials: T
authors.lastname: Matsuzawa
mesh_terms: D000388:Air; D000389:Air Conditioning / Q000379:methods; D000818:Animals; D015186:Hominidae; D006801:Humans; D006813:Humidity; D001239:Inhalation / Q000502:physiology*; D008251:Macaca; D009296:Nasal Cavity / Q000033:anatomy & histology* / Q000502:physiology*; D002679:Pan troglodytes; D012212:Rheology / Q000379:methods*; D013045:Species Specificity; D013696:Temperature