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.

Nasal airflow simulations suggest convergent adaptation in Neanderthals and modern humans

“…Here, we aim to circumvent this problem by reconstructing and analyzing the internal nasal morphology of MHs and Neanderthals, including its mucosa surface, to compare the performance among the Neanderthal nasal model and MHs from two different climates/populations. Then, we simulate the respiratory cycle under several conditions using computational fluid dynamics (CFD) methods (21,22). To complement this performance approach from a model-bound evolutionary perspective, we test if nasal variation on different clades containing species inhabiting in cold climates is compatible with neutral versus nonneutral evolution.…”

Genome-wide Association study of facial morphology identifies novel genetic loci in Han Chinese

“…Double homozygous mutations in Sfrp1 and Sfrp2 result in severe shortening of the thoracic region (Satoh, 2006). Remarkably, DCHS2 and SFRP2 were differentially expressed in human and chimp CNCCs, which might make the human nose big and thereby unique among primates (Nishimura et al, 2016). We found the new locus, rs16897517, located in the intron of VPS13B.…”

Computational Fluid Dynamics Simulations of Chimpanzee Nasal Airflow for Different Respiratory Modes

“…Recently, there has been an increase in the number of computational fluid dynamics (CFD) simulations involving realistic nasal cavities of Old World monkeys [11], [12] and chimpanzee [12]. [11] investigated the contribution of the maxillary sinus to air-conditioning performance, using CFD simulations based on 3D topological computer models, derived from computed tomography (CT) scans, of the nasal cavity of four Japanese macaques, two rhesus macaques, and a savanna monkey.…”

“…[11] investigated the contribution of the maxillary sinus to air-conditioning performance, using CFD simulations based on 3D topological computer models, derived from computed tomography (CT) scans, of the nasal cavity of four Japanese macaques, two rhesus macaques, and a savanna monkey. [12] used a CFD model to compare nasal air-conditioning principles and performance in humans, chimpanzees, and macaques; based on digital topological models of the nasal passage reconstructed from medical imaging of six human volunteers, four chimpanzees, four Japanese macaques, and two rhesus macaques.…”

“…Even though [12] carefully evaluated nasal airconditioning performance among the three genera (human, chimpanzee, and Old-World monkeys) by simulating varied ambient atmospheric conditions, they have only reported constant respiratory flow models which may not represent what actually occurs during unstable tidal airflow. Furthermore, they assumed that nasal airflow is laminar whereas turbulent models have been used for airflow simulations in human nasal cavities [13], [14].…”