Abstract The evolutionary reasons for variation in nose shape across human populations have been subject to continuing debate. An import function of the nose and nasal cavity is to condition inspired air before it reaches the lower respiratory tract. For this reason, it is thought the observed differences in nose shape among populations are not simply the result of genetic drift, but may be adaptations to climate. To address the question of whether local adaptation to climate is responsible for nose shape divergence across populations, we use Qst–Fst comparisons to show that nares width and alar base width are more differentiated across populations than expected under genetic drift alone. To test whether this differentiation is due to climate adaptation, we compared the spatial distribution of these variables with the global distribution of temperature, absolute humidity, and relative humidity. We find that width of the nares is correlated with temperature and absolute humidity, but not with relative humidity. We conclude that some aspects of nose shape may indeed have been driven by local adaptation to climate. However, we think that this is a simplified explanation of a very complex evolutionary history, which possibly also involved other non-neutral forces such as sexual selection.

Author summary The study of human adaptation is essential to our understanding of disease etiology. Evolutionary investigations into why certain disease phenotypes such as sickle-cell anemia and lactose intolerance occur at different rates in different populations have led to a better understanding of the genetic and environmental risk factors involved. Similarly, research into the geographical distribution of skin pigmentation continues to yield important clues regarding risk of vitamin D deficiency and skin cancer. Here, we investigate whether variation in the shape of the external nose across populations has been driven by regional differences in climate. We find that variation in both nares width and alar base width appear to have experienced accelerated divergence across human populations. We also find that the geospatial distribution of nares width is correlated with temperature, and absolute humidity, but not with relative humidity. Our results support the claim that local adaptation to climate may have had a role in the evolution of nose shape differences across human populations.

Citation: Zaidi AA, Mattern BC, Claes P, McEcoy B, Hughes C, Shriver MD (2017) Investigating the case of human nose shape and climate adaptation. PLoS Genet 13(3): e1006616. https://doi.org/10.1371/journal.pgen.1006616 Editor: Greg Gibson, Georgia Institute of Technology, UNITED STATES Received: December 17, 2015; Accepted: February 3, 2017; Published: March 16, 2017 Copyright: © 2017 Zaidi et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: We have made almost all data, which were used to generate results in the paper, publicly available in a spreadsheet in the Supporting Information section (S1 File). The spreadsheet includes de-identified data, split by the analyses for which they were used. More specifically, we have provided several phenotypes (nose shape measures, skin pigmentation, height, weight, and BMI) as well as relevant covariates (ancestry components, sex, and age). Where relevant, we have also provided climate values (mean annual temperature, annual aridity index, and ultraviolet B irradiance) ascribed to each individual. These data can be used to replicate almost all the results presented in the manuscript, without compromising the privacy of the participants. Some raw data, such as the 3D photographs, genotypes, and birthplaces, have not been shared because they are highly identifiable in nature and sharing them would be in ethical and legal violation of the informed consent obtained from participants. This restriction is not because of any personal or commercial interests. Funding: This work was supported in part by the United States National Institute of Justice, the United States Department of Defense, the University of Illinois Interdisciplinary Innovation Initiative Research Grant, the Flemish Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT Vlaanderen), the Research Program of the Fund for Scientific Research - Flanders (Belgium), and the Center for Human Evolution and Development at Penn State. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction The shape of the nose, like many other parts of the face, varies both within as well as across human populations. For example, the distance between the nasal alare (wings of the nose) are significantly larger in individuals of West African, South Asian, and East Asian ancestry as compared to persons with European ancestry [1]. The nasal index (width/height of the nasal aperture of the skull) is also known to vary significantly among populations [1,2]. Whether these population differences in nose shape are due primarily to genetic drift or natural selection is unclear. A vital function of the nose is to warm inspired air to core body temperature and saturate it with water vapor before it reaches the lower respiratory tract [3]. In fact, inhaled air reaches 90% of the required temperature and humidity levels before even reaching the nasopharynx, implicating the nasal cavity as the major conditioning apparatus in the respiratory tract [4,5]. This conditioning acts to maintain proper functioning of the mucociliary apparatus, which functions in trapping particles and pathogens and removing them from the airways. Low humidity in the respiratory tract leads to impaired mucociliary function and increases the risk of both upper and lower respiratory tract infections [6]. Much of the air conditioning occurs as it passes through the turbinates, the walls of which are lined with blood vessels and mucus producing goblet cells [4]. Studies have shown that the efficacy of the conditioning process depends on the flow dynamics of the inspired air, which in turn, depend on the geometry of the nasal cavity and inlets [5,7]. Because of the function of the nose as an air-conditioning apparatus, it is hypothesized that differences in nose shape across populations may have been driven by local adaptation to climate [2,3,8]. There are several challenges in testing this hypothesis. We know there is substantial nose shape variation among human populations, in both the external morphology of the nose as well as the underlying cranial morphology [1,2,9]. While this can be explained by adaptation to local selection pressures, it could also be explained by the fact that phenotypic differences among geographically distant populations can arise simply due to genetic drift. Thus, in order to invoke divergent selection as an explanation, one must demonstrate that the observed variation in nose shape across human populations is greater than that expected under genetic drift alone. This can be carried out using the Qst statistic, which measures the degree of genetic differentiation underlying a quantitative trait [10]. In principle, the Qst of a neutrally evolving trait is expected to follow the Fst distribution of neutrally evolving loci [11]. Thus, when Qst is much greater than Fst, trait divergence exceeds neutral expectations and can be attributed to divergent selection [11]. The problem with Qst is that its calculation requires knowledge of the additive genetic variances within- and among-populations. These can only be estimated reliably using ‘common-garden’ experiments, in which environmental effects on the phenotype can be effectively controlled. Since such experiments are not possible in humans, Qst-based inference regarding divergent selection on human phenotypes necessitates making realistic assumptions about the heritabilities of the phenotypes in question. Several studies have used this approach and found that while most aspects of the skull seem to be evolving neutrally, the shape of the nasal aperture appears to be more differentiated across human populations than expected under genetic drift [9,12,13]. More recently, it was also reported that the divergence in the shape of the external nose, at least between Europeans and Han Chinese populations, also exceeds neutral expectations [14]. While this might be true, most of these studies employed anticonservative heritability assumptions, which overestimate the genetic differentiation underlying a trait, and thus, lead to incorrect conclusions regarding the relative roles of selection and drift in driving phenotypic differences among populations. Here, we carried out an exploration of external nose shape variation in light of quantitative genetic theory to investigate whether nose shape variation among populations exceeds neutral expectations. In doing so, we discuss the methodological challenges involved in tackling such questions in humans, and show how the limitations of previous studies can be addressed with recent advances in statistical genetics. As proof of principle, we compare the differentiation of nose shape with two highly heritable morphological traits, which are known to have a polygenic basis; namely height and skin pigmentation [15–18]. Both height and skin pigmentation exhibit substantial variation within and across human populations, and are thought to be under positive selection in various populations [19–22]. Skin pigmentation is also a good example of a phenotype which is known to have evolved in response to a geospatially varying selection pressure: exposure to ground level ultraviolet B radiation (UVB) [23]. Finally, we test whether clinal variation in aspects of nose shape, which appear to be under accelerated divergence across populations, covaries with geographic variation in temperature and humidity, in order to determine whether this divergence is due to climatic selection pressures.

Discussion The diversity of facial features across human populations has fascinated scientists for a long time. Even though genetic drift has played a predominant role in human evolution, external physical traits such as facial shape and skin pigmentation, because of their proximity to the environment, have also likely been influenced by natural selection. Substantial evidence has accumulated supporting the hypothesis that differences in skin pigmentation across human populations have evolved largely in response to selection pressures imposed by exposure to ultraviolet radiation [20]. How selection may have affected facial shape, a trait that is also quite variable between populations, is unclear, likely because it has received much less attention to date. Given the complexity of the face, we have chosen to study one particularly interesting and variable part of the face; the nose. The broad question driving this study is: Has climate adaptation played an important role in influencing variation in human nose shape? To answer this question, we formulated two hypotheses: i. divergent selection has been involved in the differentiation of certain aspects of nose shape across populations, and ii. climate is the agent of selection in cases where divergent selection can be invoked. To test the first hypothesis, we used Qst–Fst comparisons to investigate whether the mean difference in nose shape among populations is greater than that expected under genetic drift alone. Estimation of Qst relies on stipulating the within- and among-population components of phenotypic variance that are due to additive genetic effects. These variance components are ideally calculated through experiments in which the effects of environmental variables can be controlled [28]. This poses a practical and ethical challenge in non-model organisms, such as humans, who are not amenable to the type of experimentation required. To circumvent these limitations, studies often calculate Qst directly from phenotype data under the assumption that the within-population heritability (h2) is equal to the among-population heritability (c). Qst calculated this way is probably best considered to be the ‘minimum’ Qst proposed by Relethford (1994) [35]. However, the notion that this takes the minimum value of Qst relies on h2 always being less than c, which is anticonservative as has been noted previously [32]. In fact, genetic variation between-populations is likely to be less than the genetic variation within-populations for most phenotypes. Our approach is similar to previous studies in that we also estimated Qst assuming c = h2. However, we used sensitivity curves to evaluate the behavior of Qst to cases where c < h2. In addition, we demonstrate that both within- and between-population variation in nose shape are heritable. We carried out Qst–Fst comparisons for seven nose shape traits: nares width, alar base width, nasal height, length of the nasal ridge, nasal tip protrusion, external surface area, and nostril area. We found that the divergence in the width of the nares and alar base deviate from neutral expectations, and that these results are robust to the c = h2 assumption. This suggests that the width of the nares and alar base may have evolved across populations due to divergent selection. Next, we hypothesized that the divergence of these two traits is driven by climate adaptation. To investigate this, we tested whether the spatial distribution of these traits is correlated with mean annual temperature and humidity. Our results show that nares width is strongly correlated with temperature and absolutely humidity. The positive direction of the effects indicate that wider noses are more common in warm-humid climates, while narrower noses are more common in cold-dry climates. Nares width is not, however, correlated with relative humidity. This is not surprising since absolute humidity levels are likely more important for respiration than relative humidity [43]. Alar base width is only weakly, if at all, correlated with temperature and absolute humidity, suggesting that the signal of climate adaptation might be specific to the width of the nares. Computational fluid dynamics (CFD) studies show that the geometry of the nose is important for its respiratory functions [5,44]. Inhaled air reaches 90% of the required temperature and humidity levels before even reaching the nasopharynx, implicating the nasal cavity, especially the turbinates, as the major conditioning apparatus in the respiratory tract [4,5]. We also know that the geometry of the nasal airways influences the velocity of inspired air [4,7,45]. Narrow airways in cold-dry climates might allow better conditioning by increasing the turbulence in inspired air as it reaches the turbinates, thereby facilitating contact with the nasal mucosa [5]. However, we note that nostril area does not show unusually high differentiation across populations, which suggests that it is not the size of the nostrils but the shape that might be functionally important. Another hypothesis regarding the function of the nose that has received less attention compared to the air-conditioning hypothesis is that of selective brain cooling (SBC) [46]. Some large mammals are known to regulate brain temperatures in hyperthermic conditions, often caused by sustained exercise [47,48]. This ability is mostly reported in mammals with a carotid rete, which is a network of capillaries in a cavity under the brain called the cavernous sinus. Arterial blood leaving the rete eventually perfuses the brain through the Circle of Willis. Venous blood traveling back from the nasal mucosa interacts with the carotid rete in the cavernous sinus and is thought to cool the blood entering the brain in arid-zone mammals, such as Oryx [49], as well as in winter-acclimatized animals, such as reindeer [50]. Although, humans do not possess such a rete and instead have a single artery going through the cavernous sinus, which has led some to question the role of SBC in humans [51], proponents argue that the carotid rete is not a pre-requisite as SBC is present in some animals who don’t possess a rete, such as horses [52]. Since SBC in humans is highly debated, the role of nose shape differences in contributing to differences to this physiological process, while worth mentioning here, is highly speculative. Climate may not have been the only factor in contributing to nose shape differences across populations. In fact, we show that temperature is only weakly correlated with nares width, especially when compared with the correlation between skin pigmentation and UVB. What then could be the selective agent driving the divergence of nose shape? We mentioned earlier that all aspects of nose shape studied here are sexually dimorphic, which raises a number of questions. Why does this sexual dimorphism exist? Is it merely a by-product of circulating hormones leading to differences in growth and development in early adulthood, or does it have an adaptive function, such as signaling sex to other males and females? Sexual selection has likely played an important role in human evolution, as evidenced by the presence of sexual dimorphism in many physical traits (e.g., height, waist-hip ratio, facial hair, and breasts to name a few). Could cultural differences in perceptions of dominance and attractiveness have had a role in the divergence of nose shape [53]? Could these perceptions have arisen to select mates who signal adaptation to the local environment? Indeed, ecological selection and sexual selection could reinforce each other, accelerating phenotypic divergence across populations in spite of continued gene flow [54]. These are interesting avenues of research, which need to be considered in order to sketch out a more complete picture of the evolution of the human nose. The investigation of nose shape evolution with respect to climate adaptation, while interesting anthropologically, is also relevant medically. As humans are becoming more of a global community, the study of local adaptation is becoming more important to understanding health risks involved in living in ‘foreign’ climates. Obvious examples of such health risks are of increased risk of sunburn, skin cancer, and folate deficiency in light-skinned individuals exposed to high UVB, and of low birth weight and chronic mountain sickness associated with hypoxia at high altitudes [20,55]. Does the morphology of the external nose, or that of the inner nasal cavity affect risk of respiratory disease in different climates? It’s difficult to say at this point. While our findings provide support for the idea that differences in aspects of nose shape may have evolved across populations as a result of climate-related selection pressures, something that has been demonstrated previously using craniometric data [2,12,13,41,42], we note that the signal of climate adaptation is not very strong, especially when compared to skin pigmentation. This could be due to weaker selection pressure or selection on standing variation, but also due the sparse sampling of populations shown here, which is a limitation of this study. These results will need to be replicated in a larger set of populations. We expect that studies incorporating diverse populations who have been living long-term in a range of environments, such as the tropics, deserts, and circumpolar regions, will nicely fill in the gaps. Especially useful would be representation of populations from higher altitude regions, such as Andeans, Tibetans, and Ethiopians, who not only have to cope with the stress of a cold and dry climate, but also that of low atmospheric oxygen levels [56]. It would also be informative to study non-human primates in this context, who occupy a variety of climes and exhibit extensive variation in nose morphology. Finally, future studies should also focus on genome-wide association studies (GWASs) to identify variants contributing to nose shape. With increasing interest in identifying loci associated with facial shape, some GWASs have recently identified a number of nose shape loci [57–60]. Genetic variation at these loci will be informative about the nature of selection, as well as for inferring the timing of selection events.

Acknowledgments The authors would first and foremost like to thank the study participants, without whom none of this would have been possible. We are grateful to the Shriver Lab team, Ripan Malhi, and the members of his lab for their efforts in collection and organization of the data. We are also grateful to Sandra Beleza and Isabel Inês Araújo for their efforts in collecting and sharing the Cape Verdean data. We acknowledge Ine Saye from Peter Claes’ group for helping with morphometric analyses. We also thank Nina Jablonski, George Chaplin, George Perry, Laurel Pearson, Julie White, and Matthew Reimherr for helpful discussions on the analytical aspects of the paper. Finally, we would like to thank two anonymous reviewers for providing helpful critique and comments on the first submission of this manuscript.

Author Contributions Conceptualization: AAZ MDS. Data curation: AAZ BCM MDS BM. Formal analysis: AAZ. Funding acquisition: MDS PC CH. Investigation: AAZ BCM CH MDS. Methodology: AAZ MDS. Project administration: MDS CH BCM BM. Resources: MDS CH PC BM. Software: PC AAZ. Supervision: MDS. Visualization: AAZ PC. Writing – original draft: AAZ. Writing – review & editing: AAZ MDS PC CH BCM BM.