Neanderthals had a generally wide nose opening. In recent humans, such a feature is found mainly in the inhabitants of equatorial regions, while populations away from the tropics have a narrower nose. Neanderthals occupied a relatively cold and dry region of the world, so they ought to have relatively narrow noses, but they didn't. What accounts for this discrepancy?
To explain this phenomenon, it was proposed that Neanderthals were constrained to have wide noses by their wide mouths, especially the distance between their front canine teeth (ICB), which effectively "pulled to the sides" their midfacial region. Neanderthals may have needed to have such broad mouths to aid chewing.
Interestingly, a broad dental arch is associated with facial prognathism, i.e., the degree to which the face juts forward. So, perhaps Neanderthals possessed this trait because they retained the ancestral trait of prognathism. Prognathism can be measured by the basion-prosthion length (BPL), roughly the distance between the hole at the bottom of the skull and the point between the front two teeth.
It turns out that BPL rather than ICB explains more variance in the breadth of the nasal aperture, and hence this tends not to support the theory that Neanderthals had broad noses because they had wide palates.
So, how did Neanderthals manage to have broad noses against ecological expectation? They were "forced" to have such noses because of their substantial prognathism. But, their nasal heights were also great, and hence their nasal index (breadth/height) was fairly low. Thus, they may have dealt with their cold and dry environment by having a broad and long nose, rather than a short and narrow one.
Journal of Human Evolution doi: 10.1016/j.jhevol.2008.07.001
The paradox of a wide nasal aperture in cold-adapted Neandertals: a causal assessment
Nathan E. Holto, Robert G. Franciscus
Neandertals have been characterized as possessing features indicative of cold-climate adaptation largely based on ecogeographical morphological patterning found in recent humans. Interestingly, one character that deviates from this pattern is a relatively wide nasal aperture. The ecogeographical patterning of the nasal aperture in recent humans would predict instead that Neandertals should exhibit reduced nasal breadth dimensions. To explain this apparent anomaly it has been argued that a reduction in Neandertal nasal breadth was not possible due to dentognathic constraints on their midfaces via large anterior palatal breadth dimensions, especially large intercanine distances. A complicating factor in understanding the relationship between anterior palate breadth and nasal breadth is that both measurements are also correlated with facial prognathism. It is, therefore, unknown to what degree the relationship between anterior palate breadth and nasal breadth in Neandertals is a function of the pleisiomorphic retention of a prognathic facial skeleton. We used path analysis to test for a causal relationship between intercanine breadth and nasal breadth taking into account the potential effect of facial projection and facial prognathism (i.e., basion-nasion length and basion-prosthion length) using a large sample of geographically diverse recent and fossil Homo. Additionally, we examined the ontogenetic relationship between nasal breadth and intercanine breadth using a longitudinal human growth series to determine whether these variables exhibit similar growth trajectories. The results of these analyses indicate a weaker association between intercanine breadth and nasal breadth than expected, and that more variation in nasal breadth can be explained through basion-prosthion length rather than anterior palatal breadth dimensions. Moreover, the ontogenetic development of anterior palate breadth does not correspond to the growth trajectory of the breadth of the nose. These results explain the apparent paradox of wide piriform apertures in generally cooler climate-adapted Neandertals without resorting to dentognathic constraints, and provide additional insight into both the adaptive and nonadaptive (i.e., neutral) basis for Neandertal facial evolution.