Evidence for Morphological Constraint on Bony Ear Canal Length in Modern Humans

Abstract

Throughout hominin evolution there is a trend towards a decreased length of the ear canal. This change in length is likely due to many factors that relate to reorganization of the cranial base among hominins with a more anteriorly placed foramen magnum, coronally oriented petrous pyramids, and an enlarged brain compared to extant African great apes. Functionally, ear canal length has been postulated to influence the detection of certain sound frequencies and may also alter the accuracy of sound localization. Therefore, a change in length of the ear canal could have effects on sensory perception throughout the hominin clade. However, the factors driving this change are rather unclear.This preliminary study tests four null hypotheses stating there is no statistically significant correlation between the length of the bony ear canal and: 1) foramen magnum position, 2) foramen magnum width, 3) petrous pyramid length and 4) facial width. All of these factors have the potential to influence the length of the ear canal, but previous studies have not used quantitative methods to determine statistical correlations. We used 3D geometric morphometric landmark data, Principal Component Analyses and ordinary least squares regressions to determine correlations between specific measurements (see above) of the basicranium of modern humans, African apes and four hominin specimens (OH 5, KNM‐ER 406, STS 5 and AL 444‐2).Results show that there is a significant correlation between foramen magnum width and bony ear canal length in modern humans, specifically (R2=0.626, p<0.001), refuting the null hypothesis that there is no significant correlation between ear canal length and foramen magnum width. Interestingly, there was no significant correlation between facial width and bony ear canal length, except perhaps in extinct hominin taxa, likely due to their robust masticatory system. These results indicate that there may be a functional constraint on the length of the bony ear canal in modern humans, limiting the variability of ear canal length and its role within sound perception and localization. Future work should focus on increasing the sample sizes and using the timing of cranial base ossification patterns to aid in understanding if the growth of the foramen magnum is actually driving the medial constraint of the ear canal in humans.Support or Funding InformationThis research was supported by The Graduate Center, City University of New York through a Graduate Center Fellowship and would not be possible without the support of the New York Consortium of Evolutionary Primatology.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.