Cochlear shape distinguishes southern African early hominin taxa with unique auditory ecologies

J F Thackeray,K Jakata,B Zipfel,F E Grine,Y Feunteun,A Fradi,M Macé,C Samir,V A Zimmer,B A Wood,J Braga

doi:10.1038/s41598-021-96543-w

Abstract

Insights into potential differences among the bony labyrinths of Plio-Pleistocene hominins may inform their evolutionary histories and sensory ecologies. We use four recently-discovered bony labyrinths from the site of Kromdraai to significantly expand the sample for Paranthropus robustus. Diffeomorphometry, which provides detailed information about cochlear shape, reveals size-independent differences in cochlear shape between P. robustus and Australopithecus africanus that exceed those among modern humans and the African apes. The cochlea of P. robustus is distinctive and relatively invariant, whereas cochlear shape in A. africanus is more variable, resembles that of early Homo, and shows a degree of morphological polymorphism comparable to that evinced by modern species. The curvature of the P. robustus cochlea is uniquely derived and is consistent with enhanced sensitivity to low-frequency sounds. Combined with evidence for selection, our findings suggest that sound perception shaped distinct ecological adaptations among southern African early hominins.

Highlights

Insights into potential differences among the bony labyrinths of Plio-Pleistocene hominins may inform their evolutionary histories and sensory ecologies
The Fréchet mean[12,13] of two points along a curved surface is located at equal geodesic distances between them, whereas the standard arithmetic mean utilized in 3D geometric morphometric (3DGM) analyses is located at mid-linear distance, and lies outside the curve. This non-linear geometric framework, unlike 3DGM, allows measurements of local geometric p roperties[16], including mapping changes in curvature and torsion at any location along the length of the cochlea, from its base to the apex (“Methods”). We applied this method to microCT images of the cochlea of samples of museum specimens representing modern humans (Homo sapiens, n = 16), common (Pan troglodytes, n = 16) and pygmy (Pan paniscus, n = 16) chimpanzees, gorillas (Gorilla gorilla, n = 15), and Plio-Pleistocene hominin fossils from southern African cave sites belonging to Australopithecus africanus (n = 9), Paranthropus robustus (n = 8) and undoubted early Homo (n = 1) (Tables S1,2)
In a principal components analysis (PCA) of bony labyrinthine features, they and the other P. robustus specimens are wellseparated from early Homo on PC1 (Fig. 1 and “Methods”)

Summary

Introduction

Insights into potential differences among the bony labyrinths of Plio-Pleistocene hominins may inform their evolutionary histories and sensory ecologies. This non-linear geometric framework, unlike 3DGM, allows measurements of local geometric p roperties[16], including mapping changes in curvature (bending) and torsion (twisting) at any location along the length of the cochlea, from its base to the apex (“Methods”) We applied this method to microCT images of the cochlea of samples of museum specimens representing modern humans (Homo sapiens, n = 16), common (Pan troglodytes, n = 16) and pygmy (Pan paniscus, n = 16) chimpanzees, gorillas (Gorilla gorilla, n = 15), and Plio-Pleistocene hominin fossils from southern African cave sites belonging to Australopithecus africanus (n = 9), Paranthropus robustus (n = 8) and undoubted early Homo (n = 1) (Tables S1,2). We considered the StW 53 and StW 151 specimens as taxonomically indeterminate because of the disagreement relating to their assignation

Methods

Results

Conclusion