Disproportionate Cochlear Length in Genus Homo Shows a High Phylogenetic Signal during Apes' Hearing Evolution.

J Braga,A Riberon,D Descouens,J F Thackeray,J-L Kahn,P Balaresque,E Gilissen,J Dumoncel,K Hoffman,J-M Loubes,F De Beer

doi:10.1371/journal.pone.0127780

Abstract

Changes in lifestyles and body weight affected mammal life-history evolution but little is known about how they shaped species’ sensory systems. Since auditory sensitivity impacts communication tasks and environmental acoustic awareness, it may have represented a deciding factor during mammal evolution, including apes. Here, we statistically measure the influence of phylogeny and allometry on the variation of five cochlear morphological features associated with hearing capacities across 22 living and 5 fossil catarrhine species. We find high phylogenetic signals for absolute and relative cochlear length only. Comparisons between fossil cochleae and reconstructed ape ancestral morphotypes show that Australopithecus absolute and relative cochlear lengths are explicable by phylogeny and concordant with the hypothetized ((Pan,Homo),Gorilla) and (Pan,Homo) most recent common ancestors. Conversely, deviations of the Paranthropus oval window area from these most recent common ancestors are not explicable by phylogeny and body weight alone, but suggest instead rapid evolutionary changes (directional selection) of its hearing organ. Premodern (Homo erectus) and modern human cochleae set apart from living non-human catarrhines and australopiths. They show cochlear relative lengths and oval window areas larger than expected for their body mass, two features corresponding to increased low-frequency sensitivity more recent than 2 million years ago. The uniqueness of the “hypertrophied” cochlea in the genus Homo (as opposed to the australopiths) and the significantly high phylogenetic signal of this organ among apes indicate its usefulness to identify homologies and monophyletic groups in the hominid fossil record.

Highlights

Body size constraints, lifestyles, and other environment-related parameters moduling survival and reproduction, are key players in the evolution of mammalian features [1,2,3,4]
Since RECL represents a combination of external cochlear length (ECL) and TUR, in order to prevent redundancy, we produce a principal component analyses (PCA) using only RECL, oval window area (OWA) and TUR (Fig 2)
All the fossil hominin RECLs fall within the African hominid most recent common ancestors (MRCAs) distributions and cannot be differentiated from the moden human variation sampled in this study (Fig 3)

Summary

Introduction

Lifestyles, and other environment-related parameters moduling survival and reproduction, are key players in the evolution of mammalian features [1,2,3,4]. Because sensory organs influence optimal decisions when interacting with environmental signals, it is important to reliably assess their dependence on selection, scaling rules and phylogeny during evolution [5,6,7]. If the phylogenetic signal of a given trait is low or absent (i.e., phylogenetically-related species are not more similar than expected by chance), our ability to infer ancestral states of such an evolutionary malleable feature will be more limited. Measures of the phylogenetic signal represent a prerequisite for the study of evolutionary processes. This statistic allows comparisons of features in order to assess potential differences in patterns of evolutionary processes between them

Methods

Results

Discussion

Conclusion