Estudio comparativo de las cavidades del oído externo, medio e interno y establecimiento de la potencia sonora en homínidos

Abstract

This thesis has been done according to two main objectives: to complete the study of the ear cavities of the fossils of the Sima de los Huesos site (SH), and to extend the study of the external and middle ear cavities to the whole Homininae subfamily. For this, three-dimensional reconstructions of the ear cavities were made from several thousands of tomographic images, of 21 specimens of Gorilla gorilla, 9 of SH, and 7 of Homo neanderthalensis. In relation to the first objective, three works have been carried out that have been published in the Journal of Human Evolution. The first (Conde-Valverde et al., 2018), was a comparative study of the semicircular canals of the Aroeira 3 Mesopleistocene fossil (Cueva de Almonda, Aroeira, Portugal) with the SH specimens. The main result of this study has established that, despite its geographical and temporal proximity, the Aroeira 3 fossil has a more primitive morphology than that of the SH fossils, which already have Neanderthal features. In our opinion, this result is consistent with a demographic model of isolation of human populations of the Iberian Peninsula during Marine Isotopic Stage MIS 12. The second published work (Conde-Valverde et al., 2019a) consists of a detailed comparative study of the cochlea of SH individuals with samples of Pan troglodytes, Homo sapiens and H. neanderthalensis. It also includes data from other hominin species, such as Australopithecus africanus and Paranthropus robustus, obtained from literature. The results of the work indicate that the cochleae of the SH sample show a mosaic evolutionary pattern, featuring primitive traits shared with P. troglodytes, together with other derived traits shared with H. neanderthalensis and H. sapiens. Also proposed is the existence of homoplasies between the cochlea of the Neanderthals and that of our species in those traits shared by both species, but in which the SH fossils present the primitive state. The inferences made on the limits of the hearing of the fossils of SH, from the value of their cochlear volume, indicate that their upper limit was somewhat displaced towards higher frequencies, in comparison to those of H. neanderthalensis and H. sapiens. The third published work of this thesis is the study of the auditory capabilities of Cranium 4 of SH (Conde-Valverde et al., 2019b). This specimen presented bilateral exostosis of both external auditory canals and it had been diagnosed with deafness. It was the oldest known case of deafness in human evolution. In this study, threedimensional models of the outer and middle ear cavities of this specimen have been made, and the sound power transmission in both ears has been calculated. The results have been compared with those corresponding to the rest of the specimens of the SH sample, as well as with a theoretical model of Cranium 4 without pathology. The results of the work clearly show that the hearing of the individual represented by Cranium 4 was not affected by the exostosis of his external auditory canals, thus the diagnosis of deafness has been refuted. The studies carried out in relation to the second objective constitute the unpublished section of this thesis. This features a comparative study of the anatomy of the outer and middle ear of specimens of G. gorilla, P. troglodytes, SH fossils, H. neanderthalensis and H. sapiens. As a result of the study, the external and middle ears of the different species have been characterized and a proposal of features of taxonomically diagnostic value has been made. The analysis of allometries of different ear structures in the subfamily Homininae has also been performed and features of phylogenetic analytical value have been proposed. Moreover, the values of the sound power transmission and those of the bandwidth occupied in G. gorilla have been established. These are the first known data on the auditory patterns of this species. These parameters have also been calculated in six new SH specimens, which has allowed us to more accurately characterize the auditory patterns of this population. In addition, variations in the length of the occupied bandwidth have been related to changes in the proportions of the different structures of the outer and middle ear in the subfamily Homininae, and a model of the sequence of changes of these parameters in the evolution of the subfamily has been proposed. Finally, this study also tests the hypothesis that, in the subfamily Homininae, the value of the length of the occupied bandwidth of a species is directly related to the social complexity and also to the number and diversity of the vocalizations of that species.