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, 101 (27), 9976-81

Auditory Capacities in Middle Pleistocene Humans From the Sierra De Atapuerca in Spain

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Auditory Capacities in Middle Pleistocene Humans From the Sierra De Atapuerca in Spain

I Martínez et al. Proc Natl Acad Sci U S A.

Abstract

Human hearing differs from that of chimpanzees and most other anthropoids in maintaining a relatively high sensitivity from 2 kHz up to 4 kHz, a region that contains relevant acoustic information in spoken language. Knowledge of the auditory capacities in human fossil ancestors could greatly enhance the understanding of when this human pattern emerged during the course of our evolutionary history. Here we use a comprehensive physical model to analyze the influence of skeletal structures on the acoustic filtering of the outer and middle ears in five fossil human specimens from the Middle Pleistocene site of the Sima de los Huesos in the Sierra de Atapuerca of Spain. Our results show that the skeletal anatomy in these hominids is compatible with a human-like pattern of sound power transmission through the outer and middle ear at frequencies up to 5 kHz, suggesting that they already had auditory capacities similar to those of living humans in this frequency range.

Figures

Fig. 1.
Fig. 1.
Measurements of the middle and external ear (A) and ear ossicles (B). A and B are not drawn to the same scale. A is based on the CT images of Cranium 5. VMA, volume of the mastoid antrum and connected mastoid air cells, measured from its limit with the aditus ad antrum (dotted line 1); VAD, volume of the aditus ad antrum, measured from its limit with the middle ear (dotted line 2) to its limit with the mastoid antrum (dotted line 1); VMEC, volume of the middle ear cavity measured from its limit with the aditus ad antrum (dotted line 2) to the edge of the tympanic groove (dotted line 3); LAD, length of the aditus ad antrum, measured as the mean distance from its limit with the middle ear cavity to the entrance to the mastoid antrum; LEAC, length of the external auditory canal, measured from the superior point of the tympanic groove to the spina suprameatum. Dotted line 4 marks the level at which the cross-sectional area of the external auditory canal (AEAC) was measured. B is based on the profiles of the malleus and incus from the temporal bone AT-1907 and the stapes from Cranium 5. LM, functional length of the malleus, measured as the maximum length from the superior border of the short process to the inferior-most tip of the manubrium; LI, functional length of the incus measured from the lateral-most point along the articular facet to the lowest point along the long crus; AFP, measured area of the footplate of the stapes.
Fig. 2.
Fig. 2.
Sound power (dB) at the entrance to the cochlea relative to P0 = 10-18 W for an incident plane wave intensity of 10-12 W/m2. Results from modern human (solid line) and chimpanzee (dashed line) individuals are shown and were obtained by using the model defined by Rosowski (17) and the cochlear input impedance (Zc) of Aibara et al. (27). Chimpanzee individual is based on the 3D CT reconstruction.
Fig. 3.
Fig. 3.
Sound power (dB) at the entrance to the cochlea relative to P0 = 10-18 W for an incident plane wave intensity of 10-12 W/m2. All individuals have been modeled by using the model defined by Rosowski (17) and the cochlear input impedance (Zc) of Aibara et al. (27). Solid blue line, modern human; solid green line, chimpanzee 3D CT; solid black line, theoretical chimpanzee-like modern human individual; dashed black line, theoretical human-like chimpanzee individual. solid red line, AT-84; dashed red line, AT-4103; dashed-dotted red line, Cranium 5; solid magenta line, AT-421; dashed magenta line, AT-1907.

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