Internal architecture and micro-biomechanical behaviour of the Neanderthal Kebara 2 hyoid bone compared with modern human hyoids: implication for speech capacity

The origin of human language and in particular the question of whether Neanderthals were capable of speech or complex language remains a highly controversial topic, of great interest to anthropologists. In 1989, Arensburg and his group discovered a well-preserved human hyoid bone from Middle Palaeolithic layers of Kebara Cave (Israel), dating from about 60,000 years BP (Arensburg et al., A Middle Palaeolithic human hyoid bone, Nature 338, 758 (1989)). More recent discoveries of other fossil hominin hyoids have generated a renewed interest in the potential of this bone to give information on the evolution of speech and complex language.

The hyoid is, in fact, the only bone of the vocal tract and therefore its only part that can fossilize (see Figure 1). The hyoid provides support to the larynx and serves as anchor for the tongue and other muscles needed, at least in Homo sapiens, in phonation. It was already known, from the study of its external morphology that the hyoid bones of Neanderthal and modern humans do not differ significantly, while their shape is definitely different  from that of other primates such as chimpanzees. However, to be able to obtain more informatio on the function of the hyoid bone, it was crucial to analyze its internal microstructure, which remodels in response to the mechanical stress to which the bone is subjected.

In the present study, several research groups, with different expertise, have adopted an approach based on the comparison between the microstructural and micro-biomechanical properties of the Kebara 2 hyoid and those of the same bone in Homo sapiens. The study was conducted by an international research team with members from the University of Chieti, ICTP (UNESCO) and Elettra in Italy, the University of New England and of New South Wales in Australia, and the University of Toronto in Canada.

The data for the study were obtained using laboratory-based computed X-ray microtomography (micro-CT), combined with Finite Element Analyses (FEA). In particular, X-ray micro-CT technique can provide, in a non-invasive way, a virtual 3D reconstruction of the bone samples, showing its histological characteristics (trabecular thickness and pattern distribution of the vascular channels) (see Figure 2). FEA modelling allowed us to evaluate the biomechanical response of the bone as a result of the external forces applied to the hyoid by the muscles (Figure 3).
X-ray micro-CT analyses were conducted at the Elettra Tomolab station while FEA modelling was performed by the Australian and Canadian teams under the supervision of Prof. Stephen Wroe. These advanced analytical techniques provided crucial information on morphology and micro-biomechanical behavior of the investigated hyoid bones. They have confirmed that the internal microstructure of the hyoid bone from Kebara is similar to that of modern humans and that, in all the samples, the histological structure is typical of a bone subjected to intense and constant metabolic activity (such as language). Comparisons based on FEA show significant similarities in the micro-biomechanical behavior.