The cochlea of the Sima de los Huesos hominins (Sierra de Atapuerca, Spain): New insights into cochlear evolution in the genus Homo

The ear of the Sima de los Huesos hominins (Atapuerca, Spain)

Previous studies on the morphology of the inner ear (semicircular canals and cochlea) in the Sima de los Huesos hominin sample have provided important results on the evolution of these structures in the Neandertal lineage. Similarly, studies of the anatomy of the external and middle ear cavities of the Sima de los Huesos hominins have also provided important data on the auditory capacities of this European Middle Pleistocene population. The present contribution provides unpublished data on three new middle ear variables from the Sima de los Huesos fossils and compares these data with values from samples of Pan troglodytes, Homo neanderthalensis and Homo sapiens. The results of this analysis are combined with those obtained in previous studies to characterize the anatomy of the outer, middle and inner ear in the Sima de los Huesos fossils, as well as to establish the order of appearance of the features that characterize Neandertal ears. As in other cranial structures, the ear region in the Sima de los Huesos show a mosaic evolutionary pattern that includes primitive traits, others shared exclusively with Neandertals, and others that are specific to the Sima de los Huesos hominins. Neandertals and Sima de los Huesos hominins share two exclusive features of the middle ear that are among the first characteristics of the Neandertal lineage: a long tympanic cavity and a large entrance and exit of the mastoid antrum. Along with these traits, the Sima de los Huesos hominins present two specialized features: large volumes of the tympanic cavity and the mastoid antrum. Finally, the middle ear of the Neandertals is characterized by the presence of small angles between the tympanic axis and the plane of the oval window.

The Cranium I: Neurocranium

The Sima de los Huesos (SH) site has provided a significant collection of hominin remains, including numerous cranial fragments, which have contributed to our understanding of the MP human population. The taxonomic classification of the SH hominins remains a topic of debate, with some studies suggesting a close relationship to Neandertals based on nuclear DNA analysis. The cranial morphology of the SH specimens exhibits a mix of Neandertal-like features and primitive traits observed in earlier Homo populations, providing insights into the evolutionary pattern of the Neanderthal lineage. This study focuses on the neurocranial traits of the SH population and describes three previously undescribed cranial individuals. The SH cranial collection now comprises 20 nearly complete crania, representing approximately two-thirds of the estimated population size. The analysis of the SH population reveals variations in robustness, frontal torus development, sagittal keeling, and occipital torus morphology, which may be related to sexual dimorphism and ontogenetic factors. The suprainiac region exhibits notable ontogenetic changes, while suture obliteration patterns do not strictly correlate with dental age. Metric measurements, particularly cranial breadths, highlight significant intrapopulation variation within the SH sample. Compared with other Middle Pleistocene (MP) hominins, the SH cranial vault displays archaic characteristics but differs from Homo erectus and Neandertals. The SH individuals have relatively short and tall cranial vaults, distinguishing them from other MP fossils. These findings contribute to our understanding of the MP human populations and their evolutionary trajectories.

The Neandertal nature of the Atapuerca Sima de los Huesos mandibles

The recovery of additional mandibular fossils from the Atapuerca Sima de los Huesos (SH) site provides new insights into the evolutionary significance of this sample. In particular, morphological descriptions of the new adult specimens are provided, along with standardized metric data and phylogenetically relevant morphological features for the expanded adult sample. The new and more complete specimens extend the known range of variation in the Atapuerca (SH) mandibles in some metric and morphological details. In other aspects, the addition of new specimens has made it possible to confirm previous observations based on more limited evidence. Pairwise comparisons of individual metric variables revealed the only significant difference between the Atapuerca (SH) hominins and Neandertals was a more vertical symphysis in the latter. Similarly, principal components analysis of size-adjusted variables showed a strong similarity between the Atapuerca (SH) hominins and Neandertals. Morphologically, the Atapuerca (SH) mandibles show nearly the full complement of Neandertal-derived features. Nevertheless, the Neandertals differ from the Atapuerca (SH) mandibles in showing a high frequency of the H/O mandibular foramen, a truncated, thinned and inverted gonial margin, a high placement of the mylohyoid line at the level of the M3, a more vertical symphysis and somewhat more pronounced expression of the chin structures. Size-related morphological variation in the SH hominins includes larger retromolar spaces, more posterior placement of the lateral corpus structures, and stronger markings associated with the muscles of mastication in larger specimens. However, phylogenetically relevant features in the SH sample are fairly stable and do not vary with the overall size of the mandible. Direct comparison of the enlarged mandibular sample from Atapuerca (SH) with the Mauer mandible, the type specimen of H. heidelbergensis, reveals important differences from the SH hominins, and there is no morphological counterpart of Mauer within the SH sample, suggesting the SH fossils should not be assigned to this taxon. The Atapuerca (SH) mandibles show a greater number of derived Neandertal features, particularly those related to midfacial prognathism and in the configuration of the superior ramus, than other European middle Pleistocene specimens. This suggests that more than one evolutionary lineage co-existed in the middle Pleistocene, and, broadly speaking, it appears possible to separate the European middle Pleistocene mandibular remains into two distinct groupings. One group shows a suite of derived Neandertal features and includes specimens from the sites of Atapuerca (SH), Payre, l'Aubesier and Ehringsdorf. The other group includes specimens that generally lack derived Neandertal features and includes the mandibles from the sites of Mauer, Mala Balanica, Montmaurin and (probably) Visogliano. The two published Arago mandibles differ strongly from one another, with Arago 2 probably belonging to this former group, and Neandertal affinities being more difficult to identify in Arago 13. Outside of the SH sample, derived Neandertal features in the mandible only become more common during the second half of the middle Pleistocene. Acceptance of a cladogenetic pattern of evolution during the European middle Pleistocene has the potential to reconcile the predictions of the accretion model and the two phases model for the appearance of Neandertal morphology. The precise taxonomic classification of the SH hominins must contemplate features from the dentition, cranium, mandible and postcranial skeleton, all of which are preserved at the SH site. Nevertheless, the origin of the Neandertal clade may be tied to a speciation event reflected in the appearance of a suite of derived Neandertal features in the face, dentition and mandible, all of which are present in the Atapuerca (SH) hominins. This same suite of features also provides a useful anatomical basis to include other European middle Pleistocene mandibles and crania within the Neandertal clade.

The child who lived: Down syndrome among Neanderthals?

Caregiving for disabled individuals among Neanderthals has been known for a long time, and there is a debate about the implications of this behavior. Some authors believe that caregiving took place between individuals able to reciprocate the favor, while others argue that caregiving was produced by a feeling of compassion related to other highly adaptive prosocial behaviors. The study of children with severe pathologies is particularly interesting, as children have a very limited possibility to reciprocate the assistance. We present the case of a Neanderthal child who suffered from a congenital pathology of the inner ear, probably debilitating, and associated with Down syndrome. This child would have required care for at least 6 years, likely necessitating other group members to assist the mother in childcare.

Anatomically and mechanically accurate scala tympani model for electrode insertion studies

The risk of insertion trauma in cochlear implantation is determined by the interplay between individual cochlear anatomy and electrode insertion mechanics. Whereas patient anatomy cannot be changed, new surgical techniques, devices for cochlear monitoring, drugs, and electrode array designs are continuously being developed and tested, to optimize the insertion mechanics and prevent trauma. Preclinical testing of these developments is a crucial step in feasibility testing and optimization for clinical application. Human cadaveric specimens allow for the best simulation of an intraoperative setting. However, their availability is limited and it is not possible to conduct repeated, controlled experiments on the same sample. A variety of artificial cochlear models have been developed for electrode insertion studies, but none of them were both anatomically and mechanically representative for surgical insertion into an individual cochlea. In this study, we developed anatomically representative models of the scala tympani for surgical insertion through the round window, based on microCT images of individual human cochleae. The models were produced in transparent material using commonly-available 3D printing technology at a desired scale. The anatomical and mechanical accuracy of the produced models was validated by comparison with human cadaveric cochleae. Mechanical evaluation was performed by recording insertion forces, counting the number of inserted electrodes and grading tactile feedback during manual insertion of a straight electrode by experienced cochlear implant surgeons. Our results demonstrated that the developed models were highly representative for the anatomy of the original cochleae and for the insertion mechanics in human cadaveric cochleae. The individual anatomy of the produced models had a significant impact on the insertion mechanics. The described models have a promising potential to accelerate preclinical development and testing of atraumatic insertion techniques, reducing the need for human cadaveric material. In addition, realistic models of the cochlea can be used for surgical training and preoperative planning of patient-tailored cochlear implantation surgery.

Human cochlear microstructures at risk of electrode insertion trauma, elucidated in 3D with contrast-enhanced microCT

Cochlear implant restores hearing loss through electrical stimulation of the hearing nerve from within the cochlea. Unfortunately, surgical implantation of this neuroprosthesis often traumatizes delicate intracochlear structures, resulting in loss of residual hearing and compromising hearing in noisy environments and appreciation of music. To avoid cochlear trauma, insertion techniques and devices have to be adjusted to the cochlear microanatomy. However, existing techniques were unable to achieve a representative visualization of the human cochlea: classical histology damages the tissues and lacks 3D perspective; standard microCT fails to resolve the cochlear soft tissues; and previously used X-ray contrast-enhancing staining agents are destructive. In this study, we overcame these limitations by performing contrast-enhanced microCT imaging (CECT) with a novel polyoxometalate staining agent Hf-WD POM. With Hf-WD POM-based CECT, we achieved nondestructive, high-resolution, simultaneous, 3D visualization of the mineralized and soft microstructures in fresh-frozen human cochleae. This enabled quantitative analysis of the true intracochlear dimensions and led to anatomical discoveries, concerning surgically-relevant microstructures: the round window membrane, the Rosenthal's canal and the secondary spiral lamina. Furthermore, we demonstrated that Hf-WD POM-based CECT enables quantitative assessment of these structures as well as their trauma.

Geometric morphometric analysis of the bony labyrinth of the Sima de los Huesos hominins

The bony labyrinth contains phylogenetic information that can be used to determine interspecific differences between fossil hominins. The present study conducted a comparative 3D geometric morphometric analysis on the bony labyrinth of the Middle Pleistocene Sima de los Huesos (SH) hominins. The findings of this study corroborate previous multivariate analyses of the SH hominin bony labyrinth. The analysis of the semicircular canals revealed the SH hominin canal morphologies appear closer to those of the Neandertals than to those of Homo sapiens. This is attributable to a Neandertal-like ovoid anterior canal, and mediolaterally expanded, circular posterior canal. However, the SH hominins lack the increased torsion in the anterior canal and the inferior orientation of the lateral canal seen in Neandertals. The results of the cochlear analysis indicated that, although there is some overlap, there are notable differences between the SH hominins and the Neandertals. In particular, the SH hominin cochlea appears more constricted than in Neandertals in the first and second turns. A principal component analysis of the full bony labyrinth separated most SH hominins from the Neandertals, which largely clustered with modern humans. A covariance ratio analysis found a significant degree of modularity within the bony labyrinth of all three groups, with the SH hominins and Neandertals displaying the highest modularity. This modular signal in the bony labyrinth may be attributable to different selective pressures related to locomotion and audition. Overall, the results of this study confirm previous suggestions that the semicircular canals in the SH hominins are somewhat derived toward Neandertals, while their cochlea is largely primitive within the genus Homo.

Cochlear tuning and the peripheral representation of harmonic sounds in mammals

Albert Feng was a prominent comparative neurophysiologist whose research provided numerous contributions towards understanding how the spectral and temporal characteristics of vocalizations underlie sound communication in frogs and bats. The present study is dedicated to Al's memory and compares the spectral and temporal representations of stochastic, complex sounds which underlie the perception of pitch strength in humans and chinchillas. Specifically, the pitch strengths of these stochastic sounds differ between humans and chinchillas, suggesting that humans and chinchillas may be using different cues. Outputs of auditory filterbank models based on human and chinchilla cochlear tuning were examined. Excitation patterns of harmonics are enhanced in humans as compared with chinchillas. In contrast, summary correlograms are degraded in humans as compared with chinchillas. Comparing summary correlograms and excitation patterns with corresponding behavioral data on pitch strength suggests that the dominant cue for pitch strength in humans is spectral (i.e., harmonic) structure, whereas the dominant cue for chinchillas is temporal (i.e., envelope) structure. The results support arguments that the broader cochlear tuning in non-human mammals emphasizes temporal cues for pitch perception, whereas the sharper cochlear tuning in humans emphasizes spectral cues.

Cochlear morphology of Indonesian Homo erectus from Sangiran

Homo erectus s.l. is key for deciphering the origin and subsequent evolution of genus Homo. However, the characterization of this species is hindered by the existence of multiple variants in both mainland and insular Asia, as a result of divergent chronogeographical evolutionary trends, genetic isolation, and interbreeding with other human species. Previous research has shown that cochlear morphology embeds taxonomic and phylogenetic information that may help infer the phylogenetic relationships among hominin species. Here we describe the cochlear morphology of two Indonesian H. erectus individuals (Sangiran 2 and 4), and compare it with a sample of australopiths, Middle to Late Pleistocene humans, and extant humans by means of linear measurements and both principal components and canonical variates analyses performed on shape ratios. Our results indicate that H. erectus displays a mosaic morphology that combines plesiomorphic (australopithlike) features (such as a chimplike round cochlear cross section and low cochlear thickness), with derived characters of later humans (a voluminous and long cochlea, possibly related to hearing abilities)-consistent with the more basal position of H. erectus. Our results also denote substantial variation between the two studied individuals, particularly in the length and radius of the first turn, as well as cross-sectional shape. Given the small size of the available sample, it is not possible to discern whether such differences merely reflect intraspecific variation among roughly coeval H. erectus individuals or whether they might result from greater age differences between them than currently considered. However, our results demonstrate that most characters found in later humans were already present in Indonesian H. erectus, with the exception of Neanderthals, which display an autapomorphic condition relative to other Homo species.