Comparative morphology of the hominin and African ape hyoid bone, a possible marker of the evolution of speech

Evolution of the human tongue and emergence of speech biomechanics

The tongue is one of the organs most central to human speech. Here, the evolution and species-unique properties of the human tongue is traced, via reference to the apparent articulatory behavior of extant non-human great apes, and fossil findings from early hominids - from a point of view of articulatory phonetics, the science of human speech production. Increased lingual flexibility provided the possibility of mapping of articulatory targets, possibly via exaptation of manual-gestural mapping capacities evident in extant great apes. The emergence of the human-specific tongue, its properties, and morphology were crucial to the evolution of human articulate speech.

Comparative morphology and physiology of the vocal production apparatus and the brain in the extant primates

Objective data mainly from the comparative anatomy of various organs related to human speech and language is considered to unearth clues about the mechanisms behind language development. The two organs of the larynx and hyoid bone are considered to have evolved towards suitable positions and forms in preparation for the occurrence of the large repertoire of vocalization necessary for human speech. However, some researchers have asserted that there is no significant difference of these organs between humans and non-human primates. Speech production is dependent on the voluntary control of the respiratory, laryngeal, and vocal tract musculature. Such control is fully present in humans but only partially so in non-human primates, which appear to be able to voluntarily control only supralaryngeal articulators. Both humans and non-human primates have direct cortical innervation of motor neurons controlling the supralaryngeal vocal tract but only human appear to have direct cortical innervation of motor neurons controlling the larynx. In this review, we investigate the comparative morphology and function of the wide range of components involved in vocal production, including the larynx, the hyoid bone, the tongue, and the vocal brain. We would like to emphasize the importance of the tongue in the primary development of human speech and language. It is now time to reconsider the possibility of the tongue playing a definitive role in the emergence of human speech.

Morphological disparity and evolutionary transformations in the primate hyoid apparatus

The hyoid apparatus plays an integral role in swallowing, respiration, and vocalization in mammals. Most placental mammals have a rod-shaped basihyal connected to the basicranium via both soft tissues and a mobile bony chain-the anterior cornu-whereas anthropoid primates have broad, shield-like or even cup-shaped basihyals suspended from the basicranium by soft tissues only. How the unique anthropoid hyoid morphology evolved is unknown, and hyoid morphology of nonanthropoid primates is poorly documented. Here we use phylogenetic comparative methods and linear morphometrics to address knowledge gaps in hyoid evolution among primates and their euarchontan outgroups. We find that dermopterans have variable reduction of cornu elements. Cynocephalus volans are sexually dimorphic in hyoid morphology. Tupaia and all lemuroids except Daubentonia have a fully ossified anterior cornu connecting a rod-shaped basihyal to the basicranium; this is the ancestral mammalian pattern that is also characteristic of the last common ancestor of Primates. Haplorhines exhibit a reduced anterior cornu, and anthropoids underwent further increase in basihyal aspect ratio values and in relative basihyal volume. Convergent with haplorhines, lorisoid strepsirrhines independently evolved a broad basihyal and reduced anterior cornua. While a reduced anterior cornu is hypothesized to facilitate vocal tract lengthening and lower formant frequencies in some mammals, our results suggest vocalization adaptations alone are unlikely to drive the iterative reduction of anterior cornua within Primates. Our new data on euarchontan hyoid evolution provide an anatomical basis for further exploring the form-function relationships of the hyoid across different behaviors, including vocalization, chewing, and swallowing.

Differential DNA methylation of vocal and facial anatomy genes in modern humans

Changes in potential regulatory elements are thought to be key drivers of phenotypic divergence. However, identifying changes to regulatory elements that underlie human-specific traits has proven very challenging. Here, we use 63 reconstructed and experimentally measured DNA methylation maps of ancient and present-day humans, as well as of six chimpanzees, to detect differentially methylated regions that likely emerged in modern humans after the split from Neanderthals and Denisovans. We show that genes associated with face and vocal tract anatomy went through particularly extensive methylation changes. Specifically, we identify widespread hypermethylation in a network of face- and voice-associated genes (SOX9, ACAN, COL2A1, NFIX and XYLT1). We propose that these repression patterns appeared after the split from Neanderthals and Denisovans, and that they might have played a key role in shaping the modern human face and vocal tract.