The origin of speech differs from the origin of language because language is not necessarily spoken; it could equally be written or signed. Speech is a fundamental aspect of human communication and plays a vital role in the everyday lives of humans. It allows them to convey thoughts, emotions, and ideas, and providing the ability to connect with others and shape collective reality.Oxford English Dictionary
Many attempts have been made to explain scientifically how speech emerged in humans, although to date no theory has generated agreement. A striking feature of language is that it is modality-independent. Should an impaired child be prevented from hearing or producing sound, its innate capacity to master a language may equally find expression in signing. Sign languages of the deaf are independently invented and have all the major properties of spoken language except for the modality of transmission. From this it appears that the language centres of the human brain must have evolved to function optimally, irrespective of the selected modality. No other primates exhibit this ability for sign language.
Non-human primates, like many other animals, have evolved specialized mechanisms for producing sounds for purposes of social communication.Kelemen, G. (1963). Comparative anatomy and performance of the vocal organ in vertebrates. In R. Busnel (ed.), Acoustic behavior of animals. Amsterdam: Elsevier, pp. 489–521. On the other hand, no monkey or ape uses its tongue for such purposes. The human species' unprecedented use of the tongue, lips and other moveable parts seems to place speech in a quite separate category, making its evolutionary emergence an intriguing theoretical challenge in the eyes of many scholars.
Modality-independence
The term modality means the chosen representational format for encoding and transmitting information. A striking feature of language is that it is modality-independent. Should an impaired child be prevented from hearing or producing sound, its innate capacity to master a language may equally find expression in signing. Sign languages of the deaf are independently invented and have all the major properties of spoken language except for the modality of transmission.Stokoe, W. C. 1960. Sign language structure: an outline of the communicative systems of the American deaf. Silver Spring, MD: Linstock Press.Bellugi, U. and E. S. Klima 1975. Aspects of sign language and its structure. In J. F. Kavanagh and J. E. Cutting (eds), The Role of Speech in Language. Cambridge, Massachusetts: The MIT Press, pp. 171 203. From this it appears that the language centres of the human brain must have evolved to function optimally, irrespective of the selected modality.
Animal communication systems routinely combine visible with audible properties and effects, but none is modality-independent. For example, no vocally-impaired whale, dolphin, or songbird could express its song repertoire equally in visual display. Indeed, in the case of animal communication, message and modality are not capable of being disentangled. Whatever message is being conveyed stems from the intrinsic properties of the signal.
Modality independence should not be confused with the ordinary phenomenon of multimodality. Monkeys and apes rely on a repertoire of species-specific "gesture-calls" – emotionally-expressive vocalisations inseparable from the visual displays which accompany them. Humans also have species-specific gesture-calls – laughs, cries, sobs, etc. – together with involuntary gestures accompanying speech.Darwin. C. 1872. The Expression of the Emotions in Man and Animals. London: Murray.Ekman, P. 1982. Emotion in the Human Face, 2nd Edition. Cambridge: Cambridge University Press.McNeil, D. 1992. Hand and Mind. What gestures reveal about thought. Chicago and London: University of Chicago Press. Many animal displays are polymodal in that each appears designed to exploit multiple channels simultaneously.
The human linguistic property of modality independence is conceptually distinct from polymodality. It allows the speaker to encode the informational content of a message in a single channel whilst switching between channels as necessary. Modern city-dwellers switch effortlessly between the spoken word and writing in its various forms – handwriting, typing, email, etc. Whichever modality is chosen, it can reliably transmit the full message content without external assistance of any kind. When talking on the telephone, for example, any accompanying facial or manual gestures, however natural to the speaker, are not strictly necessary. When typing or manually signing, conversely, there is no need to add sounds. In many Australian Aboriginal cultures, a section of the population – perhaps women observing a ritual taboo – traditionally restrict themselves for extended periods to a silent (manually-signed) version of their language. Then, when released from the taboo, these same individuals resume narrating stories by the fireside or in the dark, switching to pure sound without sacrifice of informational content.
Evolution of the speech organs
Speaking is the default modality for language in all cultures. Humans' first recourse is to encode their thoughts in sound – a method which depends on sophisticated capacities for controlling the lips, tongue and other components of the vocal apparatus.
The speech organs evolved in the first instance not for speech but for more basic bodily functions such as feeding and breathing. Nonhuman primates have broadly similar organs, but with different neural controls. Non-human apes use their highly-flexible, maneuverable tongues for eating but not for vocalizing. When an ape is not eating, fine motor control over its tongue is deactivated. Either it is performing gymnastics with its tongue or it is vocalising; it cannot perform both activities simultaneously. Since this applies to mammals in general, Homo sapiens are exceptional in harnessing mechanisms designed for respiration and ingestion for the radically different requirements of articulate speech.MacNeilage, Peter, 2008. The Origin of Speech. Oxford: Oxford University Press.
Tongue
The word "language" derives from the Latin lingua, "tongue". Phoneticians agree that the tongue is the most important speech articulator, followed by the lips. A natural language can be viewed as a particular way of using the tongue to express thought.
The human tongue has an unusual shape. In most mammals, it is a long, flat structure contained largely within the mouth. It is attached at the rear to the hyoid bone, situated below the oral level in the pharynx. In humans, the tongue has an almost circular sagittal (midline) contour, much of it lying vertically down an extended pharynx, where it is attached to a hyoid bone in a lowered position. Partly as a result of this, the horizontal (inside-the-mouth) and vertical (down-the-throat) tubes forming the supralaryngeal vocal tract (SVT) are almost equal in length (whereas in other species, the vertical section is shorter). As humans move their jaws up and down, the tongue can vary the cross-sectional area of each tube independently by about 10:1, altering formant frequencies accordingly. That the tubes are joined at a right angle permits pronunciation of the vowels [i], [u] and [a], which nonhuman primates cannot do. Even when not performed particularly accurately, in humans the articulatory gymnastics needed to distinguish these vowels yield consistent, distinctive acoustic results, illustrating the quantal (all-or-nothing) nature of human speech sounds.Stevens, K. N. 1972. The quantal nature of speech: Evidence from articulatory-acoustic data. In P. B. Denes and E. E. David, Jr. (eds.), Human Communication: A unified view. New York: McGraw-Hill, pp. 51–66. It may not be coincidental that [i], [u] and [a] are the most common vowels in the world's languages.Ladefoged, P. and Maddieson, I. 1996. The Sounds of the World's Languages. Oxford: Blackwell. Human tongues are a lot shorter and thinner than other mammals and are composed of a large number of muscles, which helps shape a variety of sounds within the oral cavity. The diversity of sound production is also increased with the human's ability to open and close the airway, allowing varying amounts of air to exit through the nose. The fine motor movements associated with the tongue and the airway, make humans more capable of producing a wide range of intricate shapes in order to produce sounds at different rates and intensities.
Lips In humans, the lips are important for the production of stops and fricatives, in addition to vowels. Nothing, however, suggests that the lips evolved for those reasons. During primate evolution, a shift from nocturnal to diurnal activity in tarsiers, monkeys and apes (the haplorhines) brought with it an increased reliance on vision at the expense of olfaction. As a result, the snout became reduced and the rhinarium or "wet nose" was lost. The muscles of the face and lips consequently became less constrained, enabling their co-option to serve purposes of facial expression. The lips also became thicker, and the oral cavity hidden behind became smaller. Hence, according to Ann MacLarnon, "the evolution of mobile, muscular lips, so important to human speech, was the exaptive result of the evolution of diurnality and visual communication in the common ancestor of haplorhines".MacLarnon, A. 2012. The anatomical and physiological basis of human speech production: adaptations and exaptations. In M. Tallerman and K. .Gibson (eds.), The Oxford Handbook of Language Evolution. Oxford: Oxford University Press, pp. 224-235. It is unclear whether human lips have undergone a more recent adaptation to the specific requirements of speech.
Respiratory control Compared with nonhuman primates, humans have significantly enhanced control of breathing, enabling exhalations to be extended and inhalations shortened as we speak. Whilst we are speaking, intercostal and interior abdominal muscles are recruited to expand the thorax and draw air into the lungs, and subsequently to control the release of air as the lungs deflate. The muscles concerned are markedly more innervated in humans than in nonhuman primates.MacLarnon, A. M. (1993). The vertebral canal. In A. Walker and R. Leakey (eds.), The Nariokotome Homo erectus skeleton. Cambridge, Massachusetts: Harvard University Press, 359–90. Evidence from fossil hominins suggests that the necessary enlargement of the vertebral canal, and therefore spinal cord dimensions, may not have occurred in Australopithecus or Homo erectus but was present in the Neanderthals and early modern humans.
Larynx
The larynx or voice box is an organ in the neck housing the vocal folds, which are responsible for phonation. In humans, the larynx is descended, it is positioned lower than in other primates. This is because the evolution of humans to an upright position shifted the head directly above the spinal cord, forcing everything else downward. The repositioning of the larynx resulted in a longer cavity called the pharynx, which is responsible for increasing the range and clarity of the sound being produced. Other primates have almost no pharynx; therefore, their vocal power is significantly lower. Humans are not unique in this respect: goats, dogs, pigs and tamarins lower the larynx temporarily, to emit loud calls. Several deer species have a permanently lowered larynx, which may be lowered still further by males during their roaring displays. Lions, jaguars, cheetahs and domestic cats also do this. However, laryngeal descent in nonhumans (according to Philip Lieberman) is not accompanied by descent of the hyoid; hence the tongue remains horizontal in the oral cavity, preventing it from acting as a pharyngeal articulator.