The Comparative Anatomy and Evolution
of the Human Vocal Tract
Margaret Clegg
Department o f Anthropology
University College London
Submitted in fulfilment o f the degree o f Doctor o f Philosophy. University o f London
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Ta b l e o f c o n t e n t s 1
Ta b l eo ff ig u r e s 4
Ta b l eo ft a b l e s 6
Ac k n o w l e d g e m e n t s 7
Ab s t r a c t 8
In t r o d u c t io n 9
Ch a p t e r On e. An a t o m yo ft h ev o c a lt r a c t 14
Description of vocal tract anatomy 15
The nasal and oral cavities 15
The pharynx 16
The basicranium 21
Vocal tract length and shape 23
Theories o f speech production and perception 24
Limitations o f linguistic models 28
Explanations of vocal tract enlargement 29
The present study 30
M ethod 32
Material 32
Procedure 32
Body mass calculations 32
Vocal tract measurements 32
Distance between the soft palate and epiglottis 34
Analysis 35
Results 36
Vocal dissections 36
Dissection summary 38
Vocal tract measurements 39
Vocal tract length 40
Oral cavity length 4 0
Tongue length 41
Tongue length compared to oral cavity length 42
Oral cavity and vocal tract lengths 43
Position o f the larynx and hyoid bone in the vocal tract 45
Discussion 4 7
Vocal tract dissections 4 7
Vocal tract Measurements 48
Vocal tract length 48
Oral cavity length 50
Tongue length 50
Tongue length and oral length 50
Epiglottis and soft palate contact 51
Oral length as a proportion o f vocal tract length 52
Relationship between the vocal tract and crania 52
Conclusions 54
Ch a p t e r Tw o Th e Hy o id Bo n e 55
The human hyoid bone 55
Hyoid position 55
Description o f the hyoid bone 57
Hyoid bone muscles 58
Ossification o f the hyoid bone 59
Final fusion o f the hyoid bone 60
The position o f the great ape hyoid bone 62
Relationship between the hyoid bone and the crania 63
Measurements o f the hyoid bone 64
The present study 65
M ethods 68
Materials 68
The human sample 68
The A frican ape sample 68
The cranial sample 68
Equipment 69
Procedure 69
The Hyoid bone Measurements 69
Cranial Measurements 71
Age estimation 77
Error Measurements 77
Analysis 78
Results 79
Intraspecific variation 79
Humans 79
Chimpanzees 85
Gorillas 86
Interspecific variation 87
Relationship between the hyoid bone and crania 97
Discussion 102
.Intraspecific Variation 102
Humans 102
The A frican Apes 107
Interspecific variation 109
Relationship between of the hyoid bone to crania 110
Conclusions 112
Ch a p t e r Th r e e Th e La r y n x 113
General description 113
Laryngeal cartilages 114
Vocal Folds 115
The laryngeal ventricle 117
Comparison of the ape and human hyoid 117
The position of the larynx in the vocal tract 121
The role of the larynx 122
The ontogeny o f the larynx 126
Primate development 128
The present study 128
M ethods 130
Materials 130
Procedure 130
Results 132
Laryngeal dissections 132
Laryngeal Measurements 133
Laryngeal size 133
Vocal cord length 134
A nterio-posterior length and body mass 13 5
Maximum diameter o f the larynx and body mass 136
Vocal fo ld and laryngeal length 136
Laryngeal length and height 13 7
The height o f the larynx and vocal fo ld length 13 8
Discussion 140
Laryngeal dissections 140
Metric measurements of the larynx 141
Ch a p t e r FOUR Sw a l l o w i n g 143
The present study 146
M ethods 147
Materials 147
Procedure 147
Analysis 148
Results 149
Analysis o f mortality data
Death rates from choking on fo o d compared to death rates from other causes 149
Comparison o f death rates from choking on fo o d across age groups 150
Age specific death rates 151
Sex differences in mortality from choking on fo o d 155
Average death rates over the 100years 156
Sex ratios 157
Discussion 159
Conclusions 165
Ch a p t e r Fi v e Re c o n s t r u c t i n g EARLIER HOMiNiN HYOID b o n e s 166
The present study 168
M ethods 170
Materials 170
Procedure 172
Results 173
Hyoid bone predictions 173
Kebara hyoid compared to humans and chimpanzees 173
The Kebara hyoid compared to hominin estimated hyoid bone dimensions 178
Discriminant function analysis 184
Comparison of Kebara hyoid with modem human hyoid morphology 185
Comparison with known age sample 185
Qualitative hyoid bone morphology 186
Discussion 189
Conclusions 194
Ch a p t e rs ix Ge n e r a ld i s c u s s i o n 195
Reconstructing earlier hominin vocal tracts 201
A new model o f vocal tract evolution 205
Future work 207
R e f e r e n c e s 209
A p p e n d ix 1 226
A p p e n d ix 2 228
A p p e n d ix 3 232
A p p e n d ix 4 234
A p p e n d ix 5 245
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Figure 1.1 Sagittal section of a human and chimpanzee vocal tract 17
Figure 1,2 The pharyngeal constrictor muscles in humans 19
Figure 1.3 Posterior view of the pharynx 20
Figure 1.4 The shape o f the vocal tract during the production of three quantal vowels 26
Figure 1.5 Model of quantal sounds 28
Figure 1.6 Vocal tract Measurements 34
Figure 1.7 The distance between the uvula and hyoid bone 35
Figure 1.8 The relationship between vocal tract length and body mass 40
Figure 1.9 The relationship between oral cavity length and body mass 41
Figure 1.10 The relationship between tongue length and body mass 42
Figure 1.11 The relationship between tongue length and oral cavity length 43
Figure 1.12 The relationship between oral cavity length and vocal tract length 44
Figure 1.13 The contribution of oral length to vocal tract length 44
Figure 1.14 The relationship between hard palate length and hyoid position 47
Figure 1.15 The relationship between hyoid bone and soft palate with body mass 47
Figure 2.1 The textbook view of the hyoid bone 57
Figure 2.2 The muscle attachment sites on the hyoid bone 58
Figure 2.3 The chimpanzee hyoid bone 61
Figure 2.4 The human and chimpanzee cranial base 64
Figure 2.5 The measuring points on the hyoid bone 70
Figure 2.6 Facial measurements 73
Figure 2.7 Cranial measurements 73
Figure 2.8 Palatal measurements 74
Figure 2.9 Mandibular measurements 74
Figure 2.10 Cranial base lengths 75
Figure 2.11 Cranial base measurements 75
Figure 2.12 Facial prognathism measurements 76
Figure 2.13 The relationship between chronological age and transverse diameter of the hyoid bone 82
Figure 2.14 Range o f variation in greater horn fusion in known age humans 85
Figure 2.15a Human hyoid with fused synchondrosis 84
Figure 2.15b Human hyoid with un fused synchondrosis 84
Figure 2.16 Discriminant function analysis o f hyoid bone measurements 90
Figure 2.17 The overlap between chimpanzees and humans when juveniles are included 91
Figure 2.18 Principal component analysis o f hyoid measurements in all three species 92
Figure 2.19 The relationship between transverse diameter and anterio-posterior thickness 95
Figure 2.20 The relationship between anterio-posterior thickness and the depth of the posterior depression 96
Figure 2.21 The principal component plot after size removed 97
Figure 2.22a Hyoid bone breadth greater at the mid point of greater horns 104
Figure 2.22b The hyoid above is wider at the distal ends of the greater horns 104
Figure 3.1 Diagram of the larynx anterior view 114
Figure 3.2 Superior view o f the larynx 115
Figure 3.3 Coronal section through the larynx 117
Figure 3.4 Laryngeal height measurement 131
Figure 3.5 Laryngeal measurements 131
Figure 3.6 The relationship between laryngeal height and body mass 134
Figure 3.7 The relationship between vocal fold length and body mass 135
Figure 3.9 The relationship between maximum laryngeal diameter and body mass 136
Figure 3.10 The relationship between vocal fold length and anterio-posterior length 137
Figure 3.11 The relationship between laryngeal height and anterio-posterior length 138
Figure 3.12 The relationship between laryngeal height and vocal fold length 139
Figure 4.1 Comparison o f deaths from choking on food with deaths from cancer and infectious diseases 149
Figure 4.2 The death rate from choking on food compared to the death rate from all causes 150
Figure 4.3 Percentage contributed by each age group to the total number of deaths from choking on food 151
Figure 4.4 A comparison of infant death rate for choking on food with infent mortality rate 151
Figure 4.5 The age specific death rate in under 1 year olds for choking on food 152
Figure 4.6 The age specific death rate for children aged 1-4 years from choking on food 153
Figure 4.7 The age specific death rate for children aged 5-14 years wiio choke to death on food 153
Figure 4.8 The age specific death rate for adults 15-49 years who choke to death on food 154
Figure 4.9 The age specific death rate for over 50 year olds who choke to death on food 155
Figure 4.10 The average age specific death rate for those choking on food 157
Figure 4.11 The sex ratio o f choking to death on food for the 100 year period 158
Figure 4.12 Comparison of the decline in breastfeeding with numbers of in Ants choking to death on food 162
Figure 5.1 The Kebara hyoid bone 166
Figure 5.2 The anterio-lateral view o f the domestic pig hyoid 167
Figure 5.3 The Kebara hyoid bone compared to human percentiles 174
Figure 5.4 The Kebara hyoid bone measurements compared to chimpanzee percentiles 174
Figure 5.5 Hyoid measurements predicted using human cranial measurements and chimpanzee equations 175
Figure 5.6 Hyoid bone measurements predicted using chimpanzee cranial measurements and human 176
equations
Figure 5.7 The observed and predicted Kebara hyoid bone measurements compared to human percentiles 178
Figure 5.8 The observed and predicted Kebara hyoid bone measurements compared to chimpanzee 178
percentiles
Figure 5.9 The estimated hyoid bone for P. boisei compared to human percentiles 180
Figure 5.1 OThe estimated hyoid bone for P. boisei compared to chimpanzee percentiles 180
Figure 5.11 The predicted measurements for P. boisei compared to gorilla percentiles 181
Figure 5.12 Estimated hyoid measurements for A. cfricanus compared to human percentiles 182
Figure 5.13 Estimated hyoid measurements for A. africanus compared to chimpanzee percentiles 182
Figure 5.14 Estimated hyoid measurements for Homo sp. compared to human percentiles 183
Figure 5.15 Estimated hyoid measurements for Homo sp. compared to chimpanzee percentiles 184
Figure 5.16 Homo erectus estimated hyoid measurements compared to human percentiles 185
Figure 5.17 Homo erectus estimated hyoid measurements compared to chimpanzee percentiles 185
Figure 5.18 Discriminant function analysis o f hyoid bone measurements 186
Figure 5.19 Transverse diameter compared to chronological age 187
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Table 1.1 Description of vocal tract measurements 33
Table 1.2 Regression analysis components o f the vocal tract 39
Table 1.3 Regression analysis o f hard palate length and hyoid position 45
Table 2.1 Hyoid bone measurements 70
Table 2.2 Description o f scoring o f hyoid bone features 71
Table 2.3 Stages o f fusion of the greater horns to the hyoid body 71
Table 2.4 Cranial, facial and mandibular measurements 72
Table 2.5 Stages of dental development 77
Table 2.6 Sex differences in hyoid bone measurement in the human sample 80
Table 2.7 Correlations between hyoid bone measurements in humans 79
Table 2.8 Partial correlations between hyoid measurements controlling for sex and age group 81
Table 2.9 Correlations of hyoid measurements in the known age sample 82
Table 2.10 Sex differences in the known age and sex sample 83
Table 2.11 Relationship between transverse diameter and fusion or non fusion in the known age sample 84
Table 2.12 Sex differences in hyoid measurements in the chimpanzee 85
Table 2.13 Correlations of hyoid bone measurements in the chimpanzee 86
Table 2.14 Sex differences in hyoid bone measurements in the gorilla 87
Table 2.15 Correlations between hyoid bone measurements in the gorilla 87
Table 2.16 Descriptive statistics of the three species 88
Table 2.17 Anova results for interspecific differences in hyoid bone measurements 89
Table 2.18 Classification discriminant function analysis adult specimens 90
Table 2.19 Correlations of predictor variables with discriminant function for adult specimens 90
Table 2.20 Classification discriminant function analysis all specimens 91
Table 2.21 Correlations of predictor variables with discriminant function for adult and juvenile specimens 91
Table 2.22 Prin cipal component analysis o f all three species 92
Table 2.23 Interspecific correlations of hyoid bone measurements 93
Table 2.24 Interspecific partial correlations between hyoid bone measurements 94
Table 2.25 Regression analysis of hyoid bone measurements 95
Table 2.26 Correlations between the size corrected hyoid measurements 96
Table 2.27 Principal component analysis o f the size corrected measurements 97
Table 2.28 Factor loadings for principal component analysis o f hyoid and cranial measurements 99
Table 2.29 The cranial measurements included in the regression analysis 100
Table 2.30 Multiple regression equations for predicting human hyoid bone dimensions 101
Table 2.31 Multiple regression equations for predicting chimpanzee hyoid bone dimensions 101
Table 3.1 Laryngeal measurements 130
Table 3.2 Regression analysis o f laryngeal measurements 133
Table 4.1 Total number of deaths from choking on food 147
Table 4.2 Sex differences in deaths from choking on food 156
Table 4.3 Differences within sexes between the high and low mortality years 156
Table 4.4 The sex ratio for choking to death on food 157
Table 5.1 Cranial measurements for humans, chimpanzees and fossil hominines 171
Table 5.2 Kebara hyoid measuremaits compared to observed and predicted human and chimpanzee 173
measurements
Table 5.3 The Kebara hyoid bone measurements compared to Neanderthal predicted measurements 176
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c k n o w l e d g e m e n t s
My supervisor Leslie C. Aiello, for her support, advice and encouragement. Simon Strickland, my second supervisor, for general advice and encouragement. The Anatomy Department UCL for allowing me to use their material, and store my specimens in their laboratory. Wendy Birch and Derek Dudley o f the Anatomy Department UCL for preparing specimens and arranging space for working, without their help and advice the dissections would not have been possible. Louise Humphrey and Theya MoUeson at the Natural History Museum, for access to the Spitalfields Collection and much discussion and advice. Andrew Kitchener at the Royal Museum o f Scotland for providing the primate specimens for dissection. John Hartman at the Powell-Cotton Museum, not only for access to the African ape material but for his many kindness’ to me. John Rubin, David Howard and Annette Kelly at the Royal National Throat Nose and Ear Hospital London, not only for allowing access to X-ray material but, answering so many o f my questions and letting me sit in on clinic sessions. In addition, the staff in the X-ray department at this hospital for help in interpreting the X-rays. Helen Chatterjee at The Grant Zoology Museum UCL for access to the Negus sagittal heads. Louise Scheuer, for access to St. Brides and St. Bamabus and for giving me so much support and encouragement.
Yoel Rak for making me a cast o f the Kebara hyoid and answering questions about the bone not in the journal articles. Hartley Odwak for photographing the Kebara hyoid bone. Will Harcourt-Smith for bringing back the cast o f the Kebara hyoid from Israel. Ignatio Martinez for discussions on hyoid bones. Joy Reidenberg gave me much encouragement, advice and long discussions whenever we met. Dan Lieberman and Rob McCarthy for many discussions. Solveg Bulb and Volker Sommer for help with translations. Gwen Hewitt for hours o f discussion. Christophe Soligo, for advice on handling and dissecting frozen specimens. Cathy Key, Mark Collard, Claire Imber, Tina Coast, Helen Chatterjee, Hartley Odwak, Will Harcourt-Smith, Helen Wood, Fred Brett, and the rest o f the Department o f Anthropology at UCL. Barry Alcock, who sparked my interest in Palaeo- anthropology. Also to the many other people who have expressed interest in this project and offered suggestions for ways o f exploring the data.
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b s t r a c t
During the course o f human evolution a major re-organisation o f the upper respiratory tract, or vocal tract, appears to have taken place. This re-organisation is easily observed when humans and other mammals are compared. All mammals, including non human primates, are reported as having larynges positioned high in the neck, with the epiglottis and soft palate in close approximation. Human adults, on the other hand, have a low laryngeal position with the epiglottis and soft palate widely separated.
This thesis sets out to investigate inter and intra specific variation in, and relationships between, various hard and soft tissue features o f the vocal tract in humans (n=20), great apes (n=3) and several other non-human primates (n=18), comprising 13 Macaca mulatta, 2 Colobus guereza, 1 Hylobates muelleri, 1 Leontopithicus rosalia and
1 Hapalemur sp. The main aim o f the thesis is to gain a better understanding o f how the shape and size o f the human vocal tract correlates with speech abilities and when these abilities evolved during human evolution. This thesis also aims to extend our knowledge o f the anatomy and morphology o f the larynx, vocal tract and hyoid bone. The
morphology and ossification o f the hyoid bone are little documented. This thesis attempts to rectify this by a full analysis o f the morphology and ossification o f human (n=l 15) and Afi'ican ape (n=90) hyoid bones. The functional disadvantage o f a low larynx is explored through analysis o f historic demographic data on mortality fi-om choking on food in England and Wales over a period of 100 years.
This thesis questions commonly held assumptions regarding the morphology of the vocal tract and the possible selection pressures resulting in the descent o f the human larynx. This thesis offers no support for the hypothesis that the ability to produce speech sounds was the main selection pressure for a low larynx. A more parsimonious
explanation is that the lower larynx and consequent longer vocal tract in humans is a response to changes elsewhere in the cranium. The evidence fi'om mortality statistics shows that choking to death on food is a rare event. Almost all cases appear to be
I
n t r o d u c t i o n
Since earliest times, we have been fascinated by why humans alone o f all animals have the ability to speak. With the discovery o f fossil hominins, people became interested in how and when language evolved. Many theories o f language evolution have been proposed. Most modem theories for the evolution o f language rely on the anatoniical differences between humans and other mammals that are associated with the ability to produce the range o f sounds o f human speech. The proponents o f these theories accept one major premise, that during the course o f human evolution a major re-organisation o f the upper respiratory tract, or vocal tract, appears to have taken place. This re-organisation is easily observed when humans and other mammals are compared. All mammals, including non-human primates, are reported as having larynges positioned high in the neck, with the epiglottis and soft palate in close approximation. Human adults, on the other hand, have a low laryngeal position with the epiglottis and soft palate widely separated.
The descent o f the human larynx is age related, with the first re-positioning occurring at about two years of age (Westhorpe, 1987). It has been supposed that this descent sets humans apart and is proposed as essential for producing the sounds o f speech. However, ontogenetic studies o f macaques (Fliigel and Rohen 1991) have shown that they too have an age related descent o f the larynx in relation to the cervical vertebrae, similar to that seen in human children. This descent has also been proposed for the chimpanzee (Avril, 1963; Jordan, 1971). It would seem that our re-organised vocal tract with its low laryngeal position is not a unique feature of humans, but part o f a trend in primates to adjust laryngeal position with age, possibly to maintain the relationship o f the hyoid bone to the mandible. Humans can therefore be regarded as having extended this propensity for laryngeal descent. A similar trend is shown in increased brain size in human and non-human primates (Stephan et al.
1970).
small sample sizes, most usually n=l. It is therefore not surprising that these disagreements exist. It seems most probable that the differences documented by researchers are actually intraspecific variation. Studies that include larger numbers o f animals show that there is a range o f variation similar to that found in modem humans.
The first three chapters o f this thesis are concerned with the anatomy o f the vocal tract, hyoid bone and larynx. These chapters aim to extend our knowledge o f the anatomy and morphology o f the larynx, vocal tract and hyoid bone. It is hoped to answer several questions regarding our assumptions o f vocal tract re-organisation, inter and intraspecific variation in hyoid bone morphology, and muscle attachments. It is widely assumed that the human vocal tract has lengthened in response to a shortening of the oral cavity. The increase in length is seen as essential to maintain the integrity o f the sounds produced. So we may therefore ask if the re-organisation o f the vocal tract does produce a shorter than expected oral cavity. The muscle attachment sites on the hyoid bone and cranial base are seen as determining the position o f the hyoid and larynx in mammals. Are the differences between humans and non human primates sufficient for this to be correct? The hyoid bone has a pivotal role in laryngeal and tongue movements. It is therefore important in the production o f speech. Is it possible to distinguish human from non-human primate hyoid bones?
It has been proposed that because o f our low laryngeal position our method of swallowing differs from that o f other mammals. In humans all food passes across the laryngeal opening, therefore our low larynx is seen as compromising our ability to swallow safely. This view is widely accepted and regarded as a common cause o f death. If we were at risk from choking to death on the food we eat then it would be expected that infant deaths would be low and that deaths would be highest at times o f laryngeal descent. Chapter 4 o f this thesis analyses mortality statistics for a 100 year period in England and Wales to determine the death rates from choking on food for different age ranges o f modem humans.
Reconstructions o f vocal tracts are problematic. The soft tissue o f the vocal tract does not fossilise. It is true that laryngeal cartilages ossify or more properly calcify with age, but these structures are fi-agile and unlikely to be preserved in the fossil record. The finding o f a hyoid bone is rare. This is not as popularly thought because this bone is fi’agile but because the morphology and ossification are little documented and often unknown to archaeologists. This thesis attempts to rectify this by a full analysis o f the morphology and ossification o f the human and Afi’ican ape hyoid bones.
This lack o f fossil evidence prevents us knowing the size and shape o f earlier hominin vocal tracts or the position o f the larynx and hyoid bone. Therefore, we are reliant on the associated hard tissue to provide markers for the size and shape o f the vocal tract as well as the position o f the structures within it. This thesis sets out to determine the relationship between the vocal tract and its related structures to the honey landmarks o f the cranial base, mandible and hyoid bone. This will be attempted through a re-evaluation o f the hypotheses o f Lieberman and Laitman:
1. Cranial base flexion and length are important in determining laryngeal position. 2. The selection pressure for speech ability is so strong that it can produce a functional
disadvantage, through compromising our ability to swallow safely.
3. The length o f the vocal tract is important for the range o f sounds produced. 4. The hyoid bone alone can tell us nothing about its anatomical position.
5. Analysis o f the hyoid bone can not distinguish the human hyoid fiom those o f other mammals.
The reconstrutions o f Lieberman and his co-workers are a two-stage process. The first stage relies on information regarding length and position o f the vocal tract based on
anatomical markers. The second uses the reconstructed vocal tract to model the type of sounds produced by the reconstructed vocal tract. This thesis proposes a model for the
reconstruction o f the vocal tract o f a variety o f earlier hominins, as well as inferring the hyoid bone morphology. The sounds produced will not be directly modelled but information on mammalian sound production in living animals will be used to determine the range o f sounds possible.
To understand the evolution o f the human vocal tract requires a firm understanding o f the anatomy o f the upper respiratory tract (vocal tract) in both human and non-human
construct a model vocal tract, which is used to generate the sounds o f speech that could be made by that particular hominin. If the proposed vocal tract is incorrect, or if a theory that only partly explains human speech is used then the model is likely to be inaccurate.
This thesis sets out to investigate inter and intra specific variation in, and relationships between, various hard and soft tissue features of the vocal tract in humans, African apes and several other non-human primates. The main aim o f the thesis is to gain a better
understanding o f how the shape and size o f the human vocal tract correlates with speech abilities and when these abilities evolved during human evolution.
This thesis is set out in six chapters. The first five chapters have an appropriate literature review, a materials and methods section, a description o f the results, and a discussion o f the results drawing appropriate conclusions. Each o f these five chapters has specific aims. These aims are set out in a subsection o f the literature review. The sixth
chapter is a general discussion chapter drawing together the findings o f this thesis and setting them in context and proposes a model o f vocal tract evolution.
• Chapter 1 : The role o f the vocal tract in sound production is explored. This chapter then explores the inter and intra specific variation in vocal tract dimensions among primates. The relationship o f vocal tract dimensions to body mass is also determined. The anatomy o f the vocal tract is described from dissection o f a range o f primate species. These
include, tamarin, lemur, macaque, colobus monkey, gibbon, chimpanzee, orang-utan, gorilla and modem humans.
• Chapter 2: This chapter aims to define hyoid bone dimensions and their variability across a range o f primate species, particularly living African apes and modem humans. The hyoid bone measurements were based on those used by Arensberg et a l (1989), but were redefined for greater accuracy and clarity.
• Chapter 3: This chapter aims to explore the inter and intra specific metric variation in laryngeal cartilage dimensions. The relationship o f these variables to body mass is also determined. It also examines the variation in intrinsic musculature o f the primate larynx. The role o f the larynx in respiration and sound production is described. The species included are those used in chapter 1.
tested through analysis o f historic demographic data on mortality from choking on food in England and Wales over a period o f 100 years. The methods used are standard
epidemiological and demographic techniques.
• Chapter 5: This chapter aims to tests the hypothesis that the Kebara hyoid bone is human like in its morphology. The results o f chapter 2 are used to determine the grouping that the metric measurements o f the Kebara 2 hyoid bone fall into using discrimanant function analysis. The results from chapter 2 are also used to make predictions based on regression equations taken from the regression analysis o f the cranial measurements outlined in chapter 2. These predictions o f hyoid bone dimensions are based on the La Chapelle-aux- Saints Neanderthal cranium, an average Neanderthal cranium, as well as predictions based on P. boisei, A. africanus, Homo sp. and Homo erectus. These predicted hyoid bone dimensions are compared to the actual Kebara 2 hyoid dimensions and to human and chimpanzee hyoid bone percentiles to determine the degree o f accuracy o f the equations.
• Chapter 6: This chapter draws together and discusses the results from the earlier chapters. An evaluation o f the hypotheses tested will be made. The limitations o f the study will be considered. The implications are discussed in light o f previous researchers' work. A model o f vocal tract evolution will be proposed.
This thesis sets out to test four hypotheses:
1. The ability to produce speech sounds is the main selection pressure for a low larynx. 2. Our low larynx confers a functional disadvantage in the form o f choking to death on the
food we eat.
3. The Kebara hyoid bone is human-like in its morphology.
4. The length o f the cranial base is the most important factor in determining the position o f the larynx.
This thesis concentrates on the anatomical aspects o f the vocal tract and its
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The vocal tract is the most common term applied to the upper respiratory or supralaryngeal tract. It is essentially a tube stretching from the larynx to the lips. At the larynx, the tube branches to form the trachea, which sits below the larynx and allows air into the lungs, and the oesophagus, which lies posterior to the larynx. The oesophagus is the route all food takes to enter the stomach and thence the digestive system. All food ingested must therefore pass over, or in the case o f non-human primates is considered to pass round, the larynx (Lieberman et al. 1971,1972,1985; Wind, 1976; Laitman et al. 1978, 1982; Laitman & Reidenberg, 1993; Harrison, 1995). The vocal tract therefore has three functions, respiration, ingestion and vocalisation. The term vocal tract is used because we attach so much importance to the third function o f this region, vocalisation. Although all primates have similarities in the organisation, structures and muscles o f the vocal tract one major difference found in humans is regarded as unique by many
researchers. In humans, the larynx is positioned low in the neck, rather than immediately beneath the soft palate. This configuration is thought to be the reason that humans alone can make the sounds o f human speech (Lieberman et al. 1971,1972,1985). This ability is not the same as having language, although the two terms are often used interchangeably in the literature. This study is concerned with the anatomical aspects o f speech rather than language. Speech is the conversion o f language into sound (Borden, Harris & Raphael, 1994). Definitions o f linguistic terms including speech, language and vocalisation can be found in appendix 1. Our vocal abilities have developed from structures originally adapted for the functions o f respiration and ingestion, rather than evolving special sound
production units such as seen in songbirds (Negus, 1949; Kirchner, 1993; Fitch, 1997).
The basic anatomy o f the vocal tract described here will be based on human anatomy. Where differences exist between humans and one or more non-human primate species they will be detailed.
The shape and size o f the vocal tract is determined by the three cavities o f which it is composed, the oral cavity, the nasal cavity and the pharynx. Its shape is also thought to be determined by the angulation o f the cranial base (Dubrul & Laskin, 1961 ; Reidenberg & Laitman, 1991).
De s c r i p t i o n o f Vo c a l t r a c t a n a t o m y
Th en a s a la n d o r a l c av i t i e s
In humans, the boundary between the oral cavity and the pharynx is arbitrary (Berkovitz & Moxham, 1988). This is because the epiglottis and soft palate are no longer in contact with each other (Fig. 1.1). For this reason, adult humans do not breathe when they swallow, whereas all other primates are considered able to breathe and swallow at the same time (but see chapter 4 for a review o f the literature on swallowing and breathing). Non-human primates on the other hand have retained the contact between the soft palate and epiglottis and therefore have an oral cavity discrete from the pharynx (Fig. 1.1). In humans however, the oral cavity can be considered to extend as far as the pillars o f the fauces (Berkovitz & Moxham, 1988). The pharynx can be regarded as beginning inline with the anterior margin o f the mandibular ramus.
forming the anterior wall o f the pharynx. Non-human primates are generally considered to have their tongue wholly within the oral cavity (Lieberman, 1971, 1972, 1992; Falk 1975). However, Negus (1949), Harrison (1995), Swindler and Wood (1973) all show a small portion o f the tongue as part o f the anterior wall o f the pharynx in great apes. This difference in tongue position may be due to the older age o f the apes used by Negus (1949), Swindler and Wood (1973) and Harrison (1995). Avril (1963) and Jordan (1971) both speculated that the position o f vocal tract structures varied with age. Fliigel and Rohen (1991) have shown that an age related descent o f vocal tract structures occurs in macaques. Therefore, age is an important factor in determining the position o f vocal tract structures in both human and non-human primates.
The tongue is attached to both the mandible and hyoid bone by ligaments and muscles, including the mylohyoid muscle that forms the floor o f the mouth (Falk, 1975; HoUinshead, 1982; Berko vitz & Moxham 1988; Houghton, 1993; Arensburg et al.\99QÎ). The anterior portion o f the mylohyoid muscle is attached to the mandible along the mylohyoid line. The central posterior portion o f the mylohyoid muscle is attached to the hyoid bone, the rest o f the posterior portion is unattached. The other muscles attached to the mandible and tongue are described in the section on the hyoid bone later in chapter 2.
The nasal cavity is continuous posteriorly with the nasopharynx (HoUinshead, 1982). The floor of the nasal cavity is the hard palate (Berkovitz & Moxham, 1988). The nasal floor is therefore essentiaUy horizontal and flat. Both humans and non-human
primates can close off the nasal cavity from the vocal tract, particularly during swaUowing (Giest, 1933; Hiiemae & Crompton, 1985). The abiUty to open and close the nasal cavity is used in human speech. The role played by different parts o f the vocal tract in the
production of speech sounds is described in appendix 2.
Th e PHARYNX
The pharynx extends superiorly in humans from the laryngeal inlet to the posterior o f the mouth and nose. This is iUustrated in figure 1.1. The pharynx is usuaUy described in three sections: The laryngopharynx, which extends from the larynx to the base o f the tongue, the oropharynx, from the base o f the tongue to the soft palate and the
position of the larynx (Laitman & Reidenberg, 1993). The posterior wall is muscular and has two groups of muscles, the constrictors and the longitudinal muscles, that attach to the larynx.
N asal Cavity
N asal Cavity S oft P a la te
H ard P a la te H ard P a la te
T o n g u e
S o ft P a la te P h a ry n x
T o n g u e
Hyoid B o n e
Epiglottis Epiglottis
Larynx
Vocal Fold Vocal Fold
T ra ch e a
O e s o p h a g u s O e s o p h a g u s
Figure 1.1 Sagittal section o f the chim panzee (A ) and human (B ) vocal tract. The diagram show s the position and main features o f the vocal tract. By perm ission JT Laitman
Th e \ a s o p h a r y n x
The nasopharynx is adjacent to the sphenoid and occipital bones and contains the pharyngeal tonsils (adenoids). The primary function of the nasopharynx is respiration (Morris, 1988). The basicranium forms both the roof and posterior wall o f the
nasopharynx. The nasopharyngeal floor is the superior surface of the soft palate (Berkovitz & Moxham, 1988). The nasopharynx is rigid, contributing to the patent airway, only the nasopharyngeal floor is moveable (Berkovitz & Moxham, 1988). This is formed by the soft palate, which can be raised and lowered to close or open the nasopharynx.
The Eustachian or auditory tube enters the nasopharynx on the lateral walls, linking the ear and the pharynx. The cartilaginous end of the Eustachian tube ends in the tubal elevation, two rounded prominences on the lateral walls o f the nasopharynx (Berkovitz & Moxham, 1988). The Eustachian tube gives attachment to the palatine muscles, the levator veli palatini and the tensor veli palatini. The former arises fi*om the auditory tube and the petrous part of the temporal bone. The latter arises Ifom the scaphoid fossa and the
cartilage of the auditory tube. These muscles pass between the pterygoid plates and turn at a right angle round the hamulus and insert on the lateral side of the soft palate. The origin of these muscles is more medial in the great apes (Dean 1985) arising Ifom the temporal bone at the Eustachian process. Therefore, the human nasopharynx is wider than that of
the apes. In great apes the palatine muscles run almost parallel to the basicranium (Dean 1985). In macaques, the levator veli palatini arises from the petrous part o f the temporal bone near the auditory tube (Geist, 1933). The tensor veli palatini arises on the sphenoid and cartilaginous part o f the auditory tube. The origin in the macaque is therefore more like that in humans than in the great apes (Geist, 1933). However, these muscles insert more dorsally on the palate than in humans. In macaques, therefore the nasopharynx is slightly wider than in the great apes, more closely resembling humans (Geist 1933). The levator veli palatini and the tensor veli palatini control the movements o f the soft palate. This is true for both humans and non-human primates.
Th e OROPHARYNX
The oropharynx is inferior to the soft palate and forms the middle section o f the pharynx (Berkovitz & Moxham, 1988). In humans, it is continuous with the oral cavity. In mammals, including non-human primates, it is separated from the oral cavity by the
continuity o f the soft plate and epiglottis (Lieberman et a l 1971, 1972, 1992; Negus, 1949; Harrison, 1995). In humans the anterior wall o f the oropharynx is the pharyngeal portion o f the tongue (HoUinshead, 1982). In mammals, including non-human primates, the anterior wall is the epiglottis. The posterior wall o f the oropharynx and nasopharynx are continuous in all mammals. The inferior border o f the oropharynx is the root o f the tongue and the superior is the tip o f the epiglottis. It is continuous with the
laryngopharynx. The lateral walls o f the oropharynx are formed by two prominent folds (the pillars o f fauces) The anterior fold or palatoglossal arch, is formed by the
palatoglossal muscle. It is often regarded as the junction between the oral cavity and oropharynx in humans (Berkovitz & Moxham, 1988). The posterior fold contains the palatopharyngeus muscle, which passes from the soft palate to the pharyngeal wall.
Between the pillars o f fauces are the palatine tonsils. The pillars are often referred to as the tonsiller pillars.
THE LARYNGOPHARYNX
THE P HA R YNG E AL WALL
The pharyngeal wall comprises six pairs of muscles. They can be divided into two groups. One group comprises three pairs of constrictor muscles that run transversely across the pharynx. The other group is composed o f three pairs o f muscles that run longitudinally (Berkovitz & Moxham, 1988).
The superior constrictor muscle arises, and the inferior and middle constrictors insert on the sides of the head and neck. All three constrictors insert posteriorly into the pharyngeal raphe, which joins the paired muscles from each side of the pharynx
(HoUinshead, 1982) (Fig 1.2). The constrictor muscles form an overlapping series of muscles, the superior, middle and inferior constrictors (HoUinshead, 1982). The constrictor muscles generate a wave of contractions that carry the food bolus into the oesophagus (Berkovitz & Moxham, 1988).
pharyngobasilar fascia pharyngeal tubercle
y t
superior \ constrictor
middle constrictor
V" inferior
/ constrictor
pharyngeal
/ raphe
oesophagus
Figure 1.2 The pharyngeal constrietors m useles in humans
The superior constrictor is a quadrilaterally shaped muscle. It arises from the hamulus of the medial pterygoid process (Fig 1.2). In humans, it attaches to the skull only in one place, the pharyngeal tubercle. In great apes, however, the superior constrictor attaches to the periphery o f the roof o f the nasopharynx (Dean, 1985). The superior constrictor meets the buccinator muscle in a raphe or it may be continuous with this muscle (HoUinshead, 1982).
The middle constrictor is a fan shaped muscle (HoUinshead, 1982). It arises from the greater and lesser horns of the hyoid bone. The middle constrictor overlaps the superior constrictor posteriorly (HoUinshead, 1982).
The inferior constrictor arises on the lateral surface o f the thyroid cartilage and the posterior portion of the cricoid cartilage (Berkovitz & Moxham, 1988; HoUinshead, 1982). It therefore has two parts, the thryopharyngeus and the cricopharyngeus. Superiorly the inferior constrictor overlaps the middle constrictor. Inferiorly, this muscle joins the circular muscles of the oesophagus (HoUinshead, 1982).
The longitudinal muscles of the pharynx attach to the larynx. They elevate the pharynx and larynx during swaUowing. The longitudinal muscles comprise the
palatopharyngeus muscle, the salpinopharyngeal muscle and the stylopharyngeus muscle (Fig. 1.3).
levator veli palatini . . .
cartilage of auditory tube
tensor veli
\ y t palatini
^
J
V I , ./4 V , r ; / X hamulus
, f •
; ' " Stylo
superior
constrictor -'^..palato
• ‘ pharyngeus
uvula ' ÿ V ^ ' salpingo
\
' pharyngeusiV.
: I
Figure 1.3 Posterior view o f the phar>Tix show ing the longitudinal m uscles
muscles. It inserts on the thyroid cartilage. The palatopharyngeus muscle elevates the larynx and the lateral edge o f the tongue. The inferior portion o f the palatopharyngeus muscle may also depress the soft palate.
The salpinopharyngeus muscle arises from the cartilaginous portion o f the Eustachian tube (HoUinshead, 1982). It produces the salpinopharyngeus fold. It then merges with the palatopharyngeus muscle (Fig. 1.3). Berkovitz and Moxham (1988) state that it elevates the pharynx during swaUowing. However, according to HoUinshead (1982) although the elevation o f the pharynx is the usuaUy given as the role o f the
salpinopharyngeal muscle, it is often poorly developed or even absent in humans. Some researchers, (for example, de Paula Assis, 1947) suggest that it is actuaUy part o f the palatopharyngeus muscle. It seems Ukely that variation 'within humans gives rise to the disagreement in whether there are two muscles or two parts o f a single muscle.
The stylopharyngeus muscle arises on the medial surface o f the styloid process of the temporal bone (Berkovitz & Moxham, 1988). It passes inferiorly and mediaUy between the external and internal carotids. The stylopharyngeus muscle enters the pharynx between the superior and middle constrictors (HoUinshead, 1982). It inserts on both the inferior constrictor and the thyroid cartUage (Berkovitz & Moxham, 1988). It also elevates the pharynx and larynx.
Th e Ba s i c r a n i u m
Many o f the muscles o f the pharynx, including those attached to the hyoid bone, as weU as the muscles o f the soft palate, have their origin on the cranial base (Dean &
Pegington, 1996; Berkovitz & Moxham, 1988). Much o f the neck sits below the cranial base, from the entrance o f the foramen magnum to the muscles that attach the mandible to the hyoid bone. Immediately anterior to the spinal column are the three muscles that form the posterior and lateral waUs o f the pharynx, the superior, posterior and middle
constrictor muscles. These muscles in humans insert into a median raphe, which descends from the pharyngeal tubercle. In the great apes, the pharyngeal constrictor muscles insert over a much wider area than in modem humans (AieUo & Dean 1990). The whole o f the superior margin o f the superior constrictor attaches to the edge o f the nasopharynx rather than to the pharyngeal tubercle (Dean 1985). Previously, Giest (1933) showed the
bone in close association with the pharyngeal wall. The stylohyoid muscle takes origin on the styloid process and inserts on the lessor horns o f the hyoid bone. The stylohyoid muscle assists in pulling the hyoid bone upwards and backwards during swallowing. In the chimpanzee the styloid process is often just a small pit and the stylohyoid muscle attaches to this pit and descends down to attach on to the lesser horns o f the hyoid bone
(Zucherman, 1962). The gorilla has the same attachment o f the stylohyoid muscle as found in humans (Raven, 1950). In macaques, although it arises fi'om a very short styloid
process, the attachment on the hyoid bone is to both the greater and lesser horns, probably due to the smaller size o f the hyoid bone (Geist, 1933).
The posterior belly o f the digastric muscle has its origin fi'om the mastoid or digastric notch, on the medial side o f the mastoid process. Although studies show that in humans the digastric is always attached to the digastric notch, Mckee and Helman (1991) show some variability in the area over which origin extends. In almost half their dissections the digastric was also attached to the medial side o f the mastoid process, in one case this extended to include the anterior surface o f the process. Several individuals had the muscle attached to the juxamastoid eminence, rather than attaching by the more usual tendon. McKee and Helman (1991) suggest that this was determined by the size o f the muscle and by the size o f the mastoid process as well as the juxamastoid eminence. In humans the digastric leaves a deep groove on the cranial base, its wider origin in the great apes leaves no bony landmarks (Dean 1984). Dean proposes that the change in position in humans caused by the anterior position o f the foramen magnum forced the muscle into a deep groove. Although McKee and Helman (1991) found that the digastric muscle could take origin over a wider area than previously thought, in all cases humans had a digastric notch that was the primary site o f origin.
The tympanic plate, which is in fact part o f the temporal bone, gives origin to the levator palatini muscles. The tensor palatini are positioned laterally to the sphenoid process. These muscles control movements o f the soft palate. The narrower nasopharynx o f the great apes places both the levator and tensor palatini muscles on the Eustachian process (Dean, 1984).
Vo c a l t r a c t l e n g t h a n d s h a p e
through the vocal folds is described in the section on vocalisation in chapter 3. Lieberman and co-workers have had a great influence on theories o f language evolution, based on their reconstructions o f earlier hominin vocal tracts. Their work in this context is discussed in chapter 6.
Th e o r i e s o fs p e e c hp r o d u c t i o na n dp e r c e p t i o n
SOURCE FILTER THEORY
First proposed by Müller (1848), source filter theory was extended by Fant (1960). The basic concept is still very similar to that o f Müller (1848). The vibration o f the vocal folds is the source; the vocal tract is viewed as an acoustic tube that, depending on length, filters the source in different ways. The source generates a fundamental frequency and the vocal tract acts as a time varying filter, which suppresses the passage o f sound energy at certain frequencies while allowing the passage o f other frequencies. Formants are those frequencies at which the vocal tract sustains sound energy. These formant frequencies are determined, in part, by the overall shape, length and volume o f the vocal tract. The detailed shape o f the filter function is determined by the whole vocal tract serving as a resonance system.
The theory can best be illustrated for vowel sounds, which are produced without the obstructions in the vocal tract that characterise voiced consonants. The linguistic alphabetic notation is used for the four vowels described in this section. The full notation and a description o f the position in the vocal tract where the sound is produced can be found in appendix 2
The model produced is based on a male vocal tract o f about 17.5cms in length, called the idealised vocal tract. The tube is open at one end as in the human vocal tract. A tube that vibrates at one end and is open at the other will have frequencies at which it prefers to vibrate. The tube will resonate with maximal amplitude a sound whose wavelength is 4 times the length o f the tube. To calculate the resonating frequencies o f such a tube the speed o f sound (35,000 cms per second) is divided by 4 times the length o f the tube.
times higher than the first, the next is 5 times higher and so on. Any wavelengths at different fi*equencies will be filtered out by the vocal tract, as they will not resonate in a tube o f this length.
When such a tube filters the laryngeal tone, the resulting resonant peaks will be imposed on the acoustic spectrum o f the tone. This can be modelled either mechanically with an appropriate sound source and an open tube or simulated by computer. The artificially generated spectrum is similar to the spoken one and sounds like the vowel [9].
Because the resonance fi-equencies o f the vocal tract are so important for
recognising vowels, they are called formants. The first formant for [9] is 500 H the second is 1,500 H, the third 2,500 H. It is the formant fi-equencies not the fundamental fi'equencies that are essential for intelligible speech.
Changing the shape o f the vocal tract, to make it resonate at different fi-equencies produces other vowels. Moving the tongue down and back in the mouth produces the vowel [a] as in the English word ‘father’. This enlarges the oral cavity and constricts the pharynx. The general effect can be approximated by a double tube that is narrow at the source end and wide at the open end (Fig. 1.4). The vowel [i], which occurs in ‘feet’, is produced by the opposite arrangement. The tongue moves forward and up enlarging the pharynx and constricting the mouth (Fig. 1.4). The vowel [u] as in ‘boot’ is also produced using a high tongue position but the tongue is drawn back with the open end o f the vocal tract reduced by constricted lips. This partial closure lowers the resonant fi-equencies (Fig.
1.4). These alterations in shape produce the two tube vocal tract.
The physiological, acoustic and perceptual features o f all English vowels could be produced in this way. However, these four vowels are particularly important. The vowel [9] can be thought o f as the neutral vowel or point o f origin. This sound is produced when nothing alters the resting position of the vocal tract (however, see chapters 2 and 3 for a discussion o f resting position). The other three vowels are the most extreme variations o f this neutral position that can be produced by moving the tongue and lips. All other English vowels are intermediate between these extremes. Although the number o f vowels differs widely across languages, all seem to have at least two o f these extreme vowels (Pickett
/a/ 'ah'
/i/ 'e e '
/u/ 'oo'
1000 2000 3000 *000
1000 2000 3000 4000
ENERGY
1000 2000 3000 *000
FREQUENCY (HERTZ)
Figure 1.4 The shape o f the vocal tract during the production o f three quanta! vow els
The main problem with this model is that it uses an idealised vocal apparatus, which corresponds to a notional male speaker. Laver (1991) states that the human female and child are not just scaled down versions of the male, but that there is a different length ratio of pharynx to oral cavity. Laver (1991) says “we know less than is useful about the adult female and child". More recently, the female vocal tract has begun to be studied (e.g. Klatt & Klatt, 1990; Hanson & Chang, 1999), but source filter theory is still based on the original male model.
The concentration on an idealised vocal tract has left us relatively ignorant about acoustic differences between speakers based on anatomy rather than accent (Laver, 1991). However, humans have no difficulty dealing with this variation. The standard model is merely a convenience. It has no implications of normality or any suggestion of a representative resting position (Laver, 1991).
Qu a n t a l t h e o r y
Stevens (1972,1989) developed quantal theory. The theory proposes that vocal tract configuration and acoustic output have a non-linear relationship that favours the
establishment o f sound categories. That is, as the shape o f the vocal tract changes continuously, the resulting change in sound may be either large or small. The auditory system is predisposed to respond to these quantal changes. The auditory system responds with a similar pattern o f change and stability to steadily changing acoustic parameters. Stevens (1972, 1989) proposes that the range o f sounds in each language is chosen from these regions of acoustic stability.
The theory is Stevens’ (1972, 1989) response to two questions. First, why are some sounds so common in most languages and other sounds found in only a few? Second, why do a large number o f sounds resolve into a small number o f classes in our minds?
Stevens (1972) points out that acoustic theory predicts that for a given articulatory parameter, there will be some regions in which a small articulatory change leads to a significant change in acoustic output. In others, an equally small change produces no noticeable difference (Fig. 1.5). The most important point is that the curve is non-linear. It is probably best thought o f as a ‘quantal jump’ from one relatively stable state to another. Anywhere in I there will be little change but as one moves along the acoustic parameter into II, there will be large changes in acoustic output. Once past II and into III the acoustic output is again relatively insensitive to change. Stevens (1972, 1989) suggests that I and III are regions o f stability and form the basis o f the range o f sounds used in languages, forming sound contrasts in the acoustic space. The regions o f stability allow some
flexibility in articulation (thus allowing for individual variation) while giving the listener a relatively stable signal.
Quantal theory is a simple and elegant explanation for the production o f speech sounds. However, its very simplicity has been criticised as oversimplifying a complex system. Stevens, has deliberately kept the theory simple to allow study o f the parts o f the articulatory-auditory relationships in which he is most interested. This therefore is
is not a stable vowel in quantal terms because its formants are widely spaced, so it should be rare, but is quite common.
œ 0)
E
CO CTJ Q.
O
<
An articulartary parameter
Figure 1.5 Model o f quanta! sounds after Stevens (1972)
Lim it a n o N S o f l i n g u i s t i cm o d e l s
Both commonly used theories have limitations. The most important is that they describe only the individual sounds not the fluent, rapid flow of speech. Context and the shared knowledge of the speaker and listener are as important as the sounds themselves. Speech sounds are themselves modified by the sounds that surround them (Borden et al. 1994; Pickett, 1999). It has been proposed that the discrete sounds o f speech are in fact linguistic constructs, defined by linguists to study language (Port, 1996b). Traditionally, production and perception of speech has been described by linguists as having the following properties:
• Human speech is heard as discrete sound categories called phones (see appendix 1). • Each phone is a simultaneous combination of phonetic features
• Each phonetic feature is static with a simple articulatary and auditory specification • There is a closed set of such features for human speech
• Individual languages employ subsets of this universal set
• Children learning their native language employ these units to organise speech perception and the grammar o f the language.
The traditional approach is based on three assumptions. First, that the limited perceptual resolution o f humans forces a limit on the number o f distinctions and thus supports discrete categories (Watson, 1987). Second, the phenomenon o f categorical perceptions is well tested and suggests that distinct boundaries are necessary to speech perception (Lieberman et a l 1967; Hamad, 1987, Repp, 1984). Finally, quantal theory shows that discreteness is natural for many phonetic categories (Stevens, 1972, 1989).
Port (1996b) suggests that this discreteness is actually produced by the method o f testing perception. The standard theory of linguistic phonetics requires that all phonetic contrasts be presented discretely. Most commonly presented are vowels. Yet, categorical perception is weakest for vowels. Indeed, according to Port (1996b) categorical
perception o f vowels can only be obtained under special conditions. Port and co-workers (1996a) did not ask the participants in their experiment to identify discrete sounds, but to identify which word was said. The listener therefore used all the acoustic evidence to decide what was said, a task closer to the perception o f speech. They propose that speech sounds are a continuum rather than discrete sounds. This is intuitively plausible as speech is produced on a continuous flow o f air, see chapter three. That sounds are fluctuations within this stream is not a new idea. It has been shown for over fifty years that speech sounds distribute themselves smoothly over a wide range o f variables (Pickett, 1999). Port (1996b) stresses that speech does contain some discrete ‘sound objects’, for example, between articulation and voicing, but that the use o f phonetics alone will not explain them.
Ex p l a n a t i o n s o f v o c a l t r a c t e n l a r g e m e n t
The human vocal tract bends sharply at right angles to the cranial base. The right angle is formed by the cranio vertebral junction. This is the junction between the upper pharynx and spine with the cranial base. In humans, the spine sits directly below the cranial base forming a right angle. This is a distinctive feature o f humans. All other mammals have an elongated cranial base against which the oropharynx is only slightly bent. This
difference is considered by some researchers as the main reason for the low position o f the human larynx (DuBml & Laskin, 1961). Although, non-human primates have a more angled vocal tract than other mammals, probably relating to their more upright posture.
almost no supralaryngeal portion o f the pharynx (Negus, 1949; Harrison, 1995: Lieberman et al. 1992). Few studies however, exist to show the comparative lengths o f the vocal tract in non-human primate species. Much of the more recent work on the vocal tract is
concerned primarily with the relationship between vocal tract length and formant frequencies. It has also been suggested that reduction in the oral cavity necessitated the descent o f the larynx, to maintain recognition call integrity. It has also been proposed that the reduction in facial prognathism drove laryngeal descent during human evolution (Owren, 1996).
Th e p r e s e n t s t u d y
Most studies o f the vocal tract have looked at components o f the system rather than the system as a whole. The relationship o f the pharynx to the major muscles attaching it to the cranial base will influence the width and length o f the vocal tract. The size o f the oral cavity and position o f the larynx as well as the placement o f the pharyngeal muscles is likely to determine the length o f the tongue. The relationship between the different
components must be investigated as a whole if the evolution o f earlier hominin vocal tracts is to be understood. This part o f the study o f the vocal tract is concerned with a general overview o f the comparative anatomy o f the vocal tract. The hyoid bone and larynx are considered separately. Dissections, x-rays, and skeletal material are used to examine four main areas. First, the muscle attachment sites o f the main pharyngeal, oral and hyoid muscles are determined and described in humans and in non-human primates, ranging from lemurs to chimpanzees. Then the oral length across humans and a wide range o f non human primates was measured and comparisons made. The length o f the tongue was measured in both humans and non-human primates. The length o f the vocal tract and the relationship between the component parts was measured and compared across species. Finally, the relationship o f the vocal tract components to the surrounding hard tissue was investigated. All comparisons also take account o f the animals’ bodyweight.
The main aims o f the present chapter are:
• To compare the variation in vocal tract muscle attachments between the lemur, colobus monkey, macaque, gibbon, tamarin and human.
• To determine if the human vocal tract has a shorter oral cavity than that found in non human primates.
• To determine the relationship between oral cavity length and tongue length in human and non-human primates.
M
e t h o d sM a t e r i a l s
The specimens used in this section include: twenty adult humans, (eight living people measured from x-rays in the Department o f Laryngology, University College London at the Royal National Throat Nose and Ear Hospital and twelve cadavers from the Anatomy Department University College London); thirteen Macaca mulatta, from the Department o f Anatomy University College London; and three great apes, one Pan troglodytes, one Gorilla gorilla, one Pongo pygmaeus, are the specimens prepared by Negus for his 1949 study o f the comparative anatomy and physiology o f the larynx, the head and neck o f each animal have been sagittally sectioned. The specimens are currently housed in the Grant Museum o f Zoology, University College London. A further five primates, one Leontopithicus rosalia (golden lion tamarin), one Hylobates muelleri (gibbon), one Hapalemur sp. (lemur), two Colobus guereza (black and white colobus monkeys, one male, one female), are from the Royal Museum o f Scotland Edinburgh. Ten adult macaques {Macaca mulatta) from the study by Flügel and Rohen (1991) are also included.
Pr o c e d u r e
Bo dym a s sc a l c u l at io n s
The body weights used were as far as possible the actual body weights o f the individuals. These are taken from records at the Museums or relevant departments. For those individuals with no known body weight, weight was estimated using femoral head diameter or area o f the eye orbit. Body mass is determined using the femoral head measurement used in Aiello and Wood (1994), and Ruff et al. (1997) and eye orbit area (Aiello & Wood, 1994). If weight could not be predicted then the published average weight for the species was used. Appendix 3 gives the full data set and indicates for which specimens actual, estimated or literature based bodyweights were used.
Vo c a l Tr a c t Me a s u r e m e n t s
measured from the sagittal section. Eight macaques had the crania cut sagittally to give the outline o f the vocal tract, and for these the length o f the vocal tract is measured
directly on the specimen using calipers (Fig 1.6 for measurement points). Sectioned heads from the Grant Museum were used for the chimpanzee, gorilla and orang-utan vocal tracts. The measurements were taken in the same way as for the macaques (Fig 1.6). It was not possible to section the other specimens as they were on loan and could not be damaged.
The measurements used are based on those used by Fitch (1997) and Fitch and Giedd (1999). The measurements below are illustrated in Figure 1.7.
Table 1.1 Description of vocal tract measurements
Measurement Description
Vocal tract length Measured from the posterior surface of
the upper incisors to the caudal margin o f the uvula, from the caudal margin o f the uvula to the caudal margin o f the glottis.
Oral cavity length Measured from the posterior surface o f
the upper incisors to the caudal margin o f the uvula.
Tongue length Measured from the tip o f the tongue to
the cranial border o f the hyoid bone. Distance between hyoid bone and uvula Measured from the caudal margin o f the
uvula to the cranial edge o f the hyoid bone.
Distance between hyoid bone and hard palate Measured from the posterior o f the hard palate to the superior border o f the hyoid body
Hard palate length Measured from the posterior surface o f
soft )palate
Uv. to e p ig lo ttis
Uvula to g lo ttis
Figure 1.6 Vocal tract measurements, from the posterior surface o f the upper incisors to the caudal margin o f the uvula, from the uvula to the superior margin o f the vocal folds
DISTANCE BETW EEN THE SO F T PALA TE AND HYOID BONE,
The human x-rays and the published data on macaques from Flügel and Rohen
(1991) were used to determine the space between the soft palate and hyoid bone. The
human distance was measure using the grid method described above, (Fig. 1.7). Flügel and
Rohen do not give this measurement in their paper. The distance between the soft palate
and hyoid bone in the macaques was calculated by subtracting the distance from the hard
palate to soft palate from the hard palate to hyoid bone distance. The soft palate extends
further back in the oral cavity than does the hard palate.
^ p a l a te J
fi>dength S
/ Lfvula to hyoid
Figure 1.7 The distance between the uvula and hyoid bone and the length o f the hard palate
An a l y s i s
Regression analysis was used to determine the relationship between the various
vocal tract measurements and their relationship with body mass. The regressions were
graphed to show relationships. The analysis was performed using Excel 98 and SPSS 6.1
