Possible Preadaptations to Speech.
A Preliminary Comparative Approach
Marc Verhaegen and Stephen Munro
Studiecentrum Antropologie, 2580 Putte, Belgium
marc.verhaegen@village.uunet.be
Human Evolution 19: 53–70, 2004
Abstract
Human language is a unique phenomenon and its evolutionary origins are uncertain. In
this paper we attempt to explore some of the preadaptations that might have contributed to the
origin of human speech.
The comparative approach we use is based on the assumption that all features of a species
are functional, and that all features can be compared with those of other animals and correlated
with certain lifestyles. Using this method we attempt to reconstruct the different evolutionary
pathways of humans and chimpanzees after they split from a common ancestor.
Previous results from comparative studies suggest human ancestors may not have evolved
on the open African savannas as was once believed, but more probably were coastal omnivores
feeding on plant matter and easy to catch invertebrates such as shellfish from beaches and
shallow waters. Fossil and archaeological data suggest this coastal phase occurred at the
beginning of the Pleistocene, when Homo ergaster-erectus dispersed between East-Africa,
North-Africa, South-Asia and Indonesia.
This paper presents comparative data suggesting the various human speech skills may
have had their origins at different times and may originally have had different functions. Possible
preadaptations to speech include, for instance, musical skills present in a variety of primate
species (sound production); airway closure and breath-hold diving for collecting seafood
(voluntary breath control); and suction feeding adaptations for the consumption of fruit juice or
certain seafoods (fine control of oropharyngeal movements). The different evolutionary
pathways of chimpanzees and humans might explain why chimpanzees lack language skills and
why human language is a relatively recent phenomenon.
Key words
Speech origins, language evolution, hominid diet, human evolution, aquatic theory,
musical abilities, diving abilities, suction feeding, consonants, Homo erectus, seafoods,
comparative biology.
Introduction – Comparative Anthropology
Three major components of human language – phonology, semantics, and syntax – are
acquired successively from about the first, the second and the third year of life (Hirsch-Pasek &
Golinkoff 1996). This succession may reflect the human linguistic evolutionary stages: prelanguage, one-word sentences, and grammatical or ‘true’ language. This paper discusses the first
stage, the phonetic pre-adaptations for language, and is based mostly upon comparative data with
other mammals.
In constructing human evolutionary models, paleoanthropologists tend to focus on the
fossil evidence, but the comparative method (comparing the anatomy, physiology, behaviour,
DNA etc. of living animals) is probably more secure, systematic and reliable.
By using the comparative data we adopt an analytical and functionalistic approach.
Biological features are generally inherited independently of each other (Mendel’s Laws), due to
the crossing-over and independent assortment of chromosomes during gametic reproduction.
This recombination of genetic material may not only explain how features can evolve in parallel,
i.e., apart from each other, but also why selection, working on different features in parallel in all
members of a population, can be so efficient. Since most biological features are polygenetic
(influenced by more than one gene) they can be ‘fine-tuned’ through the processes of
recombination and selection. All species have gone through an immense period of recombination
and selection – not one of all our millions of ancestors died before it had produced fertile
offspring – therefore every feature must have had one or more functions. These functions are not
always obvious, for instance, features can have multiple functions and functions can change over
time (evolutionary opportunism), but by comparing the similar features of different species it is
often possible to identify general trends and correlations between certain features and particular
lifestyles or environments.
The comparative approach can be used for the anatomical, physiological and behavioural
features of all animals. According to biomolecular data (DNA and proteins), chimpanzees are
our closest relatives. By comparing the features of humans and chimpanzees, therefore, it may be
possible to work back towards the Last Common Ancestor of Homo and Pan, and attempt to
reconstruct its likely habitat and behaviour. It may also be possible to determine which features
humans have since acquired, and the type of environmental factors that may have been
responsible for these acquisitions.
Perhaps the main drawback of the comparative approach is the insufficient data available
for the great majority of species. Many anthropologists, for example, are not particularly familiar
with the anatomies, physiologies and behaviours of non-primate species, and have shown a
preference for the hominid fossil evidence rather than the comparative evidence. However, while
the fossil record can provide additional insights, its importance can also be overstated. Fossils are
incomplete – typically they are fragmented pieces of bone without soft parts – and are usually of
uncertain relation to living species. Frequently, species, age and sex are unknown, and
sometimes the geological age and palaeo-environment are uncertain.
In this paper, we first briefly outline the results of our comparative studies and present a
hypothesis for human evolution: a waterside scenario (Verhaegen & Puech 2000; Verhaegen et
al. 2002). Then, more specifically, we compare the anatomy and physiology of the food and
airways of humans, chimpanzees and other mammals, provide a list of possible functions, and
attempt to determine how and why the human peculiarities associated with speech evolved.
A Waterside Scenario
Most researchers agree that our remote primate ancestors lived in trees, but there is some
disagreement as to how humans became independent of trees. Recently, there has been a steady
accumulation of evidence suggesting that humans may not have evolved in a warm and dry
environment as was once commonly believed, but instead may have evolved in warm and wet
conditions, at the edge between land and water (Hardy 1960; Ellis 1991; Morgan 1997; Bender et
al. 1997; Tobias 1998; Verhaegen et al. 2002). Anatomical, physiological, biochemical and
palaeo-environmental data tend to support this view. At the 1999 symposium on Water and
Human Evolution, in Ghent, Belgium (Vaneechoutte 2000), it was proposed that our ancestors
were coastal or riverside omnivores who not only consumed terrestrial plants and animals, but
also collected food from shallow waters. This scenario is supported by a comparison of some
typically human features with those of other mammals.
Bipedality: climbing-wading origin? Primates, perhaps because they are traditionally
climbing animals, have a tendency to adopt an erect posture, and this behaviour is
accentuated in primate species that frequently wade through shallow water. Proboscis
monkeys, for example, cross shallow stretches of water on two legs when moving from one
mangrove tree to another (Napier & Napier 1967). Lowland gorillas wade on their hindlimbs
through forest swamps in search of wetland plants and sedges (Chadwik 1995; Doran &
McNeilage 1997). Dental microwear and isotopic evidence suggest the australopithecine diet
may have also included such plants (Puech et al. 1986; Puech 1992; Sponheimer & LeeThorp 1999). Therefore it is likely that Pliocene hominids also regularly waded bipedally in
the shallow waters of forest clearings, gallery forests or mangrove areas, possibly in search of
floating fruit, wetland plants, reed sedges, fish or shellfish (DuBrul 1977; Ellis 1991;
Broadhurst et al. 1998; Verhaegen 1992; Verhaegen et al. 2002). This is corroborated by the
recent discoveries of the 7- to 6-million year-old, probably bipedal-and-climbing, hominids
Sahelanthropus tchadensis and Orrorin tugenensis in shallow perilacustrine environments
(Brunet et al. 2002; Senut et al. 2001).
Thick enamel and stone tool use: hard-shelled foods? A combination of thick molar
enamel and stone tool use is known to have existed in various Homo species, and exists today
in capuchin monkeys and sea otters. Sea otters have large, flat cheekteeth, which resemble
those of australopithecines (Walker 1981), and use stones to crack open shellfish while
floating on their backs. Capuchins open nuts with stones and use oyster shells to remove
shellfish from the trunks of mangrove trees (Fernandes 1991). Chimpanzees, which have
thinner molar enamel, manipulate stones to crack open hard-shelled nuts. Human Pliocene
ancestors, perhaps in the same way as mangrove capuchins, might have used stones or other
hard objects to remove coconuts from palm trees, to crack hard nutshells, or to remove and
open oysters from the trunks of mangrove trees (Verhaegen et al. 2002).
During the late Pliocene or early Pleistocene, members of the genus Homo – as opposed
to our more distant relatives the australopithecines – might have also learned how to duck the
head underwater and to dive and collect underwater shellfish as well as other aquatic resources.
Humans have much more efficient diving capabilities than nonhuman primates (Schagatay 1996;
Morgan 1997; Bender 1999; Verhaegen et al. 2002). Indeed, Homo fossils – as opposed to
australopithecines – are typically found near shellfish (Chiwondo, Chemeron, Nariokotome,
Zhoukoudian, Boxgrove, Terra Amata, Rabat, Hopefield, Gibraltar and others). Although sea
level rises and the actions of tides and waves have drastically reduced the chances of discovering
hominid fossils at sea beaches, Homo erectus remains have been discovered amid shellfish,
barnacles and corals, from the early Pleistocene skull of Mojokerto at Java (Ninkovich &
Burckle 1978), to the late Pleistocene Acheulean tools of Eritrea (Walter 2000; Walter et al.
2000). Stone tools discovered on Flores suggest Homo erectus crossed a 19 km wide, deep
oceanic channel more than 800,000 years ago (Morwood et al. 1998; Tobias 1998). We have
argued that the fast dispersal of Homo erectus at the beginning of the Pleistocene between
Algeria (Aïn-Hanech), Israel (Yiron, Ubeidya) and Java (Mojokerto) occurred along the
Mediterranean and Indian Ocean coasts, where foods could be gathered from both the land and
sea (Verhaegen et al. 2002). From the coasts, different Homo sidebranches could have migrated
up rivers into the interiors of Africa and Eurasia, where fossilisation chances may have been
more likely. Initially restricted to the edges of rivers, swamps and lakes, some Homo populations
later moved to areas further from permanent water.
Whereas stone tool use for cracking hard-shelled foods may have been a preadaptation
for the development of lithic technologies, the diving abilities of our ancestors might have been a
preadaptation for the development of voluntary speech (Morgan 1997; Diller 2000).
Like Darwin (1871), however, we believe human sound production probably has deeper
roots, beginning at a time when our ancestors were still arboreal. There may be several
overlapping preadaptations for speech, including musical abilities, swallowing abilities, the
ability to close the airways, the ability to control breathing, and the ability to communicate
symbolically.
A Short Survey of Food- and Airway Adaptations in Mammals
In most animals the mouth is used primarily for feeding, and most oropharyngeal
adaptations are directly linked to feeding behaviour (foodway). In amniotes (reptiles, birds,
mammals) the nose is primarily for breathing (airway). Sound production outside the water is
normally linked to the airway, but for the production of loud sounds a wider space may be
advantageous. This is possibly why singing birds and barking dogs make a connection between
the airway and the mouth. The traditional functions of the mouth, nose and throat cavities are
discussed here under three headings: air, sound and food.
1. AIR
On land: All land mammals breathe air through their nostrils, though some can also
inhale and exhale through their mouths. A number of hypotheses have been put forward to
explain the evolutionary function of the human external nose. For example, it has been argued
that the human nose is designed to prepare air for breathing: to purify, moisturise, filter or warm
it, or to retain water from expired air, etc. (Franciscus & Trinkaus 1988a, 1988b). There are,
however, no comparative examples of mammals developing an external nose for similar reasons.
The only other primate with a well developed external nose is the mangrove-dwelling proboscis
monkey, which is well known for its swimming ability and can swim several metres under water
(Napier & Napier 1967).
Most land mammals, as opposed to humans (except babies), have their larynx positioned
high in the throat. At rest, the larynx connects with the nasal passage, its entrance well within the
nasal cavity, and acts as a barrier separating the nasal passage from the oral cavity. This means
most mammals can swallow fluids (and in some species semi-solid materials) and breathe
simultaneously (Laitman 1985; Crompton et al. 1997). Some mammals, like red deer and koalas,
have a permanently low larynx, but have evolved (at least in red deer) a long velum which
connects the nasal cavity with the larynx when at rest. Humans have a descended larynx, like
koalas and red deer, but lack an elongated velum. Thus, while most mammals, including human
babies, can swallow fluids and breathe simultaneously, humans above the age of about six
months cannot.
In water: Diving mammals must be able to close their airways underwater. They must
also have adaptations that allow for the considerable water pressure that can be placed upon airfilled cavities (middle ear, sinuses, bronchi, lungs) and for the great and sudden pressure changes
that can be experienced while diving and surfacing. Moreover, they must be able to inhale large
amounts of air rapidly when they surface in order to minimise the length of time between dives.
It has been argued that some semi- or ex-aquatic mammals, such as tapirs, elephants, hooded and
elephant seals, initially evolved elongated external noses to help them breathe and to prevent
water from entering the airways while wading or swimming (the human nose seems well
designed hydrodynamically to keep water out of the airways while swimming at the surface or
under water or while dipping the head under water or diving into water). Most aquatic mammals
normally breathe through the nostrils (the whales’ blowhole), although some, like walruses,
frequently mouth-breathe (Fay 1982).
Olfaction: Most mammals use the air they breathe for olfaction. In land mammals the
sense of smell is very important, but in many aquatic mammals this function has been reduced
and in some cases completely lost (Dehnhardt 2002).
Panting: The rapid forcing in and out of air through the mouth is a method used by many
mammals (including humans when thermoregulatory sweating does not work well, for instance,
in very humid conditions) to help reduce body temperature. In dogs, the velum (soft palate)
opens during inhalation and closes during exhalation with each pant cycle (Schmidt-Nielsen &
Taylor 1970; Biewener et al. 1985).
2. SOUND
Calls: Sound production is derived from the function of breathing. Instinctive and
territorial sound productions, like barks and roars, are seen in many mammals; humans cry and
laugh aloud when stimulated. Many of these ‘automatic’ sounds are reflexes controlled by neural
centres in the brain stem. Most are produced by the glottis slit between the vocal folds in the
larynx. The lower the larynx during sound production, the louder and more impressive the
territorial calls (Fitch 2000; Ohala 2000). An extreme example is the male hammerhead bat,
which has an extremely large and low, in fact intrathoracal, larynx (Rosevear 1965). Monkeys,
dogs, pigs and all other mammals examined so far, lower the larynx and close the velum during
loud vocalisations. In male red and fallow deer, during roaring, and probably also in other
mammals during loud calls, the vocal tract has a horizontal (oral) and vertical (pharyngeal) tube,
which is reminiscent of the permanent situation in humans. This two-tube configuration is said to
have implications for sound production, the expanded pharynx perhaps allowing humans to
produce the full range of speech sounds (Laitman 1985). Many primates, including all hominoids
except humans and smaller gibbons, have large laryngeal airsacs, possibly for making their long
calls louder or faster or for preventing hyperventilation (Hewitt et al. 2002; Ankel-Simons 2000).
Song: Musicality in animals is often correlated with an arboreal (many birds, some
primates) or aquatic (some cetaceans and pinnipeds) lifestyle. The songs of humpback whales are
particularly well-known. Primate examples include the pant-hoots of chimpanzees. Male
proboscis monkeys, who have longer external noses than females and infants, are said to use the
nose as an organ of resonance in vocalization (Ankel-Simons 2000), and even to produce a
typical double sound through the nose and mouth at the same time (Napier & Napier 1985).
More elaborate songs are seen in some monogamous primates such as indris, tarsiers, titi
monkeys and gibbons (Darwin 1871; Vaneechoutte & Skoyles 1999; Müller & Anzenberger
2002). At least in primates, these sounds appear to be largely under the emotional control of the
limbic cortex (Deacon 1997). The songs of birds, as well as mammals, differ according to the
population (dialects). Birds typically learn their songs from their fathers during a sensitive period
early in life (presong).
Speech: Speech production is uniquely human, though some birds, such as mockingbirds
and parrots, can mimic human speech (Pepperberg 2000). Human speech sounds are produced by
muscles under the voluntary control of the greatly enlarged precentral cortex (Brodmann’s Area
4, the motor cortex that controls fine skeletal muscle movements). Consonants are produced by
using the lips, tongue, jaw, velum, pharynx and glottis, although in birds (with inflexible beaks
and reduced tongues) these sounds are probably imitated by muscle contractions in the syrinx,
the vocal organ, where the right and the left bronchus come together. The only mammals known
so far that can mimic human utterances, presumably without any understanding of the meaning,
was a harbour seal that had learned to produce (albeit with a throaty voice) fragments of
humanlike speech from a fisherman, probably at a sensitive period early in its life (Ralls et al.
1985; Deacon 1997), and to a very limited extent a beluga whale (Eaton 1979). Very variable
sounds such as clicks, though not humanlike, are produced by dolphins in the nasal air passages,
probably mainly by using the larynx, which, unlike the human larynx, is very ‘ascended’, i.e.
permanently locked in the nasal cavity (Slijper 1958; Dudok van Heel 1970).
3. FOOD
Biting: The anatomy of the oral cavity in mammals is probably influenced mainly by its
primary function, the processing of food through biting, chewing and swallowing. Many fruiteaters have spatulated incisive teeth, as do most primates including humans, whereas these are
more conical in meat- or insect-eaters (for primates see Ankel-Simons 2000), and can be reduced
in terrestrial and aquatic plant-eaters. Terrestrial carni-, insecti- and omnivorous mammals
typically have long canines, whereas pure herbivores, including the aquatic sirenians, have
reduced or absent canine teeth. Aquatic carnivores such as cetaceans, and to a lesser degree
pinnipeds, generally do not have very long canines. All their teeth (front teeth and cheekteeth)
tend to be sharp and conical, probably for catching slippery fish or squid. An obvious exception
is the walrus, which has long canine tusks probably for intraspecific display (e.g. de Muizon
1995). Large front teeth can be important for attacking sexual rivals or as a defence against
predators, or as a warning signal. It has been argued that showing the front teeth, as is seen in
human laughter, can indicate the physical strength, health and self-confidence of an individual.
Chewing: Premolars and molars are for processing the food, for instance, cracking hard
foods with thick enamel, chewing calorie-poor plants with flat cheekteeth, cutting through tough
plant food or insect exoskeletons with sharp ridges of thin enamel, or slicing meat with the
carnassial teeth. Typically, mammals such as ungulates and carnivores, as well as most primates
except humans, have low, long and horizontal palates with transversal ridges of cornified
epithelium, probably for fixing food while chewing (Romer & Parsons 1977).
Swallowing: As already discussed, in most mammals the larynx at rest is engaged in the
nasal cavity, which means that swallowing fluids (mouth to esophagus) and breathing (nose to
larynx) at the same time is possible. A permanently lowered larynx makes this impossible for
humans, except babies. Not all kinds of food have to be chewed thoroughly. As opposed to
herbivores, carnivores often swallow large food boluses, which seems to require a large
pharyngeal space (laryngeal descent creates a larger pharyngeal space, see figure 1). Foods such
as fruit juice and tree exudate (some bats and primates) or insects and grubs (ant-eaters, bears)
can be sucked and swallowed without chewing. Some juice- or sap-sucking New World monkeys
show features that are believed to increase the suction drainage force, such as an angular and
highly vaulted palate in some marmosets and a humanlike closed upper dental arch with
incisiform canines in dusky titis (Hill 1957; Hershkovitz 1977; Jones & Anderson 1978). Many
pinnipeds, most notably walruses and bearded seals, have round tongues and smooth vaulted
palates, probably to enable them to more easily suck the smooth and slippery seafoods out of
shells (King 1972; de Muizon 1993). Most land mammals, including nonhuman primates, and
most aquatic mammals, including hippos, otters and furseals, have palatal ridges, but these are
lacking in some cetaceans, crabeater seals, elephant seals and walruses (Roger Crinion, personal
communication). The absence of palatal ridges may allow food to slide through the mouth
unchewed. The strong suction feeding of walruses probably requires a round and relatively
narrow oral cavity and a strong retractable tongue, and possibly an enlarged pharyngeal space
and lowered larynx (Fay 1982; de Muizon 1993). Most aquatic mammals can swallow food
underwater, although it is not clear which specific adaptations make this possible. In many
aquatic mammals such as walruses, sealions and seacows, the epiglottis, the lid that covers the
well-developed larynx of humans during swallowing and prevents food entering the trachea, is
not as well developed as it is in humans, monkeys, pigs and probably most terrestrial mammals
(Negus 1949).
Possible Explanations for the Human–Chimp Differences
Chimpanzees lack an external nose, slitlike nostrils and a philtrum (the vertical furrow in
the human upper lip), and have a shorter and more direct air passage from the nostrils to the
nasopharynx than humans (see figures 1 and 2). In humans the nasal air passage is both longer
and narrower and has an inverted U shape (with the nostrils underneath the nose instead of in
front).
Chimpanzees have a larger and more protruding mouth (prognathism) with larger canine
teeth and corresponding gaps (diastemas) in the opposite jaw, whereas humans have a smaller
mouth with everted lips, a closed and parabolic tooth row and teeth of nearly equal height
(Hocket 1967; Laitman 1985).
Chimps have a flat tongue and a long and transversally ridged palate, whereas humans
have a round, thick and bulbous tongue, a much-shortened oral space and a short, wide, deep
palatal ‘vault’. The human tongue can be shaped to fit tightly against the arched and smooth
palate.
In chimpanzees, the gap between the palate and larynx is smaller than in humans, who
have a tongue bone (hyoid) and larynx “retreated still farther down in the neck” (DuBrul 1958).
Humans have a well-developed larynx and very muscular vocal folds, but lack airsacs (Negus
1949).
Humans have a very large representation of the oral muscles in Area 4 (see above), and
only in humans does damage of Area 4 produce muteness (Deacon 1997). Humans, unlike
chimpanzees and other primates, have an Area 4 representation of the larynx and breathing
musculature, have direct fibers connecting Area 4 to the nucleus ambiguus (cortico-ambiguus
connections), and can voluntarily control the larynx muscles (nucleus ambiguus) and the
breathing muscles (brain stem).
1. SINGING – VOCAL FOLDS
Babies of two or three months produce cooing sounds. This is called vocalising and is
performed with the vocal folds in the larynx, without much oral movement. Soon thereafter, even
in deaf children, the babbling starts to include labial consonants, and syllables are produced
(consonant plus vowel). In babies older than six months, the sound pattern already resembles the
native language, and ‘dialogues’ with the mother stimulate the utterances. This sensitive period
of automatic sound production and the learning of local dialects resembles the subsong period in
birds (Deacon 1997).
The early prelingual sounds, without symbolic meaning, may correspond with the
elaborate songs of nonhuman primates like gibbons. Music powerfully affects the emotions
(anthems, hymns, marches, love songs), and is used by humans as a territorial and pair- or groupbinding behaviour, as it is by other animals. Well-developed musical abilities and duet-singing
are seen in monogamous primates such as gibbons. Bonobos (pygmy chimpanzees) engage in
group-chorusing and rival males have been observed engaging in vocal duels (De Waal 1997).
Some aquatic species such as humpback whales (polygynous) also use complex melodic
utterances for territorial behaviour. Interestingly, the harbour seal raised by a fisherman only
‘spoke’ in a humanlike way when it was not engaged in emotional or territorial behaviour
(Deacon 1997). It is known that musical training in young children induces an enlargement of the
temporal and insular cortex (planum temporale) in the left brain hemisphere, and can lead to an
improvement in a child’s ability to hear absolute tones (Schlaug et al. 1995). Intonation is an
indispensable element of all spoken languages, and almost half the world’s languages are tonal.
Ohala (2000) argues that our descended larynx could not have evolved as an adaptation
for speech, since men, who have an even lower larynx than women, are not better adapted for
speech than women, who perform better in verbal tests. In fact, the comparative evidence
suggests that laryngeal descent may have occurred in order to make the voice more impressive
(Fitch 2000; Ohala 2000). This, however, does not explain why the human larynx is incapable of
making a direct connection from the nasal passage to the larynx and cannot engage in the
nasopharynx (intranarial or suprapalatal), as it does in human babies and most land mammals.
2. DIVING – AIRWAYS
At birth, humans have a larynx that can engage in the nasopharynx, then, between four
and six months, the larynx begins to descend. One possible explanation for laryngeal descent in
humans could be the need to breathe a large amount of air in a short period of time to facilitate
diving (Morgan 1997). Whales and dolphins have permanently intranarial larynges, ascended
rather than descended.
All humans, probably unlike most nonhuman primates, can easily learn to dive, and
several human populations, such as some Indonesian and Oceanic populations, as well as the
Ama of Korea and Japan, collect shellfish through breath-hold diving (Schagatay 1996).
Diving, as seen in aquatic or semi-aquatic mammals, requires a voluntary control of
breathing. In contrast with land mammals, divers must be able to take a deep breath just before
they intend to dive, and breathe deeper and faster between dives. They must also be able to hold
their breath underwater, paradoxically at the very time when their oxygen needs are highest. In
contrast, terrestrial mammals intensify their breathing when they need more oxygen – while
running, for example. This may explain why humans are unique among primates in being able to
control the larynx and breathing musculature at will.
Diving also requires the complete closure of the airways underwater, so water can be kept
out of the lungs. Humans can close the oral (e.g. small mouth with fleshy lips, closed tooth row,
bulbous tongue and smooth palate) and nasal passages (e.g. slitlike nostrils, longer and narrower
airway) much more completely than chimpanzees can (figure 1). It has been suggested that
human ancestors might have been able to close the nostrils by pressing the upper lip (moustached
or not) against the nostrils as some people do today when they dive (figure 2). The upper lip with
the human philtrum seems “perfectly made for this. The two lines descending from the nose plug
up the holes, while the recess in the middle allows for the bridge between the holes” (Peter van
de Graaf, in Morgan 1997).
The inverted U shaped nasal passage in humans may have helped to keep water out of the
airways while ducking the head underwater.
3. FEEDING – MOUTH AND TONGUE
It has also been suggested, on the basis of comparative evidence, that our permanently
descended larynx, incapable of engaging in the nasopharynx, might have been a useful
adaptation for suction of certain slippery foods or juices (Roger Crinion, personal
communication). Animals featuring a descended larynx include not only some deer and koalas
that produce loud calls, but also so-called suction feeders such as some bats and perhaps some
sap-feeders. It is possible that laryngeal descent allows considerable retraction of the hyoid and
tongue so that the pressure in the oral cavity can be lowered, which is one possible way to
accommodate underwater suction, as in walruses, or for sucking juicy fruits, as is seen in some
bats (Sprague 1943; Rosevear 1965; Hildebrand 1974; Fay 1982).
Other adaptations seen in mammals that regularly suction-feed are a small mouth, a
smooth and vaulted palate, and a smooth and round tongue that can be shaped to fit tightly
against the palate, as well as a closed parabolic upper tooth row without long canines and
diastemas (gaps in the tooth row would hinder suction). These features, in different
combinations, are seen in sloth bears, some bats and primates that suck insects, fruit pulp or
exudates, and in particular in walruses and other pinnipeds that include shellfish, squid or fish in
their diet. These features also typically distinguish humans from apes.
The human sucking adaptations could have been used for fruits and/or for smooth aquatic
foods. Humans do not have to chew raw oysters in order to eat them, and are able to swallow
small fish whole. Moreover, humans can swallow food underwater, and can also keep their
mouths open underwater without swallowing or inhaling water. Feeding underwater requires a
fine co-ordination of the lips, mouth, tongue and throat in order to keep water out of the airways
and, at least in marine environments, to prevent ingestion of too much seawater. The human
tongue is extremely flexible and is well adapted for manipulating objects within the mouth. It is
also well designed to help expel water from the mouth.
Some of the mouth-closing and/or feeding adaptations might explain why the human
tongue is able to close the oral cavity at different places, allowing a diverse number of
consonants to be produced, for example, at the alveolar, palatal, velar and uvular articulation
places. The only nonhuman mammals, as far as we know, that are able to reproduce recognisable
pieces of human speech are the harbour seal (Ralls et al. 1985) and possibly a beluga whale
(Eaton 1979).
Concluding Remarks
The combination of comparative and fossil data suggests that by about 1.8 million years
ago human ancestors may have become more reliant on wading and diving than on climbing.
A waterside mammal might be expected to have greater control of the lips, tongue and
throat muscles for seafood consumption, as well as voluntary control of the airway and breathing
musculature for swimming and diving. This oral cavity and airway control might have been
preadaptative to the evolution of human speech, particularly in combination with the already
well-developed rhythmical, melodic and duetting abilities of our primate ancestors.
A wading-and-diving lifestyle might have also required a different method of
communication. Traditional primate communication systems such as smell and certain types of
body language (such as posture, for example, though not facial expression) may have been less
effective in a semi-aquatic milieu when compared to a purely terrestrial or arboreal one (Morgan
1997). Derek Ellis (personal communication) notes “how well sound travels over water,
compared to being muffled in forests, and even compared to grassland. Foraging beach and
lagoon apes could separate quite widely and still remain in contact by vocalising.”
It is possible that the modifications to our ancestors’ food and airway entrances coincided
with an early stage in the disproportionate expansion of the human neocortex, in particular Area
4 (precentral) and Area 44 (Broca), which control the fine movements of the mouth and throat
muscles – whether for singing, swallowing or diving. Humans, as opposed to chimpanzees and
other primates, have disproportionally large neocortical areas when compared to the brain stem
(e.g. Deacon 1997). Of these, the temporal and insular areas (including the Areas 4, 44 and
Wernicke), where sounds are produced, processed and interpreted, seem to have undergone the
greatest enlargement (Semendeferi & Damasio 2000). Perhaps in this part of the brain, the preexisting functions of song production, food consumption and airway control were integrated into
a system that could produce voluntary and articulated sounds, i.e. the beginnings of speech. The
integration of this voluntary sound production system with the symbolic powers that may have
already existed in primates (Savage-Rumbaugh 1986), might have been made possible due to the
extra brain tissue (association or integration cortex) that developed during human evolution.
Acknowledgements
We are greatly indebted to Tecumseh Fitch, Elaine Morgan and Roger Crinion for
information, corrections and ideas.
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Table 1 -- Nose-Mouth-Throat Differences with Chimps
Human feature
External nose
Inferior nostrils
Smaller mouth
Vaulted palate
Round tongue
Smooth palate
Very low larynx
Mobile larynx
Flexible glottis
Mouth breathing
Baby babbling
+ additional description
long, narrow nose passage
human philtrum in upper lip
with red everted lips
with parabolic tooth row
fits nicely in palate
less transversal ridges
no intranarial engagement
large pharyngeal space
very muscular vocal folds
under volitional control
song: sensitive period
Possible function(s) originally
Keeping water out of airways? Sexual selection?? Resonance??
Keeping water out? Nose closure (figure 2)? Acoustic??
Mouth closure. Suction of fruits? of seafood? Sexual selection??
Food suction, e.g. fruits? seafood? Underwater??
Chewing & swallowing. Food suction? Vocalisation?
Swallowing smooth and slippery seafood? Suction?
Vocalisation? Suction? Large intake of breath? Choking danger!
Vocalisation? Suction? Swallowing large food boluses?
Singing (tone height). Glottis closure for diving??
Diving? Singing? Speaking?
Singing? Speaking?
Table 2 -- Possible Explanations of the Human-Chimp Differences
Song?
Abilities
Human
vs. chimp
features
Animal
examples
Secondary
use in
speech
. Tone & rhythm
. Duetting
. vocal folds very muscular
. sensitive period & babbling
? lower larynx & louder calls
? mobile larynx
? no laryngeal airsacs
. Musicality: many birds;
gibbon, titi, tarsier, indri;
humpback whale
. Speech imitation: some
birds, harbour seal
. Tone, intonation
. Rhythm
. Dialogue
. Sound imitation
Seafood?
. Smooth & slippery
. Swallowing underwater??
? small mouth & fleshy lips
? smooth palate with less ridges
? vaulted palate & round tongue
? mobile hyoid & larynx
? large pharynx
. Suction feeding: sloth bear;
walrus & other pinnipeds;
some primates & bats
. Extreme lip, tongue & pharynx
control; dental, palatal etc.
closures easy (not only labial)
. Click sounds
Diving?
. Voluntary breathing
. Airway closure
? small mouth & everted lips
? philtrum & slitlike nostrils
? long & narrow nasal passage
? round tongue, smooth palate
?? laryngeal descent
. External nose: proboscis
monkey, tapir, elephant,
hooded & elephant seal
. Airway closure & voluntary
breathing in diving mammals
. Breathing muscles: voluntary
in- & expiration
. Airway closures at lips, tooth
row, palate, velum, glottis…
Figure 1 -- Midsagittal Section through the Heads of a Chimpanzee and a Human (after Laitman
1985). Note the protruding nose and chin, the round tongue and short, vaulted, smooth
palate, and the lowered larynx in humans.
Figure 2 -- Possible Function of Upper Lip and Philtrum: Closing the Nostrils (after Morgan
1997). Note that human ancestors were more prognathous, and possibly had moustaches,
so that the upper lip was closer to the nostrils.