Within a few hours of the naval sonar drill, reports arrived of stranded beaked whales appearing over many kilometres along the coast.  These animals showed signs of decompression sickness, also known as ‘the bends’.

Post-mortems on these animals revealed gas and fat bubbles in their bones and tissues.

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The deeper you dive, the more the pressure forces nitrogen and oxygen from your lungs to dissolve into your body tissues.  If you then surface too quickly, these gases can come out of solution and form bubbles in your blood.  These can block smaller blood capillaries, cutting off the oxygen supply to the affected tissues.  Decompression sickness is a recurrent risk amongst scuba-divers who breathe compressed air, and breath-holding ‘free-divers’ who make too many consecutive dives.

In contrast, beaked whales routinely hunt for an hour below 1000m, using echolocation.  These ‘extreme divers’ do not normally experience decompression sickness, although fossils from early in their evolutionary history show that they were not immune to these problems.  X-rays of the fossilised bones of more primitive whales show regions where bubbles formed inside a capillary, damaging the bone tissue and leaving a tell-tale signature.

Whale embryos initially develop rear limb buds, like land mammals.  These structures are reabsorbed back into the body later in development. The fossil record, along with DNA studies, reveal that whales’ closest living relatives are cows and hippos, which share their same four-legged (tetrapod), hoofed, land-dwelling ancestors.

This raises some puzzling questions:

– Why did whales’ ancestors take to the water after 300 million years on land?

– Why didn’t they re-evolve gills?

– How can they dive for so long without getting ‘the bends’?

Why did whales’ air breathing ancestors take to the water?

The land-dwelling ancestors of whales may have first waded into the sea to escape from predators on land.  Shallow coastal areas offered a relatively safe haven with little competition for the new food resources available in or near the water.  This initial stage would have enabled these semi-aquatic ancestors of modern whales to adapt their digestive systems to a marine food source.

Fossils from the early Eocene (52Ma) show a succession of increasingly aquatic forms.  From crocodile-like and otter -like amphibious hunters, developmental changes remodelled their breathing, senses, kidney function and limbs to survive better in water.  By 40Ma, these early whales had flippers, a fluked tail, and could mate, birth and suckle their young without leaving the water.

At the Eocene-Oligocene boundary (around 36Ma), movement of the continental plates opened up the deep waters of the circum-Antarctic ocean.  This offered new ecological roles for the deeper-diving whales.  Many new whale species appeared, including ancestors of the filter-feeding baleen whales and toothed whales that hunt in deep waters using echolocation.

Why didn’t whales re-evolve gills?

The ability to breathe underwater like fish seems at first like a requirement for life in the sea.  However despite their lack of gills, whales and dolphins are highly effective predators in both shallow and deep water.

Modern whales’ warm bodies enable their fast reflexes for hunting.  Whilst swordfish and tuna have some warm muscles, most of their tissues are at sea water temperature.  Were their whole bodies warm, the heat loss from their gills would be energetically too costly.

Fish gills develop from the ‘branchial arches’; bulging structures in the early vertebrate embryo.  These same tissue bulges give rise to the lower jaw, the middle ear, hyoid bone and larynx in the throat of humans and other mammals.  For whales and other mammals to form gills would require that they develop new embryonic structures; this would render redundant the lungs with their vast area of vascular tissue.

Breathing air enables whales to use vocal signals to coordinate their social groups and attract mates.  Like land mammals, the baleen whales make vocal calls by passing a controlled air flow through the larynx.  Echolocation, the alternative means of producing sound used by dolphins and other toothed whales, also requires air.  Their ‘sonic lips’ generate calls in an air-filled nasal passage.  Whilst many fish make sounds, their vocal abilities are simple and limited.

How do they dive for so long without getting ‘the bends’?

All mammals store oxygen in their muscles using a protein called myoglobin.  Sustained activity during long foraging dives requires a lot of oxygen.  Deep divers have much higher muscle myoglobin concentrations than land mammals, giving them substantial oxygen reserves.

Modern diving mammals, and deep diving fish such as tuna, have also modified their myoglobin.  As early whales began to explore the deeper waters, selection resulted in better survival from individuals whose myoglobin carried a stronger positive electrostatic charge.   Like positive magnetic poles, these ‘supercharged’ molecules repel each other.  This keeps them in solution, allowing them to function at high tissue concentrations where most other proteins would clump together.

A supercharged form and high concentration of myoglobin makes it possible for deep diving mammals to return to the surface slowly after a prolonged dive.  This behaviour avoids decompression sickness.

However when beaked whales and other species encounter naval sonar at depth, this causes them to ‘panic’ and surface too quickly, inducing ‘the bends’.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Balasse M et al. (2006) ‘Stable isotope evidence (δ13C, δ18O) for winter feeding on seaweed by Neolithic sheep of Scotland’ Journal of Zoology 270(1); 170-176

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The dolphin calf is barely three weeks old. 

His mother nuzzles at him, calling softly.  The calf responds, mimicking her call. 

This unique set of sounds is his signature; the name that mother is teaching him to recognise, and which she will use to call to her calf as she teaches her baby to hunt.  Later he will use this signature whistle so that others in his own pod will recognise him. 

Again and again as his mother calls this name, he repeats it back. 

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Words hold ideas in code.  As in all communications, the meaning of a signal must be agreed between the sender and receiver.  We give our words their meaning by shared agreement.

As we remember and recall them, we access the information they hold.  Collectively we use our words as tools to store information in symbolic form, and so bring our memories ‘to mind’.

Human languages, whether sung or spoken, produce words by controlling the pitch and articulation of these distinct sets of sounds with the lips and tongue. We process our words in the brain through the same fine motor control circuits as we (and our primate cousins) use to coordinate our hands and fingers.

This means that we use words almost as if they are tools in our hands.  At the neurological level, words are gestures to which we give meaning, and then use as tools to share that meaning.

Why did our ancestors need words?

Hearing words bring our ideas to mind, coordinating the thoughts of our social group.  New words are coined when that group agrees to associate a syllable sequence (a word), itself distinct from existing words, with a new unique meaning.  Some other animals, e.g. dogs, can learn to associate human word sounds or gestures with simple meanings.

 

However our understanding and use of words – as symbolic tools – is highly flexible.  Our application of words is often playful, for example puns and ambiguities can extend the meaning of a word, or apply it in a new way.  This furthers our use of these tools for social interaction.

Perhaps starting around 2.5 Ma, our ancestors began to experience selective forces that ultimately promoted a remarkable mental flexibility, resulting in the development of elaborate and multi-purpose manual tools.  This expanded tool use corresponds with the onset of cultural learning.  The making and development of tools is learned from our social group, as is our speech.

Are we then unique in our ability to coin new words?  Dolphins and some other whales broadcast ‘signature calls’ when hunting in murky and deep water, enabling them to stay connected with their pod.  Vocal self-identifying calls would have provided similar benefits to our hominin ancestors in dense, low visibility forest habitats, and perhaps also across large distances in open grassland habitats.  Specific word tools for sharing information, e.g. warning of snakes or poisonous fruit, would enable this group to collectively navigate their world more effectively than they could alone.

As with manual tools, the act of using words provides immediate feedback.  Our language may have a gestural basis in the brain, but our vocal-auditory speech mode is much more efficient.  Although we often move our hands when we talk, we can speak whilst conducting other manual tasks.

How did our ancestors begin to use words as tools?

– Peter MacNeilage suggests that our language arose directly as vocal speech.  Our ancestors’ circumstances may have selected for specific vocal signals, received using their auditory communication channel, whilst their hands were busy with other tasks.  This could include hunting with manual tools, foraging or attending to their young.

– William Stokoe and others argue instead that sign came first.  Hand gestures use the visual channel as a receiver.  They suggest that vocal gestures emerged later, perhaps as a combined visual and auditory signal.

In practice, we often use manual and vocal channels synchronously, but they don’t mix; we never create words that oblige us to combine hand movements with mouth sounds.  Sign languages based on gestures do arise ‘naturally’ (i.e. much like a pidgin language) usually in response to a constraint, such as where deafness is present in some or all of the population, or where other forms of common language are not available within the group.  Manual languages arising under such circumstances reveals just how flexible and adaptable our speech function really is.

Before our ancestors could assign meaning to words, however, they had to learn how to copy and reproduce the unique movements of the lips and tongue that each new word requires.

What might those first words have been?

Babies begin to control the rhythmical movements involved with both eating and vocalising as they start to babble, at around 4-6 months.  Making these movements involves coordinating the rhythmical nerve outputs of multiple Central Pattern Generator neural circuits.

Central Pattern Generators operate various repetitive functions of the body, including breathing, walking, the rhythmic arm movements that babies often make as they babble, and the baby hand-to-mouth grab reflex.

Babies begin to babble simply by moving their lower jaw at the same time as making noises with the larynx.  These sounds are actually explorations of syllables the child has already heard; around half are the simple syllables used in ‘baby talk’.

Learning to make these sounds involves mastering the simplest of our repetitive vocal movements; typically this involves opening and closing the jaw with the tongue in one position (front, central or back) inside the mouth.

Say ‘Mmmmm’, then ‘ma-ma’…  Where do you feel this sound resonating?  A suckling child’s murmuring sounds have this same nasal resonance.

Our first vocal sounds as babies show our desire to connect with our parents.  This connection is two-way; neural and hormonal responses are triggered in human parents upon hearing the cries of their child.

A baby makes nasal murmuring sounds when its lips are pressed to the breast and its mouth is full.  Perhaps as a mother repeats these soothing sounds back to her child, they become a signal that the infant associates with its mother and later mimics to call to her.  Selection may have favoured hominins able to connect with their offspring using vocal sounds.

Unlike young chimps who cling to their mothers, human babies need to be carried.  A hominin mother soothing her child by voice sounds would be able to put down her baby and forage with both hands.

There is more.  Consider walking.  Adopting an upright posture provoked a re-structuring of our hominin ancestors’ body plan.

Breathing and swallowing whilst standing up required a re-orientation of the larynx.  This organ acts as a valve controlling the out-breath, prevents food entering the trachea, and houses the vocal folds (vocal cords) controlling the pitch and volume of the voice.

 

Breathing and eating whilst standing upright also requires that the soft palate (velum) in the roof of the mouth can move up and down, closing off the nasal cavity from the throat when swallowing.  Moving the soft palate also changes the size and connection between the resonating chambers in our heads.

‘Ma-ma’ sounds are made with the soft palate in an open position and opening and closing the jaw to articulate the lips together.  Closing the soft palate shifts resonance into the mouth, producing ‘pa-pa’ from the same movement.

Most world languages have nasal and oral resonance in their ‘baby talk’ names for ‘mother’ and ‘father’.  Peter MacNeilage highlights this as the only case of two contrasting phonetic forms being regularly linked with their opposing meanings.  The desire of hominin infants to connect specifically with one or the other parent may have resulted in them producing the first deliberately contrasted sounds.

Could these sounds, perhaps along with sung tones, have been part of our first ‘words’?

How do we apply meanings to these vocal gestures? 

Chimpanzees make spontaneous vocal sounds that express emotions such as fear and pleasure, much as we do.  They also communicate intentionally, using non-verbal facial gestures e.g. lipsmacks.

We gesture with our faces, hands and voices across all languages and cultures.  Human baby babbling is also voluntary, combining sound from the larynx with lipsmack-like movements to create simple syllables.

The initiation of vocal sounds arises from different regions of the brain in humans and other primates.  Primate calls arise from emotional centres within the brain (associated with the limbic system), whereas human speech circuits are focussed around the lateral sulcus (Sylvian fissure).

Within the lateral sulcus, a zone of the macaque brain (area ‘F5’) thought to be the equivalent of Broca’s area in humans, houses nerve pathways called ‘mirror neurons’.  The mirror circuits are involved with producing and understanding grasping actions associated with obtaining food, decoding others’ facial expressions, and making mouth movements related to eating.

These circuits reveal that neurological routes link hand-and-mouth action commands.  Broca’s area in humans is essential for speech, hand gestures and producing fine movements in the fingers.

This and other higher brain areas control Central Pattern Generator circuits in the lower brain which coordinate eating movements and voice control.  The same circuits that operate grasping gestures with the hands also trigger the moth to open and close in humans and other higher primates (the automatic hand to mouth reflex of babies).

Mirror neuron networks in humans interconnect with the insula and amygdala; components of the limbic system that are involved in emotional responses.  Maurizio Gentilucci and colleagues at the University of Parma suggest that mirror neurons which link these components of the emotional brain with higher brain circuits for understanding the intention of food grasping gestures may have enabled our ancestors to associate hand or mouth gestures with an emotional content.  Tagging our observations with an emotional response is how we code our own vocal and other gestures with meaning.

Many primates vocalise upon discovering food.  Gestures then may be a bridge linking body movement to objects and associated vocal sounds.  Hearing but not seeing an event take place allows the hearer to visually construct an idea of the associated experience in their mind.  Once an uttered sound could trigger an associated memory, our hominin ancestors could then revisit that experience.

When we hear or think of words that describe an object or a movement, the same mirror neuron circuits are activated as when we encounter that object or make the movement.  Thinking of the words for walking or dancing also triggers responses in our mirror neuron network that are involved with walking or dancing movements.  When we think of doing something, and then do it, we are literally ‘walking our talk’.

Conclusions

• Words are tools produced by unique sets of movements in the vocal apparatus.  They may have developed in our hominin ancestors as a sound-based form of gesture.

• Studying how our babies learn to speak gives us some insights into how hominins may have made the transition to talking.  Our ancestors’ first word tools may have been parental summoning calls.  The vocal calls of babies assist them to bond strongly with their parents.
• Words inside the brain replicate our physical experience of the phenomena that they symbolise.
• The mental flexibility to agree new sound combinations and associate these with meaning provided our hominin ancestors with a powerful resource of vocal tools that allow us to share our learning.  This ability to share learning has many potentially selectable survival advantages.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Davis, B.L. and MacNeilage, P.F. (1995)  Reconsidering the evolution of brain, cognition, and behaviour in birds and mammals.  Journal of Speech, Language, and Hearing Research 38, 1199-1211.

Eisen, A. et al. (2013)  Tools and talk: an evolutionary perspective on the functional deficits associated with amyotrophic lateral sclerosis.  Muscle and Nerve 49, 469-477.

Falk, D. (2004)  Prelinguistic evolution in early hominins: whence motherese? Behavioural and Brain Sciences 27, 491-541.

Gentilucci, M. and Dalla Volta, R. (2008) Spoken language and arm gestures are controlled by the same motor control system. Quarterly Journal of Experimental Psychology 61, 944-957.

Gentilucci, M. et al. (2008) When the hands speak. Journal of Physiology-Paris 102, 21-30.

Goldman, H.I. (2001)  Parental reports of “mama” sounds in infants: an exploratory study.  Journal of Child Language 28, 497-506.

Jakobson, R. (1960)  Why “Mama” and “Papa”?’  In Essays in Honor of Heinz Werner (R. Jakobson, ed.) pp. 538-545.  Mouton.

Johnson-Frey, S.H. (2003)  What's so special about human tool use?  Neuron 39, 201-204.

Johnson-Frey, S.H. (2004)  The neural bases of complex tool use in humans.  Trends in Cognitive Sciences 8, 71-78.

Jürgens, U. (2002)  Neural pathways underlying vocal control.  Neuroscience and Biobehavioural Reviews 26, 235–258.

King, S.L. and Janik, V.M. (2013)  Bottlenose dolphins can use learned vocal labels to address each other.  Proceedings of the National Academy of Sciences, USA 110, 13216-13221.

King, S. et al. (2013) Vocal copying of individually distinctive signature whistles in bottlenose dolphins. Proceedings of the Royal Society of London, B 280, 20130053.

Lieberman. P. (2006)  Toward an Evolutionary Biology of Language. Harvard University Press.

Lyon, C. et al. (2012)  Interactive language learning by robots: the transition from babbling to word forms.  PLoS ONE 7, e38236.

MacNeilage, P. (2008)  The Origin of Speech. Oxford University Press.

MacNeilage, P.F. and Davis, B.L. (2000)  On the origin of internal structure of word forms.  Science288, 527-531.

MacNeilage, P.F. et al. (2000) The motor core of speech: a comparison of serial organization patterns in infants and languages. Child Development 71, 153–163.

MacNeilage, P.F. et al. (1999)  Origin of serial output complexity in speech.  Psychological Science 10, 459-460.

Matyear, C.L. et al. (1998)  Nasalization of vowels in nasal environments in babbling: evidence for frame dominance.  Phonetica 55, 1-17.

Mitani, J.C. et al. (1992)  Dialects in wild chimpanzees?  American Journal of Primatology 27, 233-243.

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Savage-Rumbaugh, E.S. (1993)  Language comprehension in ape and child.  Monographs of the Society for Research into Child Development 58, 1-222.

Stokoe, W.C. (2001)  Language in hand: Why sign came before speech. Gallaudet University Press.

Tomasello, M. (1999)  The Human Adaptation for Culture.  Annual Review of Anthropology 28, 509-529.

You are out in the dark, alone.  You become aware of a distant rhythmical clicking noise.  Sensing danger, you change direction and head for cover.  Seconds later you are punched by wave after wave of sound that hits your body like a machine gun.  The noise bears down on you, loud as an aircraft, faster and faster, blurring into a roar…. 

    …You are a moth, taken by an echolocating bat.

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Bats aren’t blind.  Their eyes work, but they live in a different world, requiring an extra sense.  Using high frequency sounds, also known as ‘biosonar’ or ‘echolocation’, they can see in the dark.  We see, hear touch, smell and taste by picking up cues in our environment.  Biosonar is completely different; there can be no signal without the call.

This really is seeing in sound.  Biosonar provides bats and some other animals, including dolphins, with a distinctive and sophisticated form of vision, giving detailed three-dimensional information over long distances in darkness or murky waters.

Bats illuminate their world with beams of directional sound

Most bats hunt using a range of frequencies, moving their heads to pulse sounds in different directions as they fly.  Lower frequencies produce wider sound cones like a floodlight, and higher frequencies focus the beam like a spotlight.

Although one lives in the forest and the other in the ocean, the way bats and dolphins use biosonar when hunting is almost identical.  Dolphins, like bats, both shift the frequency of their clicks as they scan and lock onto prey, giving a characteristic ‘buzz’ of rapid calls as they take the target.  Sound travels more quickly under water, so dolphin sonar operates over a longer range.

Bats project sound and receive echoes the way that eyes scan an image

Watch someone’s eyes as they examine something; their gaze jerks from one area to another.  These are known as ‘saccade and fixate’ movements.  They focus different parts of the image onto an area at the back of the eye called the fovea, which sees detail.  ‘Saccade and fixate’ eye patterns are found in all animals with good vision, and is a defining characteristic for how eyes focus on details.

 

When our eyes focus, they project an image onto an area at the back of our eyes known as the ‘fovea’.  This zone is specialised to pick up high levels of detail.  Any sense can be developed to pick up a high level of detail; we use sight, bats use sound, and star-nosed moles use touch.

Most bats hunt insects in the open air, listening for the returning echoes before making their next signal.  They use a combination of head movements and focussed sound cones to gain information in a classic ‘saccade and fixate’ pattern.  Wider head scanning and lower frequency calls provide a ‘wide angle’ view with less detail.  Higher frequency calls and a narrow range of head movements create an ‘acoustic fovea’; this focuses their sonic gaze onto a small area and recovers detail, much like ‘zooming in’ using a macro lens.

 

Horseshoe bats use echolocation in a different way; an adaptation to hunting in and around the spatially variable (and hence acoustically complicated) ‘surface’ of tree and shrub canopies.  They use mostly constant-frequency pulses, and listen for Doppler-shifted returning echoes.  The cochlear membranes of their inner ears respond over a broad frequency range, but have particular sensitivity within a very narrow bandwidth (their acoustic fovea).  When locking the sound pulse onto their prey, they alter their emitted call in order to keep the frequency of the returning echoes constant.  This allows them to focus on the frequency modulations caused by the fluttering movements of large insects.

 Echolocation reveals shape and form using ‘stereo’ images

Just as the brain uses the different view from our two eyes to build a stereo image, our brains use the differences between what is heard by our ears to understand something of the direction and distance of a sound source.  Bats use this difference to understand and construct a ‘stereo image’ of their surroundings in three dimensions.  This enables them to understand something of the shapes and textures of objects in their environment.

Some tropical bats are nectar feeders.  Flowers hidden amongst the leaves are in an acoustically cluttered environment.  Many flowering plants using bats to assist with their pollination have evolved floral structures that act as ‘sound beacons’.  These are usually dish-shaped (parabolic) and bounce back an unique echo pattern that makes them become acoustically visible.

Parabolic shapes also make excellent receivers.  The Search for Extra-Terrestrial Intelligence (SETI) project’s radio telescope is made of many parabolic ‘ears’, listening for radio transmissions from outer space.  Whilst we have invented technologies enabling us to hear bat calls and signals from beyond our planet, our own ears’ convoluted parabolic shape also captures sounds, and funnels them to our receiver; the ‘ear drum’ .

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Fenton, M.B. (2013)  Questions, ideas and tools; lessons from bat echolocation.  Animal Behaviour 85, 869-879.

Connor, W.E. and Corcoran, A. J. (2012)  Sound strategies; the 65 million year old battle between bats and insects.  Annual Review of Entomology 57, 21-39.

Ghose, K. et al. (2006)  Echolocating bats use a nearly time-optimal strategy to intercept prey.  PLoS Biology 4, 865-873.

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Simon et al (2011) ‘Floral acoustics; conspicuous echoes of a disc-caped leaf attract bat pollinators’  Science 333: pp631-633

Schnitzler, H-U. and Denzinger, A. (2011)  Auditory fovea and doppler shift compensation: adaptations for flutter detection in echolocating bats using CF-FM signals.  Journal of Comparative Physiology, A 197, 541-559.

Surlykke, A. et al. (2009)  Echolocating bats emit a highly directional sonar sound beam in the field.  Proceedings of the Royal Society of London, B 276, 853-860.

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