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What makes tones sufficiently useful that it would lead to the evolution of tonal processing?

What makes tones sufficiently useful that it would lead to the evolution of tonal processing?


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I will define a "tone" as a steady periodic sound. As an example, I consider a sinusoidal wave to be a tone. By "tonal processing", I mean ratio relationships between the notes. For instance, if I play 220Hz sine wave then a 440Hz wave will be perceived as an octave higher.

I don't understand why humans have the ability to perceive frequency relationships between tones. I have read several evolutionary anthropology books like The Singing Neanderthal and Finding Our Tongues and, between these two books, they suggest that the obvious evolutionary purpose of music is for communication. Each author takes a slightly different approach but the basic underlying point remains that tonal processing exists because it gives us advantageous communication abilities. But even accepting this hypothesis, it doesn't explain why humans have a tonal processing mechanism that is specific enough to perceive ratio differences between notes. The tonotopic organization of both the basilar membrane and the precortical auditory pathways might partially account for the machinery used for tonal processing and why the advanced forms in music could have developed. But this still leaves us with the question as to why would such information be useful?

I remember reading somewhere one person hypothesized that the purpose of perceiving these differences was to identify "relative pitch", i.e. the ability to identify or re-create a given musical note by comparing it to a reference note and identifying the interval between those two notes. But this hypothesis doesn't really make complete sense to me because it doesn't explain why the brin can have such specificity when identifying the ratio between two notes.

Another phenomena that seems pertinent is the idea of a "drifting reference point" which Glimcher discusses in Foundations of Neuroeconomic Analysis. In Chapter 12, he explains how our perception of color isn't fixed but depends on context and in this sense drifts. IIRC, there is a literature that discusses possible neural mechanisms for this. Perhaps something similar would be relevant here for explaining why humans can process tones so precisely.

My Question:

What are the accepted arguments for why would ratios between tones would be be useful information for social communication?

Just to clarify, I am expecting the answer to this question to involve a discussion of the physical properties of tones and why a hominid brain would find this useful information for communication purposes.


I think the whole issue is that you put all your focus into music. The ability to tell apart different tones hasn't evolved with the culture of music playing. It is much older.

Ability to distinguish different tones in non-human animals

Humans are of course not the only animals that are able to recognize tones. Mice for example can as well accurately tell the difference between two close tones (ref.).

Perceiving its environment

So why is the ability to tell different tones apart important. It is important because it allows one organism to better perceive the elements in its surrounding. For example, consider being in the savanna and you can't tell the difference between a lion roaring and a water fall. It seems to be a pretty important handicap.

Tones recognition in social contexts

A very large part of animal's communication is based on tons. Very primitive forms of language are mostly based on tones. Courtship display for example is often based on the tone of the other gender (the tone is correlated with the size of the organ producing the tone). A very large part of human's communication is based on tone as well. You might want to have a look at Albert Mehrabian.

If you have more questions about the importance of non-verbal communication you will need to ask your questions to psychologists (on CognitiveScience.SE) and not to biologists. Note btw, that the condition of being unable of recognizing different tones is called tone deafness.


In short, I suspect that the answer is not so much that it is useful to recognize tonal relationships, but that it is easy.

It sounds like why or how we can distinguish pitch at all is a separate question which you have already accepted. So let's take as a given that whatever purpose our hearing serves, what we have ended up with is machinery which can usually tell higher tones from lower tones.

Now, it turns out that this by itself is not a simple problem to solve. Real, natural sound is never a simple sinusoidal wave. If you hear a pure sinusoidal wave played over a speaker, it sounds synthetic, like maybe a pitch fork, a singing bowl, or a computer - definitely something man-made. That's because absolutely nothing natural makes a sound like that. Consider any natural thing that for even a moment holds a single tone, like a long tone from within birdsong, or the long tone at the peak of a wolf's howl, or wind whistling through a cavern. If you heard samples of each at exactly the same pitch, they would still be distinguishable, just as a clarinet is distinguishable from a flute, or in fact, to the trained ear, two clarinets are distinguishable from one another. This is because the wave that reaches your ear is actually much more complex than a simple sine wave.

In fact, if you saw the high-fidelity digital wave representation of almost any sound which we would consider to be a single tone of one steady pitch, you might have a lot of trouble identifying the period of that wave (which correlates to it's pitch). In digital processing, to identify the tone in a digital wave, or for that matter, the many tones represented in a piece of music, we run a complicated calculation on the wave called a Fourier Transform.

Our aural machinery came up with a fascinating solution to the problem of distinguishing frequencies. Rather than discovering the mathematical method for calculating a "Fast Fourier Transform", our inner ears contain thousands of stereocilia - little hairs of varying lengths which vibrate at different frequencies. Rather than trying to determine the periodicity of a wave by any sort of analysis, these hairs automatically vibrate when any frequency within the wave resonates precisely with their length. This allows us to hear the various frequencies within the sound wave that reaches our ear, so that we can take in the whole ensemble of our environment.

Now here's the catch. A hair that vibrates at exactly 440Hz, also vibrates pretty well at 220Hz. In a sense, this actually represents a weakness in our hearing strategy. Even though the one frequency is twice as fast as the other, they actually sound a lot alike to us. It is harder for us to distinguish middle C (C4) from the C an octave above (C5) than it is for us to distinguish middle C from the D-flat right above it. We can still recognize the difference, but when we do, part of what we recognize is their likeness. Other intervals share the same characteristic in different ways. For example, double a frequency is one octave higher, but three times that same frequency is a fifth above that.

So, from an evolutionary perspective, this is a coincidence of the handy solution found to distinguish tones. However, the joy we find in music is so serendipitous as to beg for a more divine explanation, which you will find echoed throughout our history and surely our future, and within any moment in which you find yourself appreciating the beauty of well-tempered harmony.


Did ancient Chinese have tones (声调)?

This question has puzzled me since I have no training in ancient Chinese and ancient phonetics. What is puzzling me are the following facts:

The tones in different regions are quite different. If they are from Ancient Chinese, why are they not more consistent?

When Chinese characters were borrowed by Japanese and Korean around 1000 years ago, the tones are not picked up by their languages. Did Chinese actually have fixed tones at that time, or not?

Ancient Chinese had codas (Cantonese codas like [p], [t], [k]) but modern Chinese doesn't. Is it possible that the information in the codas is transformed to tones? Otherwise, a lot of information would have been lost.


Understanding the Roots of Human Musicality

Catherine Offord
Mar 1, 2017

© ISTOCK.COM/LIUDMYLA SUPNYSKA G etting to Santa María, Bolivia, is no easy feat. Home to a farming and foraging society, the village is located deep in the Amazon rainforest and is accessible only by river. The area lacks electricity and running water, and the Tsimane&rsquo people who live there make contact with the outside world only occasionally, during trips to neighboring towns. But for auditory researcher Josh McDermott, this remoteness was central to the community&rsquos scientific appeal.

In 2015, the MIT scientist loaded a laptop, headphones, and a gasoline generator into a canoe and pushed off from the Amazonian town of San Borja, some 50 kilometers downriver from Santa María. Together with collaborator Ricardo Godoy, an anthropologist at Brandeis University, McDermott planned to carry out experiments to test whether the Tsimane&rsquo could discern certain combinations of musical tones, and whether they preferred some over others. The pair wanted to.

“Particular musical intervals are used in Western music and in other cultures,” McDermott says. “They don’t appear to be random—some are used more commonly than others. The question is: What’s the explanation for that?”

TSIMANE’ TESTS: Ricardo Godoy of Brandeis University tests the musical preferences of a Tsimane’ woman in Santa María, Bolivia. JOSH MCDERMOTT Ethnomusicologists and composers have tended to favor the idea that these musical tendencies are entirely the product of culture. But in recent years, scientific interest in the evolutionary basis for humans’ musicality—our capacity to process and produce music—has been on the rise. With it has come growing enthusiasm for the idea that our preference for consonant intervals—tonal combinations considered pleasant to Western ears, such as a perfect fifth or a major third—over less pleasant-sounding, dissonant ones is hardwired into our biology. As people with minimal exposure to Western influence, the Tsimane’ offered a novel opportunity to explore these ideas.

If these properties are absent in some cultures, they can’t be strictly determined by something in the biology. —Josh McDermott, MIT

Making use of the basic auditory equipment they’d brought by canoe, McDermott and his colleagues carried out a series of tests to investigate how members of this community responded to various sounds and musical patterns. The team found that although the Tsimane’ could distinguish consonance from dissonance, they apparently had no preference for one over the other. McDermott interprets the results as evidence against a strong biological basis for preference. 1 “If these properties are absent in some cultures, they can’t be strictly determined by something in the biology—on the assumption that the biology in these people is the same as it is in us,” he says.

But the authors’ publication of their results proved controversial. While some took the findings to imply that culture, not biology, is responsible for people’s musical preferences, others argued that the dichotomy was a false one. Just because there’s variation in perception, it doesn’t mean there’s no biological basis, says Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Vienna. “Almost everything has a biological basis and an environmental and cultural dimension,” he says. “The idea that those are in conflict with one another, this ‘nature versus nurture,’ is just one of the most consistently unhelpful ideas in biology.”

Identifying the biological and cultural influences on humans’ musicality is one of various thorny issues that researchers working on the cognitive science of music are currently tackling. The field has exploded in recent years, and while many answers have yet to materialize, “the questions have been clarified,” says Fitch, who was one of more than 20 authors contributing to a special issue of Philosophical Transactions B on the subject in 2015. For example, “rather than talking about the evolution of music, we’re talking now about the evolution of musicality—a general trait of our species. That avoids a lot of confusion.”

Researchers are beginning to break this trait into various components such as pitch processing and beat synchronization (see Glossary) addressing the function and evolution of each of these tasks could inform the broader question of where humans’ musicality came from. But as illustrated by the discussions following McDermott’s recent publication, it’s clear just how much remains mysterious about the biological origins of this trait. So for now, the debates continue.

A mind for music?

MAPPING MUSIC: Huge areas of the brain respond to any sort of auditory stimulus, making it difficult for scientists to nail down regions that are important specifically for music processing. Functional magnetic resonance imaging (fMRI) studies have taken diverse approaches to pinpointing areas involved in musical perception, providing “musical” stimuli ranging from human singing to synthesized piano melodies and other computer-generated sounds, and yielding equally varied results. Despite these hurdles, research is beginning to offer some clues about the regions of the brain involved in musical perception.
See full infographic: WEB | PDF © CATHERINE DELPHIA Musical faculties don’t fossilize, so there’s little direct evidence of our musical past (see Time Signatures). But researchers may find clues in the much older study of another complex cognitive trait: speech perception. “Music and language are both sound ordered in time they both have hierarchical structure they’re in all cultures and they’re very complex human activities,” says Fred Lerdahl, a composer and music theorist at Columbia University. “A lot of people, including me, think that music and language have, in some respects, a common origin.”

Numerous lines of evidence have supported this view. For example, Tufts University psychologist Ani Patel and colleagues showed a few years ago that patients with congenital amusia, a neurodevelopmental disorder of musical perception commonly known as tone deafness, also had difficulty perceiving intonation in speech. 2 (See “Caterwauling for Science.”) And fMRI scans of normally hearing volunteers listening to recordings have revealed that large areas of the brain’s temporal lobes—regions involved in auditory processing—show heightened activation in response to both music and speech, compared with nonvocal sounds or silence. 3 For many, these findings hint at the possibility of common neural circuitry for the processing of speech and music.

But other research points to dissociated processing for at least some components of music and language, suggesting that certain parts of the brain specialized in musicality during our evolution. Lesion studies, for example, show that brain damage can disrupt the processing of pitch in music without disrupting pitch processing in speech. 4 And multivariate neuroimaging analyses with higher sensitivity than traditional methods indicate that, despite stimulating overlapping regions of the cortex, recordings of music and speech activate different neural networks. 5 “People may take localization of activity as evidence for sharing,” notes Isabelle Peretz, a neuropsychologist at the University of Montreal. But given the low resolution of most current methods, “that’s nonsense, of course.”

McDermott’s lab recently reported more extreme dissociation. Using a novel approach to analyze fMRI data from people listening to more than 150 recordings of speech, music, nonverbal vocalizations, or nonvocal sounds, the team identified anatomically distinct pathways in the auditory cortex for speech and for music, along with other regions of the brain that responded selectively to each. 6 “We find that they’re largely anatomically segregated,” McDermott says. “Speech selectivity seems to be located primarily lateral to primary auditory cortex, while music [selectivity] is localized mostly anterior to it.”

The neural processing mechanisms themselves remain elusive, but studies like McDermott’s “clearly demonstrate that you can separate the representations for speech and music,” says Peretz. All the same, she notes, with current research continuing to present evidence both for and against a shared neural basis for music and speech perception, “the debate is still on.”

Another way researchers hope to throw more light on how the human brain has become tuned for musical perception is by looking at people’s DNA. “For me, [genetics] is the only way to study the evolutionary roots of musicality,” says Irma Järvelä, a medical geneticist at the University of Helsinki. In recent years, Järvelä’s group has researched genome-wide association patterns in Finnish families. In a preliminary study published last year, the team used standard music-listening tests to characterize participants as having either high or low musical aptitude, and identified at least 46 genomic regions associated with this variation. 7 “We asked, what are the genes in these regions, and are these genes related to auditory perception?” she explains. In addition to homologs of genes associated with song processing and production in songbirds, the researchers identified genes previously linked with language development and hearing.

Further clues about musicality’s genetic basis could come from the study of amusia. In 2007, Peretz and colleagues reported that congenital amusia runs in families. 8 And recent descriptions of high amusia incidence in patients with genetic diseases such as Williams-Beuren syndrome, a condition associated with deletion of up to 28 genes on chromosome 7, may lead researchers to additional musicality-linked genes. 9 “We are making progress along these lines, but there’s a lot more to be done,” says Peretz. “It’s really hard to do, and more expensive than neuroimaging. So we have to be patient.” But it’s progress worth waiting for, she adds, as an understanding of the genetics contributing to particular musical—or amusical—phenotypes could offer an entirely new perspective on the biological basis for musicality.

Music’s universality in humans, combined with its fundamental social and cultural roles, is convincing evidence to some that our musicality is adaptive.

Meanwhile, some researchers advocate looking to related species to answer questions about the origins of human musicality. Although nonhuman primates share our ability to distinguish between consonance and dissonance, many apes and monkeys have surprisingly different auditory processing. “Things that are fundamental to music that people thought would be ancient, general aspects of how animals process sound turn out not to be, and potentially reflect specialization in our brains,” says Patel. For example, the ability to synchronize movement to a beat, a capacity central to music, “doesn’t come naturally to our closest living relatives,” says Patel, though he adds that “it does come quite naturally to some other species,” including parrots, seals, and elephants. (See “John Iversen: Brain Beats.”)

Similarly, vocal learning—potentially a requirement for musicality—is known to be prevalent in several taxa, including some species of songbirds, parrots, whales, seals, bats, and elephants, but it is not well documented in any primate other than humans. (See “Singing in the Brain.”) “It raises the question of why,” Patel says. “What basic features of music perception are shared with other species, and what does that tell us about the evolution of those features?”

Why music?

© ISTOCK.COM/PEOPLEIMAGES As researchers continue to probe how humans have evolved to process music, many scientists, and the public, have been increasingly drawn to another question concerning musicality’s origins: Why did it evolve at all? For some, music’s universality in humans, combined with its fundamental social and cultural roles, is persuasive evidence that our musicality is adaptive. “Music is so common in all societies,” says Helsinki’s Järvelä. “There must be favorable alleles it must be beneficial to humans.”

But just what this benefit might be, and whether it did indeed influence our evolution, have been the objects of what Patel calls “one of the oldest debates in the book.” In the late 1990s, cognitive psychologist Steven Pinker famously dubbed music “auditory cheesecake”—pleasant, but hardly essential—and argued that musicality was nothing more than a by-product of neural circuitry evolved to process language and other auditory inputs. It’s become the argument to beat for researchers looking for ultimate explanations of musicality’s evolution in humans, Fitch says. “Everybody seems to want to prove that Pinker’s cheesecake argument is wrong,” he notes. “But it’s just the null hypothesis.”

One adaptationist viewpoint, that traces its roots to Darwin, is that human musicality, like birdsong, is a sexually selected trait—albeit an unusual one, prevalent as it is in both sexes. Musicality is a reliable and visible indicator of cognitive ability, the argument goes, and so informs a potential mate of an individual’s genetic quality. Some researchers have tried to generate testable predictions from this idea, but so far there’s been little evidence in its favor. One recent study went as far as assessing the self-reported sexual success—based on indicators including the number of sex partners and age at first intercourse—of more than 10,000 pairs of Swedish twins. 10 The researchers found no association between musical ability and sexual success, but cautioned against being quick to draw conclusions about the sexual relationships of our evolutionary ancestors from modern society.

Other hypotheses arise from research on music’s far more complex and still poorly understood effects on human emotion and social bonding. University of Toronto psychologist Sandra Trehub notes, for example, that babies and young children are particularly sensitive to musical communication, and that singing comes naturally to adults interacting with them. “Caregivers around the world sing to infants,” she says. “It’s not a Western phenomenon, nor a class-based phenomenon. It seems to be important for caregiving everywhere.”

She and her colleagues recently showed that recordings of singing, more so than speech, could delay the time it took for an infant to become distressed when unable to see another person. 11 And in 2014, research led by Laurel Trainor at McMaster University found that when babies just over a year old were bounced to music, they became more helpful towards a researcher standing opposite them who had been bopping along in rhythm (handing back “accidentally” dropped objects) than to people who had been bouncing asynchronously. 12

Are musical tendencies the prod­uct of culture, or have they evolved along with our abilities to produce and process music?

These and related findings have led some to propose that parent-infant bonding, or? social cohesion in general, provided a selective pressure that favored the evolution of musicality in early humans, though Trehub herself says she does not subscribe to this rather speculative view. “I have no difficulty imagining a time when music-like things would have been very important in communicating global notions and managing interpersonal relationships,” she says. “But it’s pretty hard, based on anything we look at now, to relate it to conditions in ancient times and the functions it would have served.”

Indeed, the inherent challenge of studying ancient hominin behavior, combined with the complexity of the trait itself, makes explanations for musicality’s evolution particularly vulnerable to “just-so” stories, says Trainor. “When you look at the effect that music has on people, it’s easy to think it must have been an evolutionary adaption. Of course, it’s very difficult, if not impossible, to prove that something is an evolutionary adaption.”

This intractability has led some researchers to view adaptation-based lines of inquiry into human musicality as something of a distraction. “I don’t think it’s a particularly useful question at all,” says Fitch. “It’s an unhealthy preoccupation, given how little we know.” Others have argued for a subtler view of musicality’s evolution that avoids the search for simple answers. “The evolutionary process isn’t a one-shot thing,” says Trainor. “It has many nuanced stages.”

Her work, for example, addresses how aspects of auditory scene analysis—the process by which animals locate the source of sounds in space—could have led to features currently viewed as critical for musicality in modern humans. But that doesn’t mean that music didn’t provide its own benefits once it arose. “I think parts of the long road to our becoming musical beings were driven by evolutionary pressures [for music itself],” says Trainor, “and other parts of it were driven by evolutionary pressures for things other than music that music now uses.”

But most researchers agree that understanding our musical evolution will require studying musicality in more-focused and biologically relevant ways. For example, instead of asking why musicality evolved, Fitch suggests researchers investigate why humans evolved to synchronize their movements to a beat. This approach “is what’s really important,” says Patel. “We’ve had hundreds of years of speculation. Now, I think, the real advances are being made by thinking about the individual components of music cognition and looking at them in an evolutionary framework.”

© SASCHA SCHUERMANN/AFP/GETTY IMAGES © ISTOCK.COM/ANGEAL Without physical evidence of ancient humans’ musical perception, researchers look for signs of our capacity to produce music to approximate the timescale of musicality’s evolution. One way to do this is through archaeology. The oldest undisputed musical instruments are bone flutes (pictured at right) found in caves in Germany that have been dated as more than 40,000 years old (J Hum Evo, 62:664-76, 2012). But many researchers argue that the use of the voice as an instrument likely came much earlier than that.

To put an upper limit on the age of vocal musicality, some have turned to human anatomy. Producing complex vocalizations requires both a powerful brain and specialized vocal machinery. During hominin evolution, for example, the thorax become more innervated, a change that allowed humans (and Neanderthals) to more effectively control the pitch and intensity in their vocalizations. The fossil record indicates that the first hominins with breath control like ours lived a maximum of 1.6 million years ago, which some suggest marks the first time our lineage would have been physically capable of producing vocalizations resembling singing (Am J Phys Anthropol, 109:341-63, 1999).

Genetics might also help researchers pin down when certain components of musicality appeared in our ancestors, if parts of our DNA can be linked to our capacity for perceiving and processing music. For now, however, the question of when humans first produced something we might recognize as music remains open to speculation.


To establish a modal final (which is what we call a mode's melodic keynote) as a tonic, you've really got three things that you will use:

  1. a hierarchy of root relationships within the mode that confirms the tonic
  2. conjunct melodic motion, notably by semitone, that pushes into the tonic and
  3. the stability of the most important harmonies within the mode.

The root motion that confirms a tonic most strongly is 5-1, because the chord on 1 tends to subsume the chord on 5, the overtones of 5 being also overtones of 1. Thus movement from 1 to 5 tends to create tension movement from 5 to 1 resolves it.

Root movement from 1 to 4 tends somewhat to undercut 1 for that reason: 4 subsumes 1 and acts like a resolution, so there is a balancing act involved. This is why the subdominant tends to be used as a pre-dominant, e.g., in I-IV-V-I, and why plagal (IV-I) cadences sound less conclusive than authentic (V-I) cadences. In I-IV-V-I, the relaxation of the move to the subdominant is balanced by a tension-creating move to dominant, and the final drop of a fifth confirms the tonic. The subdominant, however, is still closely related harmonically to the tonic, and, when used with materials that tend to move very strongly to the dominant, serves to counterbalance the dominant and confirm the tonic. (This is why you will often see references to the subdominant key in the recapitulations of sonatas whose expositions featured a strong move to the dominant key.)

Movement to other roots is less powerful than these root movements to a varying degree, so there is a hierarchy of strength of root movements involved.

Points 2 & 3 are fairly obvious, I think. 7-1 melodic movement (e.g., B-C in the key of C) is very strong indeed, and you already know how unstable the tonic triad is in Locrian mode.

But it is the degree that these elements line up and work together that determines how easy or rough a time you'll have tonicising the modal final, and what steps you'll need to take.

For instance, the Ionian (major) mode does make things fairly easy: the I, IV and V chords are all very stable (all major) the root relationships between I on the one hand and IV & V on the other are all perfect intervals the third of V is 7 (the leading tone) and the mode's other melodic semitone interval, 3-4, forms on the one hand a rising leading tone relationship between the third of I and the root of IV, and a falling semitone relationship between the dominant's seventh and the third of the tonic. Even the location of the mode's diminished triad is fortuitous: vii° forms the top notes of the dominant seventh chord, and makes that chord unstable enough to demand a resolution to the tonic.

Now let's consider the other end of the spectrum of commonly used modes, the Phrygian mode.

In Phrygian, the melodic semitones are between 1 and ♭2, and 5 and ♭6. This means the key degrees are i and iv (both minor), and v° (diminished).

Because of the tritone, v° is deeply ambiguous: the root of the chord isn't well-established. In the Ionian (major) mode, because it contains the leading tone and the falling seventh, the diminished chord (vii°) can stand in for a dominant 7th that's missing its root in the Phrygian mode, it is going to suggest that same Ionian dominant seventh (which, of course, wants to resolve to the Ionian tonic a minor third below the Phrygian final), and thus undercut the tonic. Also, it only contains the falling leading tone, ♭2.

So, to establish the Phrygian final as tonic, you have to use what the mode gives you: a reasonably stable subdominant and a falling leading tone. Note that the two don't go together, so you have two kinds of cadences to lead to the tonic: a fairly standard iv-i plagal cadence, and a cadence that features ♭2-1 in the bass. In pop music, the latter cadence is often ♭II-i, but that doesn't make for great voice leading using common practice harmony, so in Classical music, you use the so-called Phrygian cadence, ♭vii 6 -i.

You do have an option of musica ficta, which is the introduction of chromatic notes into a mode to create leading tones, but you have to be careful about how you go about it, or you will either undercut the tonality or undercut the mode. In the Phrygian mode, you can sharp the sixth of ♭vii 6 to produce an augmented (Italian) sixth chord that has both the rising and falling leading tones. You don't sharp ♭2 in the bass, or you lose the sense of being in the Phrygian mode.

Other modes have their own "gotchas" - in Lydian mode, for instance, you have to watch carefully that ♯4 doesn't tonicise the dominant - but, with care and an analysis of the mode's characteristics, you can stabilise a modal final as a tonic, so the stability of the various modes is relative, and, as often as not, it is as much relative to your ability to work out the requirements of a given mode as it is relative to the other modes.

That's even true of Locrian mode (see Locrian Harmony), but make no mistake: Locrian is very, very difficult to stabilise. (I actually wrote a piece to study the ramifications of the mode when I answered that question, here. I think I succeeded in making the final B major chord a logical ending for the piece, but that was one tough exercise.)


Tonal languages require humidity

The weather impacts not only upon our mood but also our voice. An international research team including scientists from the Max Planck Institutes for Psycholinguistics, Evolutionary Anthropology and Mathematics in the Sciences has analysed the influence of humidity on the evolution of languages. Their study has revealed that languages with a wide range of tone pitches are more prevalent in regions with high humidity levels. In contrast, languages with simpler tone pitches are mainly found in drier regions. This is explained by the fact that the vocal folds require a humid environment to produce the right tone.

Languages in humid regions of the Earth (light circles) are more often tonal languages (red) than in dry regions.

© MPI f. Psycholinguistics/ Roberts

The tone pitch is a key element of communication in all languages, but more so in some than others. German or English, for example, still remain comprehensible even if all words are intonated evenly by a robot. In Mandarin Chinese, however, the pitch tone can completely change the meaning of a word. “Ma” with a level pitch means “mother,” while “ma” with a falling then rising pitch would mean “horse”. “Only those who hit the tone pitch correctly can express themselves in tonal languages,” explains Seán G. Roberts, a scientist at the Max Planck Institute for Psycholinguistics in Nijmegen.

However, the climate can become a problem for the speakers of tonal languages, as the vocal folds in the larynx – commonly known as the voice box – suffer as a result. Even a temporary increase in humidity impacts upon the vocal folds: The humidity keeps the mucous membranes moist and makes them more elastic. It also changes the ion balance within the mucous membranes of the vocal folds. With good humidity, the vocal folds can oscillate sufficiently and produce the right tone.

The scientists therefore suspect that tonal languages are less common in dry regions as the wide range of tonal pitches is difficult to produce under these conditions and are more likely to result in misunderstanding. “Modern databases enable us to analyse the properties of thousands of languages. But this also brings problems because languages can also inherit their complex pitches from another language,” says Damián E. Blasi, who conducts research at the Max Planck Institutes for Mathematics in the Sciences and for Evolutionary Anthropology in Leipzig. In their study, the scientists have now shown that these effects can be disentangled from the effects of climate.

The researchers investigated the correlation between humidity and the significance of tone pitch in over 3,750 languages from different linguistic families. This indicates that tonal languages are significantly rarer in dry regions. In relatively dry Central Europe, no tonal languages have developed like those found in the Tropics, Subtropical Asia and Central Africa.

Climate apparently shapes the role of pitch tone in a language and therefore how information is exchanged. Even small effects may be amplified over the generations to produce a global pattern. The climate thus determines the development of languages. “If the UK had been a humid jungle, English may also have developed into a tonal language,” explains Roberts.


SPECIAL TOPIC: SKIN TONE GENETIC REGULATION

The melanocortin 1 receptor (MC1R) gene acts to control which types of melanin (eumelanin or pheomelanin) are produced by melanocytes. The MC1R receptor is located on the surface of the melanocyte cells (Quillen et al. 2018). Activation of the MC1R receptors may occur through exposure to specific environmental stimuli or due to underlying genetic processes. Inactive or blocked MC1R receptors results in melanocytes producing pheomelanin. If the MC1R gene receptors are activated, then the melanocytes will produce eumelanin. Thus, individuals with activated MC1R receptors tend to have darker pigmented skin and hair than individuals with inactive or blocked receptors.

The alleles of another gene, the major facilitator, superfamily domain-containing protein 12 (MFSD12) gene, affect the expression of melanocytes in a different way than the MC1R gene. Instead of affecting the activation of melanocyte receptors, the MFSD12 alleles indirectly affect the membranes of melanocyte lysosomes (Quillen et al. 2018). The melanocyte’s lysosomes are organelles containing digestive enzymes, which ultimately correlate to varying degrees of pigmentation in humans. Variations in the membranes of the melanocyte lysosomes ultimately correlate to differing degrees of pigmentation in humans.

Ancestral MFSD12 allele variants are present in European and East Asian populations and are associated with lighter pigmentation of the skin (Crawford et al. 2017 Quillen et al. 2018). In addition, this ancestral variant is also associated with Tanzanian, San, and Ethiopian populations of Afro-Asiatic ancestry (Crawford et al. 2017 Quillen et al. 2018). In contrast, the more-derived (i.e. more recent) allele variants that are linked to darker skin tones are more commonly present in East African populations, particularly those of Nilo-Saharan descent (Crawford et al. 2017 Quillen et al. 2018). The notion that ancestral alleles of MFSD12 are associated with lighter skin pigmentation is in opposition to the commonly accepted idea that our pigmentation was likely darker throughout early human evolution (Crawford et al. 2017 Quillen et al. 2018). Due to the complexity of the human genome, MFSD12 and MC1R are but two examples of alleles affecting human skin tone. Furthermore, there is genetic evidence suggesting that certain genomic variants associated with both darker and lighter skin color have been subject to directional selection processes for as long as 600,000 years, which far exceeds the evolutionary span of Homo sapiens sapiens (Crawford et al. 2017 Quillen et al. 2018). So, evolutionary processes may lead to skin becoming more darkly pigmented as well as more lightly pigmented.

Adaptation: Melanogenesis

Figure 14.11 Penetration of skin layers by UVA and UVB rays.

Although all humans have approximately the same number of melanocytes within the epidermis, the production of melanin by these melanocytes varies. There are two forms of melanogenesis (the process through which melanocytes generate melanin): basal and activated. As discussed previously, the expression of eumelanin and pheomelanin by the melanocytes is genetically regulated through the expression of specific receptors (e.g., MC1R) or other melanocyte components (e.g., MFSD12). Basal melanogenesis is dependent upon an individual’s inherent genetic composition and is not influenced by external factors. Activated melanogenesis occurs in response to ultraviolet radiation (UV) exposure, specifically UV-B (short UV wave) exposure. Increased melanogenesis in response to UV-B exposure serves to provide protection to the skin’s innermost layer called the hypodermis, which lies below the epidermis and dermis (Figure 14.11). Melanin in the skin, specifically eumelanin, effectively absorbs UV-B radiation from light meaning that it will not reach the hypodermal layer. This effect is often more apparent during periods of the year when individuals tend to be outside more and the weather is warmer, which leads to those individuals donning fewer protective garments. The exposure of skin to sunlight is, of course, culturally mediated with some cultures encouraging the covering of skin at all times.

As previously noted, hemoglobin is an iron-rich protein that binds with oxygen in the bloodstream. For individuals with lighter-colored skin, blood vessels near the surface of the skin and the hemoglobin contained within those vessels is more apparent than in individuals with darker skin. The visible presence of hemoglobin coupled with the pink-to-red tone of the pheomelanin leads to lighter-skinned individuals having a pale pink skin tone. Individuals with lighter skin more readily absorb UV radiation as their basal melanin expression is directed more toward the production of pheomelanin than eumelanin. But, why are there so many variations in skin tone in humans? To answer this question, we now turn toward an exploration of an evolutionary-based adaptation of skin tone as a function of the environment.

Adaptation: Evolutionary Basis for Skin Tone Variation

Figure 14.12 Evolutionary basis for human skin color variation.

Skin cancer is a significant concern for many individuals with light skin tone as the cumulative exposure of the epidermis and underlying skin tissues to UV radiation may lead to the development of abnormal cells within those tissues leading to malignancies. Although darker-skinned individuals are at risk for skin cancer as well, they are less likely to develop it due to increased levels of melanin, specifically eumelanin, in their skin. Even though skin cancer is a serious health concern for some individuals, most skin cancers occur in the post-reproductive years therefore, it is improbable that evolutionary forces favoring varying melanin expression levels are related to a selective pressure to avoid such cancers. Furthermore, if avoiding skin cancer were the primary factor driving the evolution of various skin tones, then it reasons that everyone would have the most significant expression of eumelanin possible. So, why do we have different skin tones (Figure 14.12)? The term cline refers to the continuum or spectrum of gradations (i.e., levels or degrees) from one extreme to another. With respect to skin tone, the various tonal shades occur clinally

with darker skin being more prevalent near the equator and gradually decreasing in tone (i.e., decreased melanin production) in more distant latitudes. For individuals who are indigenous to equatorial regions, the increased levels of melanin within their skin provides them with a measure of protection against both sunburn and sunstroke as the melanin is more reflective of UV radiation than hemoglobin. In cases of severe sunburn, eccrine glands are affected, resulting in an individual’s ability to sweat being compromised. As sweat is the body’s most effective means of reducing its core temperature to maintain homeostasis, damage to the eccrine glands may lead to numerous physiological issues related to heat that may ultimately result in death.

Even though avoiding severe sunburn and sunstroke is of great importance to individuals within equatorial regions, this is likely not the primary factor driving the evolutionary selection of darker skin within these regions. It has been proposed that the destruction of folic acid , which is a form of B-complex vitamin, by UV radiation may have led to the selection of darker skin in equatorial regions. For pregnant women, low levels of folic acid within the body during gestation may lead to defects in the formation of the brain and spinal cord of the fetus. This condition, which is referred to as spina bifida, often significantly reduces the infant’s chances of survival without medical intervention. In men, low levels of folic acid within the body lead to an inhibition in the production of sperm. Thus, in geographic regions with high UV radiation levels (i.e., equatorial regions), there appears to be an evolutionarily driven correlation between darker skin and the maintenance of fertility.

If darker skin tone is potentially correlated to more successful reproduction, then why do lighter shades of skin exist? One hypothesis is that there is a relationship between lighter skin tone and vitamin D synthesis within the body. When skin is exposed to the UV-B radiation waves in sunlight, a series of chemical reactions occur within the epidermis leading to the production of vitamin D3. Before the body can use vitamin D3, it must travel to the liver and then to the kidneys where it is converted into different forms of bioactive molecules. Ultimately, it is converted into the bioactive molecule calcitriol (Vukić et al. 2015). Within the human body there are numerous cell types with binding receptors for calcitriol, so it is capable of adhering to the DNA of those cells (Snoddy et al. 2016). Calcitriol serves as a regulator in cellular-replication processes within the body, including those for pancreatic, breast, colon, and kidney cells (Snoddy et al. 2016). Insufficient calcitriol is associated with an increased risk of: some forms of cancer (colon, prostate, etc.), autoimmune diseases (multiple sclerosis, lupus, type I diabetes, etc.), cardiovascular diseases, and infections (e.g., tuberculosis, influenza) (Snoddy el al. 2016 Chaplin and Jablonski 2009). Deficiencies in calcitriol production and absorption within the human body may be linked to underlying genetic factors, such as a mutation in the vitamin D receptors present in some of the body’s cells (Chaplin and Jablonski 2009). Alternatively, it may be linked to inadequate exposure to the UV-B rays necessary to stimulate calcitriol production or to a nutritional deficiency in vitamin D-rich foods. Regardless of the cause of the deficiency, individuals with a calcitriol (vitamin D3) deficiency may also be at risk for the development of certain skeletal abnormalities in addition to the previously mentioned health issues.

Figure 14.13 Children with rickets in various developmental stages.

Vitamin D is required for the absorption of certain nutrients, such as calcium and phosphorus, in the small intestine. These nutrients are among those that are critical for the proper growth and maintenance of bone tissue within the body. In the absence of adequate minerals, particularly calcium, bone structure and strength will be compromised leading to the development of rickets during the growth phase. Rickets is a disease affecting children during their growth phase and is characterized by inadequately calcified bones that are softer and more flexible than normal. Individuals with rickets will develop a true bowing of their femora, which may affect their mobility (Figure 14.13). In addition, deformation of pelvic bones in women may occur as a result of rickets leading to complications with reproduction. In adults, a deficiency in vitamin D will often result in osteomalacia, which is a general softening of the bones due to inadequate mineralization. This softening is the result of impaired bone metabolism that is primarily linked to insufficient levels of bioavailable vitamin D, calcium, and phosphate. In addition, it may be linked to inadequate absorption of calcium in the bloodstream. As noted, a variety of maladies may occur due to the inadequate production or absorption of vitamin D, as well as the destruction of folate within the human body so, from an evolutionary perspective, natural selection should favor a skin tone that is best suited to a given environment.

In general, the trend related to lighter skin pigmentation further from the equator follows a principle called Gloger’s Rule . This rule states that within the same species of mammals the more heavily pigmented individuals tend to originate near the equator while lighter-pigmented members of the species will be found in regions further from the equator. Gloger’s Rule applies latitudinally however, it does not appear to hold for certain human populations near the poles. Specifically, the Inuit people (Figure 14.14), who are indigenous to regions near the North Pole and currently reside in portions of Canada, Greenland, Alaska, and Denmark. The Inuit have a darker skin tone that would not be anticipated under the provisions of Gloger’s Rule. The high reflectivity of light off of snow and ice, which is common in polar regions, necessitates the darker skin tone of these individuals to prevent folic acid degradation just as it does for individuals within equatorial regions. The consumption of vitamin D–rich foods, such as raw fish, permits the Inuit to reside at high latitudes with darker skin tone while preventing rickets.

Figure 14.14 Inuit family, 1917.

Genome studies have identified a number of genes (TYR, OCA2/HERC2, TYRP1, SLC45A2, HPS6i, etc.) related to the expression of melanin and pigmentation presentation in humans. Compared to the exceptionally large number of genes within the human genome, those regulating the expression of melanin are relatively few and appear on distinct loci. The genes at these loci are generally pleiotropic in nature, so there is a relatively predictable patterning in skin, hair, and eye color combinations (Sturm and Duffy 2012). For example, some populations that are indigenous to higher latitude regions tend to have lighter skin, hair, and eye color than their counterparts from equatorial regions. Still, since the genes affecting skin, hair, and eye color are actually independent, it is possible that variations may produce many phenotypic combinations. Turning again to our example of individuals indigenous to higher latitudes, it is theoretically possible to encounter an individual with dark hair, light-toned skin, and blue eyes within this region due to the variability of phenotypic combinations.

Adaptation: Shape and Size Variations

In addition to natural selection playing a role in the determination of melanin expression related to skin tone, which is correlated to the environment, it plays a significant role in the determination of the shape and size of the human body. As previously discussed, the most significant thermodynamic mechanism of heat loss from the body is radiation. At temperatures below 20℃ (68℉), the human body loses around 65% of its heat to radiative processes however, the efficiency of radiation as a means of heat reduction is correlated to the overall body shape and size of the individual. There is a direct correlation between the ratio of an object’s surface area to mass and the amount of heat that may be lost through radiation. For example, two metal objects of identical composition and mass are heated to the same temperature. One object is a cube and the other is a sphere. Which object will cool the fastest? Geometrically, a sphere has the smallest surface area per unit mass of any three-dimensional object, so the sphere will cool more slowly than the cube. In other words, the smaller the ratio of the surface area to mass an object has, the more it will retain heat. With respect to the cube in our example, mass increases by the cube, but surface area may increase only by the square, so size will affect the mass to surface area ratio. This, in general, holds true for humans, as well.

In regions where temperatures are consistently cold, the body shape and size of the individuals who are indigenous to the area tend to be more compact. These individuals have a relatively higher body mass to surface area (i.e., skin) than their counterparts from equatorial regions where the average temperatures are considerably warmer. Individuals from hot climates, such as the Fulani (Figure 14.15a) of West Africa, have limbs that are considerably longer than those of individuals from cold climates, such as the Inuit of Greenland (Figure 14.15b). Evolutionarily, the longer limbs of individuals from equatorial regions (e.g., the Fulani) provide a greater surface area (i.e., lower body mass to surface area ratio) for the dissipation of heat through radiative processes. In contrast, the relatively short limbs of Arctic-dwelling people, such as the Inuit, allows for the retention of heat as there is a decreased surface area through which heat may radiate away from the body.

Figure 14.15a The Fula people of Burkina Faso (pictured here in 1974) are from a tropical environment where the rapid dispersal of heat is necessary to maintain homeostasis. Figure 14.15b These Inuit people from Greenland live in an arctic environment where the conservation of heat in the body’s core is of critical importance. Figure 14.16a These organisms are representative of Bergmann’s rule. The animal on the left depicts an ungulate from a cooler environment with increased body weight and decreased surface area, compared to the slender ungulate on the right.

As described above, there are certain trends related to the general shape and size of human bodies in relation to the thermal conditions. To better describe these trends, we turn to a couple of general principles that are applicable to a variety of species beyond humans. Bergmann’s Rule predicts that as average environmental temperature decreases, populations are expected to exhibit an increase in weight and a decrease in surface area (Figure 14.16a). Also, within the same species of homeothermic animals, the relative length of projecting body parts (e.g., nose, ears, and limbs) increases in relation to the average environmental temperature (Figure 14.16b). This principle, referred to as Allen’s Rule , notes that longer, thinner limbs are advantageous for the radiation of excess heat in hot environments and shorter, stockier limbs assist with the preservation of body heat in cold climates. A measure of the crural index (crural index=tibia length [divided by] femur length) of individuals from various human populations provides support for Allen’s Rule since this value is lower in individuals from colder climates than it is for those from hot climates. The crural indices for human populations varies directly with temperature, so individuals with higher crural index values are generally from regions with a warmer average environmental temperature. Conversely, the crural indices are lower for individuals from regions where there are colder average temperatures.

Figure 14.16b These animals are representative of Allen’s rule. Note the shorter limbs and ears of the rabbit on the left that you might find in cold temperatures. Note the length of the ears on the rabbit on the right that you might find in a warm climate. Rabbits do not sweat like humans, heat is dissipated primarily through their ears.

Nasal shape and size (Figure 14.17) is another physiological feature that is affected by way of an individual’s ancestors’ environments. The selective role of climate in determining human nasal variation is typically approached by dividing climates into four adaptive zones: hot-dry, hot-wet, cold-dry, and cold-wet (Maddux et al. 2016). One of the principal roles of the nasal cavity is to condition (i.e., warm and humidify) ambient air prior to its reaching the lungs. Given that function of the nasal cavity, it is anticipated that different nasal shapes and sizes will be related to varying environments. In cold-dry climates, an individual’s nasal cavity must provide humidification and warmth to the dry air when breathing in through the nose (Noback et al. 2011). Also, in that type of climate, the nasal cavity must conserve moisture and minimize heat loss during when the individual exhales through the nose (Noback et al. 2011). From a physiological stress perspective, this is a stressful event.

Figure 14.17 Human nasal morphological variation as influenced by four major climate-based adaptive zones: hot-dry, hot-wet, cold-dry, and cold-wet. Note that images are presented left-to-right in relation to the climate-based adaptive zones, respectively.

Conversely, in hot-wet environments, there is no need for the nasal cavity to provide additional moisture to the inhaled air nor is there a need to warm the air or to preserve heat within the nasal cavity (Noback et al. 2011). So, in hot-wet climates, the body is under less physiological stress related to the inhalation of ambient air than in cold-dry climates. As with most human morphological elements, the shape and size of the nasal cavity occurs along a cline. Due to the environmental stressors of cold-dry environments requiring the humidification and warming of air through the nasal cavity, individuals indigenous to such environments tend to have taller (longer) noses with a reduced nasal entrance (nostril opening) size (Noback et al. 2011). This general shape is referred to as leptorrhine, and it allows for a larger surface area within the nasal cavity itself for the air to be warmed and humidified prior to entering the lungs (Maddux et al. 2016). In addition, the relatively small nasal entrance of leptorrhine noses serves as a means of conserving moisture and heat (Noback et al. 2011). Individuals indigenous to hot-wet climates tend to have platyrrhine nasal shapes, which are shorter with broader nasal entrances (Maddux et al. 2016). Since individuals in hot-wet climates do not need to humidify and warm the air entering the nose, their nasal tract is shorter and the nasal entrance wider to permit the effective cooling of the nasal cavity during respiratory processes.

Adaptation: Infectious Disease

Throughout our evolutionary journey, humans have been exposed to numerous infectious diseases. In the following section, we will explore some of the evolutionary-based adaptations that have occurred in certain populations in response to the stressors presented by select infectious diseases. One of the primary examples of natural selection processes acting on the human genome in response to the presence of an infectious disease is the case of the relationship between the sickle-cell anemia trait and malaria.

Malaria is a zoonotic disease (type of infectious disease naturally transmitted between animals and humans covered in more detail in Chapter 16: Human Biology and Health) caused by the spread of the parasitic protozoa from the genus Plasmodium (Figure 14.18). These unicellular, eukaryotic protozoa are transmitted through the bite of a female Anopheles mosquito. During the bite process, the protozoan parasites that are present within an infected mosquito’s saliva will enter the bloodstream of the individual where they will be transported to the liver. Within the liver, the parasites multiply and will eventually be released into the bloodstream where they will infect erythrocytes. Once inside the erythrocytes, the parasites will reproduce until they exceed the cell’s storage capacity, causing it to burst and release the parasites into the bloodstream once again. This replication cycle will continue as long as there are viable erythrocytes within the host to infect.

Figure 14.18 Life cycle of the malaria parasite.

General complications from malaria infections include: enlargement of the spleen (due to destruction of infected erythrocytes), lower number of thrombocytes (also called platelets, required for coagulation/clotting of blood), high levels of bilirubin (a byproduct of hemoglobin breakdown in the liver) in the blood, jaundice (yellowing of the skin and eyes due to increased blood bilirubin levels), fever, vomiting, retinal (eye) damage, and convulsions (seizures). According to the World Health Organization, in 2016 there were 445,000 deaths from malaria globally with the highest percentage of those deaths occurring in Africa (91%) and Southeast Asia (6%) (World Health Organization 2017). In sub-Saharan Africa, where incidents of malaria are the highest in the world, 125 million pregnancies are affected by malaria, resulting in 200,000 infant deaths (Hartman et al. 2013). Pregnant women who become infected during the gestational process are more likely to have low-birthweight infants due to prematurity or growth restriction inside the uterus (Hartman et al. 2013). After birth, infants born to malaria-infected mothers are more likely to develop infantile anemia (low red blood cell counts), a malaria infection that is not related to the maternal malarial infection, and they are more likely to die than infants born to non-malaria-infected mothers (Hartman et al. 2013).

For children and adolescents whose brains are still developing, there is a risk of cognitive (intellectual) impairment associated with some forms of malaria infections (Fernando et al. 2010). Given the relatively high rates of morbidity (disease) and mortality (number of deaths) associated with malaria, it leads to reason that this disease may have served as a selective pressure during human evolution. Support for natural selection related to malaria resistance is related to genetic mutations associated with sickle cell, thalassemia, glucose-6-phosphate dehydrogenase (G6PD) deficiency, and the absence of certain antigens (molecules capable of inducing an immune response from the host) on erythrocytes. For the purposes of this text, we will focus our discussion on the relationship between sickle cell disease and malaria.

Sickle cell disease is a group of genetically inherited blood disorders characterized by an abnormality in the shape of the hemoglobin within erythrocytes. It is important to note that there are multiple variants of hemoglobin, including, but not limited to: A, D, C, E, F, H, S, Barts, Portland, Hope, Pisa, and Hopkins. Each of these variants of hemoglobin may result in various conditions within the body however, for the following explanation we will focus solely on variants A and S.

Figure 14.19 Normal and sickled erythrocytes.

Individuals who inherit a mutated gene (hemoglobin with a sickled erythrocyte variety, HbS) on chromosome 11 from both parents will develop sickle cell anemia, which is the most severe form of the sickle cell disease family (Figure 14.19). The genotype of an individual with sickle cell anemia is HbSS whereas, an individual without sickle cell alleles has a genotype of HbAA representing two normal adult hemoglobin type A variants. Manifestations of sickle cell anemia (HbSS) range from mild to severe with some of the more common symptoms being: anemia, blood clots, organ failure, chest pain, fever, and low blood oxygen levels. In high-income countries with advanced medical care, the median life expectancy of an HbSS individual is around 60 years however, in low-income countries where advanced medical care is scarce, as many as 90% of children with sickle cell disease perish before the age of five (Longo et al. 2017).

Considering that advanced medical care was not available during much of human evolutionary history, it stands to reason that the majority of individuals with the HbSS genotype died before the age of reproduction. If that is the case though, why do we still have the HbS variant present in modern populations? As covered earlier in this textbook, the genotype of an individual is composed of genes from both biological parents. In the case of an individual with an HbSS genotype, the sickle cell allele (HbS) was inherited from each of the parents. For individuals with the heterozygous genotype of HbSA, they have inherited both a sickle cell allele (HbS) and a normal hemoglobin allele (HbA). Heterozygous (HbSA) individuals who reside in regions where malaria is endemic may have a selective advantage. They will experience a sickling of some, but not all, of their erythrocytes. Unlike an individual with the HbSS genotype, someone with HbSA may experience some of the symptoms listed above however, they are generally less severe.

As noted earlier, the mechanism through which Plasmodium protozoan parasites replicate involves human erythrocyte cells. However, due to their sickled shape, as well as the presence of an abnormally shaped protein within the cell, the parasites are unable to replicate effectively in the erythrocyte cells coded for by the HbS allele (Cyrklaff et al. 2011). An individual who has an HbSA genotype and an active malaria infection will become ill with the disease to a lesser extent than someone with an HbAA genotype. Although normal erythrocytes (regulated by the HbA allele) allow for the replication of the parasite, the parasites will not be able to replicate in HbS erythrocytes of the heterozygote. So, individuals with the HbSA genotype are more likely to survive a malaria infection than an individual who is HbAA. Although individuals with the HbSA genotype may endure some physiological complications related to the sickling of some of their erythrocytes, their morbidity and mortality rates are lower than they are for HbSS members of the population. The majority of individuals who are heterozygous or homozygous for the HbS trait have ancestors who originated in sub-Saharan Africa, India, Saudi Arabia, regions in South and Central America, the Caribbean, and the Mediterranean (Turkey, Greece, and Italy) (Centers for Disease Control and Prevention 2017) (Figure 14.20).

Figure 14.20 Distribution of sickle cell and associated erythrocytic abnormalities for Africa and Asia.

With respect to the history of these regions, during the early phases of settlement horticulture was the primary method of crop cultivation. Typically performed on a small scale, horticulture is based on manual labor and relatively simple hand tools rather than the use of draft animals or irrigation technologies. Common in horticulture is swidden, or the cutting and burning of plants in woodland and grassland regions. The swidden is the prepared field that results following a slash-and-burn episode. This practice fundamentally alters the soil chemistry, removes plants that provide shade, and increases the areas where water may pool. This anthropogenically altered landscape provides the perfect breeding ground for the Anopheles mosquito, as it prefers warm, stagnant pools of water (Figure 14.21).

Figure 14.21 The effects of human horticultural activities on the balancing selection of populations in relation to sickle cell disease genotype variants.

Although swidden agriculture was historically practiced across the globe, it became most problematic in the regions where the Anopheles mosquito is endemic. These areas have the highest incidence rates of malaria infection. Over time, the presence of the Anopheles mosquito and the Plasmodium parasite that it transmitted acted as a selective pressure, particularly in regions where swidden agricultural practices were common, toward the selection of individuals with some modicum of resistance against the infection. In these regions, HbSS and HbSA individuals would have been more likely to survive and reproduce successfully. Although individuals and populations are far more mobile now than they have been throughout much of history, there are still regions where we can see higher rates of malaria infection as well as greater numbers of individuals with the HbS erythrocyte variant. The relationship between malaria and the selective pressure for the HbS variant is one of the most prominent examples of natural selection in the human species within recent evolutionary history.

Adaptation: Lactase Persistence

With the case of sickled erythrocytes and their resistance to infection by malaria parasites, there is strong support for a cause-and-effect-style relationship linked to natural selection. Although somewhat less apparent, there is a correlation between lactase persistence and environmental challenges. Lactase-phlorizin hydrolase (LPH) is an enzyme that is primarily produced in the small intestine and permits the proper digestion of lactose, a disaccharide (composed of two simple sugars: glucose and galactose) found in the milk of mammals. Most humans will experience a decrease in the expression of LPH following weaning, leading to an inability to properly digest lactose. Generally, LPH production decreases between the ages of two and five and is completely absent by the age of nine (Dzialanski et al. 2016). For these individuals, the ingestion of lactose may lead to a wide variety of gastrointestinal ailments including abdominal bloating, increased gas, and diarrhea. Although the bloating and gas are unpleasant, the diarrhea caused by a failure to properly digest lactose can be life-threatening if severe enough due to the dehydration it can cause. Some humans, however, are able to produce LPH far beyond the weaning period.

Figure 14.22 Interpolated map depicting the percentage of adults with the lactase persistence genotype in indigenous populations of the Old World. Circles denote sample locations.

Individuals who continue to produce LPH have what is referred to as the lactase persistence trait. The lactase persistence trait is encoded for a gene called LCT, which is located on human chromosome 2 (Ranciaro et al. 2014 see also Chapter 3). From an evolutionary and historical perspective, this trait is most commonly linked to cultures that have practiced cattle domestication (Figure 14.22). For individuals in those cultures, the continued expression of LPH may have provided a selective advantage. During periods of environmental stress, such as a drought, if an individual is capable of successfully digesting cow’s milk, they have a higher chance of survival than someone who suffers from diarrhea-linked dehydration due to a lack of LPH. Per Tishkoff et al. , the “frequency of lactase persistence is high in northern European populations (more than 90% in Swedes and Danes), decreases in frequency across southern Europe and the Middle East (less than 50% in Spanish, French, and pastoralist Arab populations), and is low in non-pastoralist Asian and African populations (less than 1% in Chinese, less than 5% to 20% in West African agriculturalists)” (2007: 248). Although the frequency of the lactase persistence trait is relatively low among African agriculturalists, it is high among pastoralist populations that are traditionally associated with cattle domestication, such as the Tutsi and Fulani, who have frequencies of 90% and 50%, respectively (Tishkoff et al. 2007).

Cattle domestication began around 11,000 years ago in Europe (Beja-Pereira et al. 2006) and 7,500 to 9,000 years ago in the Middle East and North Africa (Tishkoff et al. 2007). Based on human genomic studies, it is estimated that the mutation for the lactase persistence trait occurred around 2,000 to 20,000 years ago for European populations (Tishkoff et al. 2007). For African populations, the lactase persistence trait emerged approximately 1,200 to 23,000 years ago (Gerbault et al. 2011). This begs the question: Is this mutation the same for both populations? It appears that the emergence of the lactase persistence mutation in non-European populations, specifically those in East Africa (e.g., Tutsi and Fulani), is a case of convergent evolution . With convergent evolution events, a similar mutation may occur in species of different lineages through independent evolutionary processes. Based on our current understanding of the genetic mutation pathways for the lactase persistence trait in European and African populations, these mutations are not representative of a shared lineage. In other words, just because a person of European origin and a person of African origin can each digest milk due to the presence of the lactase-persistence trait in their genotypes, it does not mean that these two individuals inherited it due to shared common ancestry.

Is it possible that the convergent evolution of similar lactase-persistence traits in disparate populations is merely a product of genetic drift? Or is there evidence for natural selection? Even though 23,000 years may seem like a long time, it is but a blink of the proverbial evolutionary eye. From the perspective of human evolutionary pathways, mutations related to the LCT gene have occurred relatively recently. Similar genetic changes in multiple populations through genetic drift processes, which are relatively slow and directionless, fail to accumulate as rapidly as have lactase-persistence traits (Gerbault et al. 2011). The widespread accumulation of these traits in a relatively short period of time supports the notion that an underlying selective pressure must be driving this form of human evolution. Although to date no definitive factors have been firmly identified, it is thought that environmental pressures are likely to credit for the rapid accumulation of the lactase-persistence trait in multiple human populations through convergent evolutionary pathways.

Human Variation: Our Story Continues

From the time that the first of our species left Africa, we have had to adjust and adapt to numerous environmental challenges. The remarkable ability of human beings to maintain homeostasis through a combination of both nongenetic (adjustments) and genetic (adaptations) means has allowed us to occupy a remarkable variety of environments from high-altitude mountainous regions to the tropics near the equator. From adding piquant, pungent spices to our foods as a means of inhibiting food-borne illnesses due to bacterial growth to donning garments specially suited to local climates, behavioral adjustments have provided us with a nongenetic means of coping with obstacles to our health and well-being. Acclimatory adjustments, such as sweating when we are warm in an attempt to regulate our body temperature or experiencing increased breathing rates as a means of increasing blood oxygen levels in regions where the partial pressure of oxygen is low, have been instrumental in our survival with respect to thermal and altitudinal environmental challenges. For some individuals, developmental adjustments that were acquired during their development and growth phases (e.g., increased heart and lung capacities for individuals from high-altitude regions) provide them with a form of physiological advantage not possible for someone who ventures to such an environmentally challenging region as an adult. Genetically-mediated adaptations, such as variations in the pigmentation of our skin, have ensured our evolutionary fitness across all latitudes.

Will the human species continue to adjust and adapt to new environmental challenges in the future? If past performance is any measure of future expectations, then the human story will continue as long as we do not alter our environment to the point that the plasticity of our behavior, physiological, and morphological boundaries is exceeded. In the following chapters, you will explore additional information about our saga as a species. From the concept of race as a sociocultural construct to our epidemiological history, the nuances of evolutionary-based human variation are always present and provide the basis for understanding our history and our future as a species.


Acknowledgements

We thank everyone who attended the ‘What makes us musical animals? Cognition, biology and the origins of musicality’ workshop for the stimulating meeting that led to this special issue, especially Henkjan Honing for organizing the meeting and the other attendees that participated in the discussion relating directly to this paper on natural versus artificial studies of animal music perception, including Peter Tyack and Aniruddh Patel. We thank Sandra Trehub for her very detailed revisions as editor, and the anonymous reviewers for their insights on paper organization. We thank also Daniel L. Bowling for help drawing the figure.


Introduction

A rapidly growing body of empirical evidence suggests that brain mechanisms governing music and language processing interact and might share an important link with respect to their underlying neurophysiological processing [1], [2], [3], [4], [5]. For example, neuroanatomical regions including Broca’s and Wernicke’s area and electrophysiological markers (N400 and P600) typically associated with language-specific operations (e.g., semantic/syntactic processing) are also recruited for processing the melodic and harmonic relationships of music [4], [5], [6]. In trained musicians, frontal regions (e.g., BA 47) typically associated with higher-order language comprehension, also show activation to the complex metric and rhythmic structures of music [7]. These studies provide evidence for a common neuronal mechanism subserving the temporal coherence found in both domains and demonstrate the intimate coupling between underlying neural processes recruited for language- and music-related processing.

Recognizing the shared brain structure between language and music leads to the provocative question of whether or not music ability might impact language-related abilities and vice versa. Indeed, the extensive overlap between these domains has led many to posit that musicianship and certain language backgrounds might impact processing in the complementary domain, i.e., so-called perceptual-cognitive “transfer effects” [3], [8], [9]. Such cross-domain influences have now been extensively reported in the direction from music-to-language. Musicians demonstrate perceptual enhancements in a myriad of language-specific abilities including phonological processing [10], verbal memory [11], [12] and verbal intelligence [13], formant and voice pitch discrimination [14], sensitivity to prosodic cues [15], detecting durational cues in speech [16], degraded speech perception [14], [17], second language proficiency [18], [19], and lexical tone identification [20], [21], [22]. These perceptual advantages are corroborated by electrophysiological evidence demonstrating that both cortical [23], [24], [25], [26], [27], [28] and even subcortical [3], [29], [30], [31] brain circuitry altered by long-term music training facilitates the sensory-perceptual and cognitive control of speech information. Musicians’ brain-behavior benefits for speech and language are, presumably, mediated by a series of enhancements to both sensory and cognitive mechanisms which operate at multiple tiers of the processing hierarchy that mediate a range of function from low-level auditory processing to higher-level aspects of cognition.

To account for such music-to-language effects, Patel [32] proposed a neurocognitive model to describe how speech processing benefits might arise due to the coordinated plasticity resulting from music training. According to the OPERA (Overlap, Precision, Emotion, Repetition, Attention) hypothesis, speech-related benefits in musicians are largely attributable to the extensive overlap in brain networks engaged during speech and music listening. As an auditory activity, music places higher demands on these shared networks than typical speech communication, allowing the pathways to function with a higher “precision” of processing. Assuming emotional engagement, sufficient repetition, and focused attention during learning, the neural plasticity engendered from music training acts to benefit speech processing by promoting an increase in the magnitude, resolution, and efficiency with which brain networks register and process salient acoustic information, music, speech, or otherwise. Although not explicitly developed in its inception, the OPERA framework makes no a priori assumption that music-language transfer should be exclusively unidirectional. Interestingly, the ingredients of the model (e.g., repetition, attention, increased sensory encoding precision) are also satisfied by forms of language expertise. Indeed, as with musical training, tone language experience (e.g., Mandarin Chinese [3], [33]) and bilingualism [34] have been shown to similarly affect the neural encoding and perception of behaviorally-relevant sound. These results, cast in the context of the OPERA framework, thus allow the possibility that cognitive transfer between music and language might be bidirectional, a point that has heretofore been largely untested ([9], p.340).

Despite its theoretical and practical significance, evidence for language-to-music transfer is scarce and conflicting [35], [36]. Most studies have focused on the putative connection between tone languages and absolute pitch (e.g., [37], [38]), a rare note naming ability irrelevant to most music perception/production ([39], p.26), or its effects on amusia [40], [41], another rare phenomenon which affects a listener’s processing, memory, and recognition for pitch. A handful of electrophysiological studies have demonstrated that relative to English-speaking controls, listeners fluent in Mandarin Chinese have improved sensory encoding of simple musical pitch patterns as evident by smoother, more robust pitch tracking in their scalp-recorded brainstem responses as well as their overall cortical response magnitudes [3], [33], [42]. In contrast, behavioral studies reveal contradictory effects, reporting either very weak [41], [42], [43] or no observable enhancement [33], [36], [44], [45] in these listeners’ nonlinguistic pitch perception abilities, music or otherwise. The failure of these behavioral studies to demonstrate a clear tone-language advantage in music perception might be due to a number of methodological issues including heterogeneity in the experimental group (e.g., pooling listeners across multiple language backgrounds [43]), the ecological validity of the “musical stimuli” [33], and/or differences in experimental tasks.

Given the inconsistencies of the extant literature, we aimed to test if listeners with tone-language expertise display similar performance to musically-trained individuals on measures of auditory pitch acuity, music perception, and general cognitive ability. We employ a cross-sectional design herein examining these “auditory experts” as it is a necessary first step to verify a bidirectional relationship between music and language prior to manipulating these variables (i.e., language experience/training) in a longitudinal study. In order to increase the possibility of identifying behavioral correlates of language-to-music influences, we aimed to recruit individuals with linguistic pitch expertise whose exposure to aspects of pitch would more closely approximate that gained via musical training. Cantonese serves as our point of departure given its intricate tone system. In contrast to Mandarin, the Cantonese tonal inventory consists of six contrastive tones, most of which are level pitch patterns minimally differentiable based on pitch height [46], [47]. Importantly, the proximity of tones is on the order of a semitone [48], i.e., 6% difference in frequency, which parallels the minimum distance between adjacent pitches found in music. Given their specialization in perceiving minute changes in steady-state, level pitch [46], [49], we reasoned that Cantonese listeners would show improvements in basic auditory (e.g., pitch discrimination) as well as music perception abilities relative to untrained listeners (English-speaking nonmusicians). Thus, we assess whether or not tone-language speakers show enhanced performance on measures of music perception. Furthermore, we compared the performance of Cantonese-speakers to that of musicians, in order to contrast the behavioral benefits engendered by these two distinct forms of auditory expertise. Both language and music training have also been implicated in improving executive processing [8]. Thus, in addition to assessing between-group perceptual differences, we also assessed performance on aspects of higher-order cognition, including general fluid intelligence and nonverbal working memory.


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Watch the video: The Art of Language Invention, Episode 27: The Evolution of Tone (May 2022).