Babel's Dawn

The Evolution of the Vocal Tract

The human vocal tract is well built for maximizing the number of different sounds it can produce. Particularly notable is the way the tongue can move freely in the mouth and the modifications that support the abillity.

I'm always happy when there is a new paper by Bart de Boer. He studies the mechanics of producing speech sounds, and in the next issue of the Journal of Evolutionary Psychology he has a review paper on the human vocal tract (draft here). As it is available online, I will limit myself to reporting data that caught my eye:

Brains and Vocalization

A week ago a commenter challenged my remark that, “We have direct links between the brain and our vocal apparatus.” The commenter asked:

Animals also have vocal apparati. Aren't the links between their brains and their vocal apparati direct? If not, what is the difference between in the linkages? How does one determine neurologically whether there is voluntary control or not?

My remark had been a quick response another comment and this direct challenge sent me back to the books so that I might better know what the heck I was talking about. I was pretty sure I had gotten the information originally from Terrence Deacon’s The Symbolic Species and sure enough chapter 8 has the story in painful detail along with some useful diagrams.

Speech Circuitry

Sketch of proposed circuitry supporting phonological loop. The auditory region of the brain associated with speech (Broca's area and adjacent regions) shown under red shading is connected to the motor area controlling vocalization (Wernicke's area and neighboring regions). The connection is sketched under black shading. The numbers shown roughly identify the Broadmann regions.

Download a PDF of this post

One of the mysteries of language is the way, when it is viewed whole, there is nothing else like it in the biological world, but if the view focuses on a part—recognizing voices, making sounds, voluntary actions, voluntary attention, etc.—it seems quite like other phenomena in the animal world. The reaction of analytical thinkers to this mystery is to look about for the part that is not duplicated elsewhere in the animal kingdom—symbols, recursive syntax, displaced reference, etc. Each of these traits fails when put to a simple test: provide a sample of language that lacks the trait, and you still get something unknown to the rest of biology.

Linguistic richness has to be explained, both psychologically and physically. During the past week I read a paper suggesting the clarity can come by focusing on physical explanations.  

How Different Are Ape Brains From Ours?

How do human brains differ from ape brains? We know that our brains are a lot bigger, but so what? Presumably we are smarter, but how exactly? We divvy up the work between right and left brain hemispheres (“lateralization”), but what does that get us? For some years now Jared Taglialatela and his team have been delving into the question of ape brains and how they foreshadow human brains, and every so often this blog reports on their work (see, e.g.: Ape Cries are Complex). The March 11 issue of Current Biology includes the latest results of the team’s investigations (“Communicative Signaling Activates ‘Broca’s’ Homolog in Chimpanzees,” paper here). They have found evidence that brain structures “underlying language production in the human brain may have been present in the common ancestor of humans and chimpanzees.” Or, to emphasize the surprise, the brain’s “language areas” are older than language itself.

A Note on Mirror Neurons

This blog has not made much of a fuss about "mirror neurons," the name given to neurons that fire both when an individual does something and when the individual observes another individual do something. For example, a neuron might fire when a monkey reaches for a carrot and when the monkey sees another reach for a carrot. I have stayed away from discussing these nerves because it seemed to me the research hasn't gotton to the next stage: explaining how human mirror neurons differ functionally from monkey mirrors. Nevertheless, technical speculations about language evolution sometimes mention mirror neurons as critical to understanding actions and imitating them. So I was startled to see that in the latest issue of The Journal of Neuroscience (Oct. 29, 2008) there is a paper denying that humans even have mirror neurons. (Paper summary here.) So I suppose more time will have to pass before this blog reports on them.

Unique Properties of the Human Mind

Cats that look like humans and humans that look like cats are unknown to nature, but commonplaces of speech. How can that be?

The April issue of the Journal of Anatomy is devoted to review articles on the evolution of humans. The result is as handy as an up-to-date textbook. What’s more, all the articles appear to be free. So I suggest readers jump to the journal’s table of contents and start downloading those PDFs. The article most directly concerned with issues on this blog is “A natural history of the human mind: tracing evolutionary changes in brain and cognition” by a team from The George Washington University’s Mind, Brain and Evolution Center (Chet C. Sherwood, Francys Subiaul, and Tadeusz W. Zawidzki). The most useful part of the article for readers of this blog is probably its listings of mental traits that humans share with apes and traits that are unique to humans. Listening, sharing information, and expressing a boundless imagination all rest on the unique traits.

Broca's Area in Chimpanzees?

Broca's area was the first region of the brain identified as specializing in speech production.

Broca's area is the best known region of the brain that is critical to speech production. If damaged it produces difficulties in speaking grammatically-complex sentences. It is one of those areas whose evolution seems critical to the story of speech origins. Now comes a report from the Yerkes National Primate Research Center that chimpanzees have a homologous region of the brain that is active when they communicate. (See: Jared P. Taglialatela et al March 11 Current Biology, abstract here) The authors speculate that "the neurological substrates underlying language production in the human brain may have been present in the common ancestor of humans and chimpanzees."

Brain Changes for Language

I'm getting a little backed up as I go over the Barcelona results and prepare to review a couple of new books. Meanwhile news continues. I have a story on the backburner, but I see that another blogger has done a good job of presenting it, so why don't I just send browsers over to that site? Check out Neurophilosophy's account of brain changes to support language right here.

Mirror Mirror, on the Wall

Shakespeare's mirror was technically primitive, yet somehow he could see deep into the truth.

Poetry is emotion recollected in tranquility sayeth an old literary giant. An article in the latest Current Directions in Psychological Science takes that a step further and says all of language consists of a respit from emotional control. Two Italian neuroscientists, Leonardo Fogassi and Pier Francesco Ferrari, argue in “Mirror Neurons and the Evolution of Embodied Language” (abstract here) that a central difference between animal cries and human speech is that the cries are under the control of the brain’s emotional circuitry while speech is part of the perceptual (sensori-motor) apparatus.

call production is nonhuman primates is correlated with intense emotional states, with the main function being to signal urgent or imminent events. [p. 136]

Cheep-cheep vs Mama

Birdsong has more to tell us about the biology of speech than one might expect, according to an “eBriefing” posted by Alisa G. Woods for the New York Academy of Sciences (the eBriefing is here). It is true that birdsong serves the common communication task of controlling relations rather than directing attention, so we would expect birdsong and speech to have evolved through different selective pressures. But they both depend on social learning. The sounds of song/speech are not the inevitable result of genes and anatomy, so they are not like the sounds crickets make. Crickets need no guidance in how to make their noise, and surely they sounded the same in Caesar’s day. With speech, however, we know for a fact that the speech sounds made by modern Italians do not match those used by Caesar’s Romans, and there has likely been some drift in the songbird sounds as well.

Young songbirds and speakers must both learn, first, to recognize the sounds their parents make and, then, to make the same sounds themselves.