Human evolution; our complex speech mechanism, 2 (Introduction)

by David Turell @, Tuesday, July 03, 2018, 23:51 (163 days ago) @ David Turell

The Evolution of this system is complex:

"In On the Origin of Species, Darwin noted “the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs.” Because of this odd anatomy, which differs from that of all other mammals, choking on food remains the fourth leading cause of accidental death in the United States. This species-specific problem is a consequence of the mutations that crafted the human face, pharynx, and tongue so as to make it easier to speak and to correctly interpret the acoustic speech signals that we hear.

"At birth, the human tongue is flat in the mouth, as is the case for other mammals. The larynx, which rests atop the trachea, is anchored to the root of the tongue. As infants suckle, they raise the larynx to form a sealed passage from the nose to the lungs, allowing them to breathe while liquid flows around the larynx. Most mammalian species retain this morphology throughout life, which explains why cats or dogs can lap up water while breathing. In humans, however, a developmental process that spans the first 8 to 10 years of life forms the adult version of the SVT. First, the skull is reshaped, shortening the relative length of the oral cavity. The tongue begins to descend down into the pharynx, while the neck increases in length and becomes rounded in the back. Following these changes, half the tongue is positioned horizontally in the oral cavity (and thus called the SVTh), while the other half (SVTv) is positioned vertically in the pharynx. The two halves meet at an approximate right angle at the back of the throat. The tongue’s extrinsic muscles, anchored in various bones of the head, can move the tongue to create an abrupt 10-fold change in the SVT’s cross-sectional area. (See illustration)

***

" This gives the adult human supralaryngeal vocal tract (SVT) two parts of nearly equal lengths that meet at a right angle: the horizontal portion of the oral cavity and the vertical portion associated with the pharynx. At the intersection of these two segments occur abrupt changes in the cross-sectional area of the SVT that allow humans to produce a range of sounds not possible for infants and nonhuman animals.

"As it turns out, the configuration of the adult human tongue’s oral and pharyngeal proportions and shape allow mature human vocal tracts to produce the vowels , , and [a] (as in the word ma). These quantal vowels produce frequency peaks analogous to saturated colors, are more distinct than other vowels, and are resistant to small errors in tongue placement.5 Thus, while not required for language, these vowel sounds buffer speech against misinterpretation. This may explain why all human languages use these vowels.

"This anatomy also begins to answer long-standing questions in language research: How did human speech come to be, and why don’t other animals talk? In 1969, my colleagues and I used a computer modeling technique to calculate the formant frequency patterns of the vowels that a rhesus macaque’s SVT could produce, based on an estimated range of tongue shapes and positions. We found that even when the monkeys’ tongues were positioned as far as possible toward the SVT configurations used by adult human speakers to yield the vowels , , and [a], the animals could not produce the appropriate formant frequencies. Three years later, using X-ray videos showing the movement of the vocal tract during newborn baby cries, we refined and replicated this study and found that, although chimpanzees and human newborns (which start life with a monkey-like SVT) produce a range of vowels, they could not produce s or s. This finding has since been replicated in independent studies, including in 2017 by the University of Vienna’s Tecumseh Fitch and colleagues. Those scientists used current computer techniques that readily model every vocal tract shape that a macaque could produce, and the research team confirmed that monkey vocal tracts were incapable of producing these vowels.

***

"It is now apparent that a massive epigenetic restructuring of the genes that determine the anatomy of the head, neck, tongue, larynx, and mouth enhanced our ability to talk after anatomically modern humans split from Neanderthals and Denisovans more than 450,000 years ago. A few years ago, David Gokhman, then at Hebrew University of Jerusalem, and colleagues reconstructed the methylated genomic regions of a 40,000-year-old Neanderthal fossil, an older Denisovan fossil, four ancient humans who lived 7,000 to 40,000 years ago, and six chimpanzees, comparing these with a methylation map of human bone cells assembled from more than 55 present-day humans. This comparison enabled the team to identify differentially methylated regions (DMRs) between the human and Neanderthal-Denisovan groups, and between humans and chimps.9,10 The researchers found that the genes that were most affected were those that controlled development of the larynx and pharynx, suggesting that epigenetic regulatory changes allowed the human vocal tract to morph into a shape that is optimal for speech.

"Of course, the fact that monkeys don’t talk like humans isn’t purely due to the physical limitations of their vocal tracts. They also lack the neural networks necessary for producing and processing speech. "

Comment: See 3 which covers mutations and neural change.


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