Kanzi: The Ape at the Brink of the Human Mind (41 page)

9

According to the evidence of molecular biology, the first hominid species appeared approximately five million years ago, a bipedal ape with long arms and curved fingers that presumably was well at home in the trees. The earliest known fossil evidence of such a creature dates from three to four million years ago, and was found in Ethiopia. These early hominids, with their 400-cubic-centimeter brains, and several later small-brained species, all belonged to the genus
Australopithecus
. Only when the genus
Homo
appeared did brain size begin to increase, leaping by 50 percent in
Homo habilis
, to more than 600 cubic centimeters. The next player on the stage of protoman was
Homo erectus
, who debuted almost two million years ago. His brain size varied from 850 to 1100 cubic centimeters. Modern levels of brain size—1350 cubic centimeters—came with the evolution of archaic
Homo sapiens
, probably around two hundred and fifty thousand years ago.

Much of the increase in brain size from the ape level to the modern human level can be accounted for by an enlargement of the neocortex, the thin coat of nerve cells that forms the outer layer of our brain. This outer covering did not expand with a common equipotential over the entire surface, however; the frontal lobes, associated with planning and foresight, expanded
disproportionately. Another part of the brain, located in the lower rear part of the skull and termed the cerebellum, has also expanded disproportionately in man. This area is associated with the automatization of skills such as driving a car, riding a bike, buttoning a shirt, and so on. This human expansion pattern was absent in the australopithecine species, but it appeared in
Homo habilis
—or handy man—the first stone tool-maker.

It was once believed that Broca’s area, long thought to be the area of the brain that made language possible, was unique to humans. Located (usually) in the left frontal lobe near the temple, Broca’s area in humans is readily identified as a raised region. When evidence for Broca’s area was discovered in the cranium of a 1.8 million-year-old
Homo habilis
from northern Kenya, two decades ago, it was taken as an indication of an advanced language faculty. However, we now know that Broca’s area occurs in the brains of other animals, too, and is merely expanded in humans, not unique. Similarly, none of the other brain centers involved in language comprehension and production, including Wernicke’s area (located in the left parietal lobe) and a scattering of some dozen or so nuclei throughout the prefrontal region, represent novel structures. The difference between ape and human brains is essentially quantitative. Those who argue for a uniquely human language-acquisition device, as do proponents of the Chomskian school, do so in the absence of any anatomical evidence for its existence.

The brain, metabolically speaking, is an extremely expensive organ. It represents 2 percent of body mass but consumes 18 percent of our energy budget. There seems little reason to have such a large brain unless it somehow greatly increases the survival prospects of its bearer. Based on the nature of size increase and reorganization seen in fossil brains, anthropologist Dean Falk, along with many others at the Wenner-Gren conference, believe that it probably was language that propelled the increase in brain size. As Dean expressed it, “If hominids weren’t using and refining language, I would like to know what they
were
doing with their autocatalytically increasing brains.”
¹

Terrence Deacon, a neurologist at Belmont Hospital, Massachusetts, is equally emphatic, but from the perspective of modern brains, not fossil ones: Deacon bases his conclusion on a study of the nature of differences in connectivity in ape and human brains, and on developmental studies of monkey brains. “The brain structures and circuits most altered in the course of human brain evolution reflect some unusual computational demands by natural languages,” he notes.
²
These alterations center on the increasing dominance of output from the prefrontal region, which allows voluntary control over vocalizations.

The modern brain appeared with the first members of a group that is loosely called archaic
Homo sapiens
, which evolved some quarter of a million years ago. Were these people as linguistically sophisticated as we are today? It’s hard to say, but if the brain-size/linguistic-capacity relationship holds, as has been argued, then the answer should be yes, for these people had brains the size of our own. They differed from us only in that they retained a physical robustness that is absent from modern skeletons.

The second line of anatomical evidence—that of the vocal apparatus—tells very much the same story as the one we see with brain-size increase. The vocal apparatus consists of the larynx (or vocal organ), the pharynx (or throat), the nasopharynx (or nasal cavity), the tongue, and the lips. In all mammals apart from humans, the larynx is positioned high in the neck, a position that has three consequences. First, the larynx can be “locked into” the nasopharynx—the air space near the “back door” of the nasal cavity. When this occurs, all breathing is done through the nose, as the back of the oral cavity is closed by the overlapping of the soft palate and the epiglottis.

Second, although the vocal tracts of chimpanzees and other mammals can produce most of the human vowel sounds, it is difficult for them to make some of the sounds readily. Edmund S. Crelin, an anatomist at the Yale University School of Medicine, has done extensive modeling of both the human and the ape vocal tracts, including the construction of manipulable rubber casts, which permit him to determine the sounds
that can be produced by the physical structure of the organism. Crelin has studied this problem in detail, comparing the anatomical capacities not only of apes and humans, but also of many hominids. He has done so by reconstructing the vocal tract tissue on the basis of the available skeletal material. In order to investigate the range of sounds that a chimpanzee can make, Crelin built a rubber model of the chimpanzee vocal tract and forced pressurized air through the model as he manually manipulated its shape. These experiments led him to conclude that he could “force a rubber tract of many nonhuman mammals to produce a set of vowel-like sounds, including those of mammals with even longer snouts, such as a horse.”
³
Nonetheless, he found that it required extreme constriction of the model ape vocal tract to produce the long
e
and long
u
sounds, and that it was also nearly impossible for the chimpanzee to switch rapidly between vowel sounds.

Finally, the range of noises apes can make does not include the most important element of human speech—the consonant. This is because they have difficulty accomplishing what is called velopharyngeal closure, or the brief blocking off of the nasal passages as air is forced through the mouth. This blockage is needed for the production of consonants; it enables us to generate the brief turbulence and temporary microbursts of air that are the basis of consonants. The action of the vocal cords lays noise over these temporary perturbations and the shape of the vocal tract itself is modulated to amplify or decrease certain frequencies, thereby serving to filter the action of the basic sound produced by the vibration of the vocal folds. These filtered sounds, without turbulence, become vowels; with turbulence, generated by velopharyngeal closure or a sealing off of the nasal cavity by raising the soft palate in the back of the throat very rapidly, we are able to produce interpretable speech.

Man alone has a vocal tract that permits the production of consonant sounds. These differences between our vocal tract and that of apes, while relatively minor, are significant and may be linked to the refinement of bipedal posture and the associated need to carry the head in a balanced, erect position over the center of the spine. A head with a large heavy jaw would
cause its bearer to walk with a forward list and would inhibit rapid running. To achieve balanced upright posture, it was essential that the jaw structure recede and thus that the sloped vocal tract characteristic of apes become bent at a right angle. Along with the reduction of the jaw and the flattening of the face, the tongue, instead of residing entirely in the mouth, was lowered partially down into the throat to form the back of the oropharynx. The mobility of the tongue permits modulation of the oropharyngeal cavity in a manner that is not possible in the ape, whose tongue resides entirely in the mouth. Similarly, the sharp bend in the supralaryngeal airway means that the distance between the soft palate and the back of the throat is very small. By raising the soft palate, we can block off the nasal passageways, permitting us to form the turbulence necessary to create consonants.

An obvious question follows from my argument that the evolution of a bipedal mode of locomotion in our ancestors was important in the development of a vocal tract capable of producing consonants: Why did
Australopithecus
not follow the same evolutionary path as
Homo
in developing a humanlike larynx? I’ve referred to all species in the human family, including
Australopithecus
, as bipedal apes. This is true, in the sense of the very close genetic relationship humans have with apes. But it may be a bit misleading with respect to how efficient the different hominid species were in their bipedal locomotion.

There has been a long-running debate among anthropologists over this question, with some arguing that the australopithecine species walked just as modern humans do, that is, with a fully upright, striding gait. Others disagree, saying that the australopithecines retained many apelike adaptations, including spending a significant amount of their time in the trees and having a more shambling gait while on the ground. I support this latter argument, primarily because the australopithecines had long arms, short legs, and curved bones in their hands and feet, just as apes do. It is true that they were adapted to a degree of bipedal locomotion, but they were not fully bipedal as species of
Homo
have been, right from their first appearance two and a half million years ago. My argument over the effect of posture
on the vocal tract refers to full bipedalism, not the incomplete form that prevailed in
Australopithecus
,

Of course, there also must have been changes in the neuroanatomical systems that controlled these structures. In addition to the proper anatomical design, speech production requires extremely precise and coordinated control of many muscles. Moreover, speech is so rapid that we cannot possibly be producing each sound individually. We are, instead, coarticulating, which means that our mouths have already assumed the shape for the next sound to be made before we have finished producing the first sound. Since speech is infinitely variable, the coarticulation process is never the same from one word to the next unless one repeats oneself. This means that although words sound the same to us when we hear them from one time to the next, they are not really being said in the same manner. They are altered as a function of the speech context in which they occur. It is for this reason that it is so difficult to build a device that interprets speech. Speech is infinitely varied and currently only the human ear can readily find the meaningful units in these infinitely varied patterns. The consonants permit us to accomplish this feat.