Plasticity means that the early specialization of human brain tissue does not have to be its ultimate destiny. It’s more like a career path, with the potential for a future change of jobs. This flexibility applies not just in what the brain can do but in how it is organized. Indeed, plasticity is the way our brain responds to all learning and experience during every minute of every day, regardless of whether that experience is an Italian class or brain surgery. There is no little field linguist inside our heads dividing language up the way we do it consciously; rather, we are plastic, and with plasticity, the hardware is the software.
Plasticity is not just a human trait. In pioneering work, William Greenough at the University of California, Los Angeles, showed that the dendrites and synapses of rats and hamsters change when the creatures are placed in a stimulating environment, and in 2005 a group of Princeton researchers demonstrated that when marmoset monkeys were moved from a standard laboratory setting to a more complex, enriched environment, their dendrites and synapses likewise underwent a dramatic change. The researchers concluded that the primate brain is extremely sensitive to even small increases in environmental complexity.19 Sue Savage-Rumbaugh invokes plasticity to explain Kanzi’s and Panbanisha’s extraordinary abilities, especially in comparison to Tamuli, who was exposed to language much later in life and never really acquired it. “By being immersed in a symbol-using environment during the period of greatest brain plasticity, all the components necessary for language comprehension (and production) were put into place for Kanzi and Panbanisha,” Savage-Rumbaugh wrote. If the bonobos are exposed to linguistic information at this crucial stage, it appears that their brains can adapt and organize in such a way that they can participate in human culture, even if it’s only at the level of a child.20
In a 1991 article about Kanzi, Chomsky was quoted as saying, “If an animal had a capacity as biologically sophisticated as language but somehow hadn’t used it until now, it would be an evolutionary miracle.”21 Yet it’s clear by now that many surprising and sophisticated capacities can be acquired by individual animals that they do not necessarily use in the wild. Plasticity suggests that mental variety is a fundamental characteristic of animal life, and that different environments can elicit different brains and mental skills from the creatures within a single species. A pathologist’s examination of brains from language-trained apes may help illuminate the specific changes that language seems to induce in the plastic brain. So far only one such organ has been examined. It weighed 528 grams, much more than that of the typical chimpanzee brain.22
The ideas of Schlaggar, Dick, Krubitzer, and other researchers are generations away from the search for the one or two nuggets of difference between speaking humans and nonspeaking animals. Carving up the world into qualitative differences may make sense to us psychologically, but it is not supported by biological research. Language as a whole is a phenomenal mental and social skill, but the enormous differences between being able to speak it and not do not correspond to equally large differences in the physiology of the brain.
Lacy’s and Alex’s recoveries are shocking to us in part because of the deeply held belief that it is the size of our brains that distinguishes our species. For a long time, we have assumed that the sheer bulk of the human brain was what made it such a formidable computing machine. We assumed a simple one-to-one relationship between intelligence and brain size, such that a brain will think more if there is more of it and, accordingly, it will think less if it is smaller.
But in absolute terms humans don’t have the largest brains (whales do). What we have, rather, are the biggest brains with respect to body size of any animal on the planet. The ratio of brain size to body size is called the encephalization quotient, or EQ. This measurement is based on the assumption that you can predict how much brain tissue an animal needs given how large its body is. Any extra tissue over and above that minimum is considered a bonus and a marker of intelligence.
Lori Marino, one of the researchers on the dolphin mirror-image project, investigates the possibility of using EQ as a neutral, objective measure of intelligence across species. She has examined cranial fossils to determine the EQ of dolphins and humans over the course of history. “I’m trying to understand what the big patterns are, and whether those patterns are driven by the same processes in humans and other animals. Fundamentally, all brains operate under the same physical laws. So my view is we should be looking for general principles and then possibly the uniqueness to each group.”
Humans currently have the highest EQ of all organisms, about 7. Bottlenose dolphins have a particularly high EQ (4.2), while belugas measure a respectable 2.4. In general, cetaceans—whales, dolphins, porpoises—measure from 1 to 5, chimpanzees measure 2 to 3. New Caledonian crows have a high EQ with respect to other birds. (No one has yet investigated the EQ of insects, or even whether it would be an appropriate measure for this type of organism.)23
Encephalization is only half the picture, said Marino. “You can have two brains that are just as big as each other, but organized in different ways, and one can be a much more complex information processor.” Comparing EQs over many species is the beginning of a truly non-human-centered approach to measuring brains.
Incidentally, the ranking of animals with the highest EQ has changed a number of times over the last few million years. Mainly, humans have been jockeying for first place with dolphins. Marino points out that a number of dolphin species throughout history had very similar brain-body ratios to our ancestors—Homo habilis, around 2 million years ago, and Homo erectus, only 1.8–2 million years ago. Rankings have shifted in the blink of an evolutionary eye and perhaps, said Marino, could do so again. “The past couple of million years at most is really the only time in history that humans have been the most encephalized organisms on the planet. It just wasn’t so two or three million years ago.” Our current standing with the biggest EQ may be secure because our highly developed culture props us in first place. But then again, our position may not be as strong as we think. On a planet that’s been in existence for four billion years, and at a completely arbitrary slice of time, can one species really be certain that things won’t change? (Presumably, if they do change, the dolphins will explain to us where we went wrong.)
Terrence Deacon brings together the perspectives of neuroscience, semiotics, and biology in order to examine the ancestral human brain as it enlarged and what effects the changes in brain-body ratio had on our abilities and behavior. He argues that first we need to compare the growth rate of our brains with those of other species. It turns out that human brains are two steps removed from the general growth patterns of all mammals.
First, humans are primates, and at some point in the distant past the primate brain evolved such that it grows a bit differently from all other mammal brains. Indeed, all primates are at least twice as encephalized as other mammals. Generally, we assume that this greater encephalization results from larger primate brains being selected for greater intelligence.
But, Deacon points out, encephalization measures a relationship between brain and body. It’s not that the primate brain got bigger, he argues, but that primate bodies started to grow smaller. Deacon compared the body and brain growth rates of primates and other animals. He found that primate brains and other mammal brains grow at the same rate, but that primate bodies grow at a slower rate than other mammal bodies. So while primate brains continue to develop along the same growth trajectory as those of other animals with a similar evolutionary history, primate bodies grow more slowly and therefore, over time, got relatively smaller. As primates, our ancestors rode that wave of greater-encephalization-by-smaller-body.
Second, humans changed once again. We are three times as encephalized as other mammals and one and a half times as encephalized as other primates. This is due to the fact that our brains not only grow at the typical primate rate but grow for longer. At the point that other primate brains stop developing, human brains continue to do so, and for a significantly
longer period of time.
The mismatch between the growth rate of body and brain in humans as compared with the mammalian average can best be understood by imagining what we’d look like if our bodies grew at the same rate as our brains, says Deacon. If our body and brain growth rates matched, humans would look more like Gigantopithecus, a half-ton Asian ape that became extinct in the last few hundred thousand years.24
The work of Marino and Deacon emphasizes how important it is to take subtle and complicated relationships into account when we make comparisons across species. Simply taking the brain of one species and comparing its gross size with another’s, is, in the end, not going to answer many questions about why one brain can support a vocabulary of sixty thousand words and complicated syntax, while the other cannot. Other researchers in recent years have uncovered important commonalities in animal brain anatomy and in the function of various types of neurons.
Evidence of the ancient neurological connections between language and gesture were announced in Nature in 2001, when Claudio Cantalupo and William D. Hopkins found that a crucial part of the brain that has been linked with language in humans, Brodmann’s area 44, which is part of Broca’s area, exists in chimpanzees and gorillas as well. What was striking about this discovery was not merely the existence of the area in other primates but the similarity of its structure to that of humans.25
It’s common knowledge that the brain is divided into two hemispheres, each of which normally controls the opposite side of the body. Roughly speaking, the right side of the brain controls the left hand and leg, and vice versa. It’s also the case that particular functions and behaviors can be dominant in one hemisphere, and a significant amount of language function seems to be represented on the left side. Brodmann’s area 44 is larger on the left side than on the right in humans. So far so good—we’ve known this for a long time. But Cantalupo and Hopkins showed that the area corresponding to Brodmann’s area in ape brains is much larger on the left side as well.
Why would this be the case? Apes don’t speak. And if spoken language is a purely human phenomenon, this finding makes no sense. It does make sense, however, if we think of linguistic ability as having a heterogeneous structure. If this ability has developed piecemeal over time, then ape brains should share some of the same structures we use for language. The ape asymmetry also means, wrote Cantalupo and Hopkins, “that the neuroanatomical substrates for left-hemisphere dominance for language were evident at least five million years ago and are not unique to human evolution.”
But still, apes don’t speak. What purpose does a larger left area 44 serve for them? Cantalupo and Hopkins suggest that apes are controlling gestures with this part of the brain in a languagelike way. Humans evolved the ability to point intentionally with their body parts and then with words. Captive apes are known to point at objects with intention, and in the apes observed by Cantalupo and Hopkins, a preference was exhibited for doing so with the right hand. Since the right hand is controlled by the left hemisphere, Brodmann’s area 44 may be controlling the ability to flexibly refer to objects in the world, an ability that underpins verbal and gestural communication.
It is also the case that the apes’ bias for using the right hand was consistently greater when they were vocalizing at the same time as they were pointing. In evolutionary terms, say the researchers, this means the “brain area may be associated with the production of gestures accompanied by vocalizations.” So what started out as a meaningful gesture plus screech in apes, according to Hopkins and Cantalupo, likely became selected over time for speech and modern language in the human species. In 2002 Elizabeth Bates and Fred Dick reviewed the work done on gesture and language and found that as a child grows, these components develop at the same time in the same places in the brain.26
Another extremely striking finding about these shared brain bases came from Michael Arbib and Giacomo Rizzolatti, who discussed mirror neurons as the first real evidence of the neurological underpinnings of imitation in 1997. Mirror neurons are specialized brain cells that fire if you, say, grasp a pen; they also fire when you see someone else grasp a pen. In some sense, the brain interprets these actions as the same thing by mapping them in the same way, meaning that what the monkey can do, the monkey can see. Arbib and Rizzolatti argued that the evolution of minor neurons allowed humans to be skilled imitators: what the human can see, the human can, within reason, do. They help explain why speech is rooted in gestural communication.
Over the course of his career, Arbib’s research has involved developing computational models of the brain mechanisms that underlie language and getting them to sync with findings in psychology, philosophy, and linguistics. In the 1980s he began a research program at the University of Southern California for computational modeling of mechanisms in the monkey brain and started collaborating with Giacomo Rizzolatti on how the brain used vision to control hand movements. He was thus on the spot when Rizzolatti’s research group in Parma, Italy, discovered mirror neurons. It was this work that led him to mirror neurons in monkeys. Arbib began a collaboration with Scott Grafton, a colleague who was an expert in PET imaging, and together they ran some PET experiments to look for mirror neurons in humans, which they eventually found in many regions of the brain.
At first, mirror neurons were thought to underlie only visual recognition of hand actions. But then Evelyn Kohler and others in Parma began to look at their use in the auditory domain, finding that the monkey mirror system was much more sophisticated than originally thought. Mirror neurons fire when stimulated by distinctive sounds as well. For example, if a monkey sees another cracking a nut, certain neurons will fire. If the monkey only hears the breaking shells, some of the same neurons—the audiovisual mirror neurons—will fire. This is a long way from speech, but it does show that mirror neurons can link to auditory input, so some basic mechanisms for grounding the evolution of speech analysis were, presumably, already in place in the brains of our common ancestor with monkeys, who lived twenty million years ago. One aspect of language for which the mirror system may be responsible is the repetition of pronunciation and words. It may also be a foundation for word acquisition, in which repetition is a relatively stereotyped performance.
In his comparative work on mirror neurons, Arbib said his challenge is to ask, “What is the minimal set of requirements for our brain which would make it possible for us to acquire language?” He uses the slogan “the language-ready brain” to suggest that a brain “might not have language, but might be ready to learn it—just as we have computer-ready brains and today we can use computers.” He added, “Nobody would claim that our biology was in any way influenced by the use of computers.”
So far most researchers have studied one relatively small local area of the brain. Arbib has examined the interaction of mirror neurons in the neocortex, and he’s done a fair bit of work on the basal ganglia, the same area of the brain that fascinates Philip Lieberman. Lieberman argued that the kind of sequencing that the basal ganglia controls is as fundamental to language as it is to dancing. And Arbib is inclined to agree. “The mirror system won’t explain all of language,” he said. “The next big step is to pull together all these brain areas that are very important for language, and in particular for understanding how language is created and understood on the fly. The brain is a big place.”
Currently, Arbib is working on a scene description study. “I’m asking, ‘How do you look at a scene, where you do start?’ If I give you a video clip, you’re going to pay visual attention to it, and you’re going to create a visual representation. Then you’re reading part of it out as a sentence as you describe it to me.” When people do this, there’s no sense that they are developing a syntactic structure first and then popping words into it as they describe it, but are literally making it up as they go along. The subjects have a very complex mental picture, and they have to translate from the mental picture to the meaning and then to the words of language.
“It’s not just the sequence
but the skill,” said Arbib. As he reached for a cup of coffee, he said: “Take an example from manual skill. We’ve actually done models of the cerebellum where we reach for a cup. What you’ll see is just one smooth movement where my opposing fingers reach the cup at the same time.” But he explained that if the cerebellum was damaged, it would not be so easy:
You’d have to decompose it in two movements, because you can’t coordinate the timing. So if you tried to do it, you might end up having the fingers too close when you hit the cup, or too far apart when you reach the cup. In other words, you run the risk of knocking the cup over. So instead, what do you do? You very quickly compensate for your understanding of your deficit, and you reach out and you get, let’s say, thumb contact, and then you will close the hand. In other words, you break the thing down into pieces that you know you can succeed with, and then you resynthesize the sequence that will get you to your goal. But each gesture is itself less skillful than it would be if you executed it with [an undamaged] cerebellum.
This implicates yet another part of the brain. “You can get the sequence right without the cerebellum, but if you want a smooth performance, you need a cerebellum. It cues each movement, and it coordinates the movements,” he said. “You can’t do language without a cerebellum.”
11. Your genes have human mutations
There is a family in England known in the medical literature as the KE family. Its twenty-nine members are spread over three generations, and fourteen of them have severe difficulties with speech and language, as well as some general cognitive problems that are less severe. Faraneh Vargha-Khadem, the cognitive neuroscientist at the Institute of Child Health in London who has studied the family for over two decades, explains that their disorder causes immobility in the lower portion of the face, including the lips, tongue, and mouth.
The First Word: The Search for the Origins of Language Page 22
