The First Word: The Search for the Origins of Language (24 page)

Brad Schlaggar, a pediatric neurologist and a professor at Washington University in St. Louis, says that the best way to think of plasticity is as a support structure. When he gives a talk about plasticity, he always shows students slides of the St. Louis Arch. “As the structure goes up,” he explains, “the relationship between the scaffolding and the leading edge of the two sides of the arch changes as they rise up to meet in the middle. The relationship between the scaffold and the emerging mature structure is dynamic, as opposed to a scaffold that surrounds a building and then comes down again.” So if damage occurs to the brain of a seven-year-old child, it occurs in a completely different context than if the child were much older or younger. “The scaffolding idea means that even in adults, the organization of the brain for learning a novel task or a challenging task is different from the organization of implementing that task once you have acquired the skill.” The scaffolding for language seems to be particularly flexible. Fred Dick describes the development of language as a moving target. If damage is sustained in one area, language may move, morph, and settle into another.

In his doctoral work Schlaggar transplanted the visual cortex of one fetal rat brain into another, placing it in the spot where the somatosensory cortex, which normally controls the body as it moves through space, typically develops. Schlaggar found that the transplanted visual cortex grew into a fully functioning somatosensory cortex. The inputs into the new region came from the body as it moved in space, and as a result that neural tissue became wired to process that kind of information.

We tend to think of the brain as developing on a completely separate trajectory from that of the body. Traditionally researchers imagined that the brain had some kind of central developmental controller instructing different parts to assume responsibility for different abilities (the visual cortex develops particular types of neurons, while the auditory cortex develops differently specialized neurons, and so on). But recent research has cast grave doubts on the existence of any kind of central controller. It looks as if the brain tissue that ends up becoming part of different specialized regions is not necessarily fated to end up that way, and that input to the brain coming through the filter of the body contributes to its architecture.

Leah Krubitzer, a professor of psychology at the University of California, Davis, also demonstrated how the immature brain isn’t fated to be mapped into the specific regions that are typical of the adult brain. She removed a big chunk of the brain of newborn marsupials, and then let them grow up and develop normally. After they reached adulthood, she took another look at their brains. The cortices had organized themselves into exactly the same areas as a normal brain would, all in the same spots relative to each other, but they were all slightly smaller, so as to fit within the smaller brain. While there is a default optimum map, it appears that the map can be drawn over different kinds of neural terrain.

For all the apparent complexity of the human language-brain relationship, it’s important not to lose sight of the fact that some hard-to-pin-down behaviors and preferences appear to be completely controlled by the way genes have built the brain. In 2001, in a strange complement to the experiment in which chickens with transplanted bits of quail brain ended up producing some species-specific quail calls, Evan Balaban and colleagues at the Neurosciences Institute in San Diego transplanted a piece of brain from a Japanese quail into the brain of a domestic chicken, and likewise placed a piece of chicken brain into the head of a Japanese quail. With their new chimeric brains, the birds continued to produce the calls of their own species, but instead of responding to the maternal calls of their own species, they showed interest in the calls of the other.
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There’s no reason to believe that processes like these aren’t also relevant to the human experience, even if they can’t fully explain the complexity of language.

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.
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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.
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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.”
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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.
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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.)
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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.
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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.
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