The Evolving Brain: The Known and the Unknown

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THE EVOLVING BRAIN

Prometheus Books

Chapter One

A brain is like an anthill. Tiny neurons are the ants, performing their circumscribed tasks in a manner determined by the role they were born to serve. They look no farther than their nearest neighbors, they understand nothing of the larger structure that results from their persistent action. Although there are an enormous variety of tasks to perform, each neuron, like each ant, can do a limited number of tasks, each in a limited way. And yet complexity emerges, in an anthill as in a brain, in ways that cannot be predicted.

The subtle and adaptive response of the brain to its environment is possible, in part, because of the high degree of specialization brain cells demonstrate. In a similar way, the complex response of the anthill to its environment is partially a result of the astonishing degree of specialization in different ant castes. Most people are familiar with the idea that a typical ant colony has workers, soldiers, and a queen, with an apportionment of tasks among the castes that is appropriate to the bodily form of the members of that caste. But even this simple separation can astonish: the Asian marauder ant shows a division of body typesso extreme that a soldier can be as much as 500-fold larger than a worker ant. A worker ant could easily ride on a soldier of the same species, like an infant clinging to an elephant.

The Power of Specialization

The human brain is a marvel of specialization: certain neurons in the visual cortex will only respond to straight lines oriented at a specific angle in the visual field. The specialization of ants is also far more specific than simply a division of castes into workers, soldiers, and a queen. For example, leafcutter ants perform a complex task that has been apportioned among worker ants in a particular way. Certain ants make foraging trips to growing plants, where they clip off leaves or leaf fragments. These leaf bits are brought back to the colony and macerated into pulp, and the leaf pulp is used as fertilizer for a fungus. This fungus, which grows into a gray blob the size of a kitchen sponge, is tended as carefully as an asparagus farmer tends his crop, providing food for the entire colony. The whole process, from clipping and macerating of leaves to the careful tending of the fungus, is done in assembly-line fashion, with each step of the process done by a specific type of worker ant. Each ant is slightly different in form and function from the other ants elsewhere on the assembly line.

Specialization in an anthill or in a brain allows the component tasks of life to be performed with greater efficiency. The foraging leafcutter ants that venture from the nest to cut leaves are the largest ants involved in the whole process; they are nearly three times larger than the smallest ants at the other end of the production line. Forager ants are large enough to bring huge leaf pieces back to the nest, carrying them above their heads like green parasols. When the forager ant reaches the nest chamber, leaf pieces are passed off to worker ants of slightly smaller size, who clip the large leaf piece into tiny fragments. As soon as this process of creating leaf confetti has been completed, smaller ants begin to mulch the confetti into moist pellets of leaf mush. Workers even smaller in size then take the moist pellets and insert them into the growing mass of mold, like a farmer fertilizing a crop. Macerated leaf pellets are placed near active sites of fungal growth so that fungal strands can be loosened and draped over the fresh leaf mulch. The very smallest and most abundant worker ants then tend the fungal mass, plucking out stray spores and strands of unwanted fungal types so that the desired fungus can grow as quickly as possible. Dwarf worker ants actually move through tiny corridors that penetrate deep within the growing fungal mass. Occasionally, the dwarf ants will tear away tufts of fungus, which resemble tiny stalked cabbages, and bring these morsels out to their nest-mates. It is these fungal tufts that provide food for the rest of the colony.

To be clear, we are not implying that the human brain is a mass of fungus or that our brains are filled with cells that crawl about, carrying knowledge like ants carry leaf mulch. It is perhaps too easy to abuse the analogy of the anthill in thinking about the complexity of the human brain. Yet the anthill and the brain have some very deep parallels, and it is possible that careful consideration of the anthill analogy will lead us to new ideas about the human brain. At a prosaic level, the anthill is nourished by worker ants that forage and bring food back to the colony; this seems analogous to how oxygen and glucose are brought to the brain by the circulatory system. But there are far deeper parallels.

Parallels between the Anthill and the Brain

Both the anthill and the brain behave as a unit, are composed of subunits of varying types, undergo a cycle of growth and senescence, and respond to their environment in adaptive ways. Both anthill and brain show subtle yet significant idiosyncrasies of structure and function that make each individual unlike any other of the same type. Both anthill and brain are rather ancient, at least in their incipient form, so there has been ample time for a relentless process of change to produce what is now a highly sophisticated structure.

Both anthill and brain lack a clear chain of command: there is no "boss" in a brain, just as there is no "king" in an anthill. The ant queen merely produces young, rather than controlling the moment-to-moment action of her subjects. All decisions are made at a local level, and decisive action can be taken without any centrally placed or "integrative" functions. A clear example of this is how an insect intruder into an anthill can be attacked and destroyed without a general alarm being sounded. The soldier ants already in the area coalesce to dispose of the intruder. In much the same way, an immune response can be mounted in the brain, relying upon locally available immune cells to attack an intruder.

Local responses can be amplified, in both anthill and brain, and the speed and scale of a response can be increased in both. Soldier ants in a colony can be broadly stimulated to respond to a major threat to the colony, just as a systemic immune response can be mobilized to fight a potent threat to the brain. The only way to insure that a local response remains adaptive is for there to be efficient communication among the responding units, as well as some form of feedback that can turn off a response when that response is no longer needed. Feedback loops, especially those negative feedback loops that help diminish a response before it escalates out of control, are central to maintaining a balance within the whole-in the anthill and in the brain.

It is an astonishing property of the collective, whether that collective is a brain or an anthill, that a local response can generate action appropriate at the systemic level. Moreover, decisions, made at the level of the individual, can nonetheless benefit the whole. For example, you can withdraw your hand from scalding water before a message of pain reaches your brain because a local reflex successfully mediates the systemic response well before pain is perceived. The same property, of a local response benefiting the whole, is also seen in an anthill. If the colony comes under attack from a slave-making species of ant, larval ants in the nursery can be saved by the workers tending them, even if there is no general alarm that recruits the colony to respond. The actions of individual ants may be nonadaptive, as some ants will pick up larvae and run about in a seemingly random fashion, but the response of ants in the aggregate is adaptive, so that the larvae are quickly saved. An adaptive response at the systemic level is therefore produced by autonomy at the individual level, modulated by appropriate feedback controls.

Another key property of the collective is that the reliability of individual units, be they ants or neurons, is lower than the reliability of the whole. The enhanced reliability of the collective is possible only because of functional redundancy: if a single ant or a single neuron fails to perform a required function, there are other units still capable of generating an appropriate response. Redundancy of function not only makes the anthill and the brain more flexible and more reliable, it may also free the collective to experiment somewhat. Over time, new functions and new ways of interacting can arise, and this may explain why both anthill and brain have been so successful through time.

The Concept of Emergent Complexity

Perhaps the most exciting parallel between the anthill and the brain is that both entities can demonstrate emergent complexity-a synergy in the system that has grown over time. Emergent complexity is shown by those properties of the whole that could not have been predicted from an intimate knowledge of the parts. No matter how deep our understanding of the brain at the level of neurons, it is simply not possible to predict that a collection of neurons could write For Whom the Bell Tolls. It may be that emergent complexity is a real-world outcome of chaos theory, in that unpredictability is inherent to any highly complex system, even if that system runs by rigid rules. But, whatever the cause, it is clear that the whole can be far more than the sum of its parts.

We are all familiar with examples of emergent complexity in human endeavor, but examples of emergent complexity in an anthill are much less familiar. Nevertheless, emergent complexity seems to be the only way to explain how an ant colony is able to perform certain remarkable feats. Ants, for example, prefer a warm environment, avoiding temperature extremes; most ant species are unable to remain active if the air temperature dips below about 20° centigrade (which is roughly equivalent to 70° Fahrenheit). Certain ant species that live in either cool or very hot climates can sometimes find a congenial microclimate by digging their nests deeply into the soil. But other ant species are able to maintain a congenial nest temperature by a much more sophisticated means than by merely exploiting the constant temperature found several meters below the soil surface. The European wood ant builds a mound nest that projects above the ground surface, yet the nest maintains a stable internal temperature as much as 10° C (22° F) warmer than the air above it or the soil below it. This constant temperature is maintained by stuffing the nest full of decaying vegetation. As the vegetation decomposes, it releases heat, just as the middle of a haymow is warm in a frigid Siberian winter. Furthermore, the nest is built to retain heat, so that the heat of decomposition and the heat created by the ants as they move about is retained within the mound. What this means, in essence, is that ants adapt their behavior to maintain a constant nest temperature despite changing environmental temperatures.

Emergent complexity is postulated to be a property not of ants, but of the colony, and not of neurons, but of the neuronal collective. Individual neurons are unable to generate the complex behavior possible for a brain, just as individual ants cannot build an anthill. The complex behavior so characteristic of humans cannot be produced by a simple brain but, rather, requires the sophistication of a mind that is able to perceive the subtleties of the world. In short, emergent complexity is a property of the mind, not of the brain. A brain is simply a collection of neurons, each of which behaves in a certain way. A mind is what arises when a huge number of neurons are free to perform their specific functions. The mind is what interposes between the brain and behavior; mind may be evidence of brain, but brain is the source of mind. A mind has thoughts and feelings, opinions and beliefs, strengths and weaknesses: a mind has a personality.

How Can We Explain the Parallels between Ants and Neurons?

We have noted the similarities between ants and neurons without asking why such parallels exist. There can be one reason and one reason alone why ants in an anthill function so much like neurons in a brain.

The energetic cost of having a brain is extremely high; the energy required for brain function in a resting adult is roughly 20% of the total energy intake for the entire body. This high demand for energy arises because, while it may be inefficient to maintain the brain in a state of constant readiness to respond when no threat is approaching, it is catastrophic to be unprepared for such peril. Even if a neuron is not conveying information, it must stand ready to do so, and the energetic cost of readiness is extraordinarily high. This places a very strong selective pressure on the brain, since resources are limited in virtually every environment. If a brain were to become too inefficient, if the benefits of having such a complex structure did not outweigh the costs, then the cockroach brain might well be the most advanced brain in the world. Similarly, ants that waste precious resources will not survive for long, and every anthill must act to conserve space, materials, time, and energy. An anthill that profligately wastes resources would lose out over time to another more efficient ant species.

In short, ants and neurons, as different as they seem, are under similar selective pressures, and they have evolved similar responses. The more we learn about the brain, the more we come to appreciate that it was shaped by the same subtle evolutionary pressures that act on an anthill. This is one of the strongest possible demonstrations of the power of evolution to explain widely disparate and seemingly unrelated observations.

Evolution: The Basics

Some people are uncomfortable with the idea that the human brain arose by an evolutionary process. The standard objection is that a random process like evolution cannot have given rise to an engineering marvel like the brain. Yet this objection is based on a misunderstanding of the true nature of evolution. Evolution is far from a random process. Evolution is a result of two complementary processes: mutation and natural selection. Mutation is a random process of change at the level of the genes. However, these genetic mutations are winnowed out in the most nonrandom process imaginable-natural selection. This process is the merciless culling of unsuccessful mutations through predation or premature death.

To discuss evolution with the rigor and the dispassionate fairness that it deserves, we must have a clear understanding of exactly what it is. Evolution is simply a gradual process of change. To a biologist, evolutionary change takes place by a mechanism that can be explained in fairly simple terms. The following four precepts are sufficient to generate gradual change in an animal stock that any biologist would recognize as evolution:

1) Variation exists. This observation is so simple that it seems incontrovertible. Any large gathering of people will include people who are slim and fat, old and young, tall and short, weak and strong, healthy and ill. It could perhaps be argued that most variations are meaningless in an evolutionary sense, especially in a human gathering, and this may well be true. Yet variation exists in all organisms of all species in all places.

2) Some variants are more successful than others. Imagine a herd of antelope in which some are slim and some are fat, some are old and some are young, some are weak and some are strong. Clearly, if a lion were stalking that herd, then the fat or the old or the weak would be more likely to die. Predation is not random; lions risk injury every time they hunt, so they always seek the weakest prey. Even then, lions are not always successful, since they are sometimes unable to kill any prey. Nevertheless, over time, there is a stronger selection pressure working against the weak than against the strong.

3) Variation is heritable. Everything we know about our own families convinces us that certain traits are likely to run in families: tall parents tend to have tall children just as near-sighted parents tend to have near-sighted children. Everything we know about genetics concurs that certain traits are passed down to offspring, often with a high degree of fidelity. This simple truth can have terrible consequences, as certain families are ravaged by hereditary illness.

4) Successful variants tend to become more abundant over time. Because certain individuals are more likely to survive long enough to reproduce, and because these individuals are able to pass specific traits on to their offspring, these traits will tend to be well represented in ensuing generations. In contrast, other individuals may have traits that are more likely to lead to premature death, so these traits are less likely to be passed down. Over time, the successful traits will tend to increase in the population whereas the unsuccessful traits will gradually decrease. While this process is inexorable, it is not as effective as one might imagine. For example, heart disease can kill people at an early age, but usually not so early that those with a weakened heart are unable to have children. Thus, there is effectively no selective pressure against heart disease in humans, unless the survival of the children is somehow impaired.

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Excerpted from THE EVOLVING BRAIN by R. GRANT STEEN Copyright © 2007 by R. Grant Steen. Excerpted by permission.
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