Deric Bownds

Chapter 2

Origins of Mind

Most of the issues raised in Chapter 1 are unique to human minds. We don't imagine that chimpanzees spend much of their time thinking about thinking. To start considering the processes that ultimately generated our self conscious thinking minds, we have to go back to a beginning when everyone agrees there could have been no minds as we know them. Before biological evolution generated bacteria and eventually chimps and humans, there was the earth. Before the earth there was the universe, which cosmologists tell us started with the Big Bang. This is thought to have occurred about 8--20 billion years ago, depending on which cosmologist you listen to. Our solar system arrived about 4.5 billion years ago, and life appeared on earth about 3.5 billion years ago. Descriptions of our universe are usually offered with an authoritative ring, but arguments over some minor details can have the feel of medieval theological debates. One of the problems to explain is how our galaxy, the Milky Way, can spin so fast without flying apart, for Newton's laws say there is not enough mass present to provide the gravity to hold it together. One solution is to postulate the existence of "dark matter," a mysterious substance that actually accounts for 90% of the mass of the universe. These issues of physics and cosmology are an interesting backdrop to the question of whether organic evolution might have occurred in other solar systems similar to our own, but we will stick to this planet. We have our hands more than full trying to figure out what happened here.

A tiny fraction of the earth's history is occupied by humans like ourselves. If we chart the total span on a single year's calendar, then each day represents 12 million years. In January the world was presumably unknown and inexperienced. The first forms of bacterial life arose sometime in March, about 3.5 billion years ago. During a very narrow window of time in early November, about 570--550 million years ago, virtually every phylum of marine invertebrate animal appeared. The first fish appeared around the end of November. The dinosaurs came around December 10 and departed on Christmas Day. Modern mammals began appearing over the last 4--6 days. Social primates, most of whom are tree-living herbivores and insectivores, appeared December 30--31. The first of our ancestors recognizable as humans didn't 'show up until the afternoon of December 31, 3--4 million years ago. Our species, Homo sapiens, appeared about 11:45 {sc}p.m.{esc} All of recorded human history unfolded in the final minute of the year. All of the hard-wired mental machinery that is specified by our genes was in place long before that final minute. Our nerve cells and nerve signals are fundamentally similar to those of invertebrates such as lobsters and beetles. We share with lower mammals brain circuits for eating, sleeping, sex, fighting, and fleeing. We have a social organization (hierarchy and division of labor) in common with langurs, baboons, and chimpanzees. We socially transmit learned behaviors like monkeys. The past is very much present in us.



We carry in our bloodstream the ionic composition of a primordial sea, and we share fundamentally similar nervous control mechanisms with the simplest of the invertebrate animals.


The theory of biological evolution, essentially as outlined by Charles Darwin, is the cornerstone of our current understanding of how we came to be the way we are. Its description of how humans evolved from apes over the past several million years reflects the best efforts of modern science. It is a story, supported by a vast amount of evidence, that is universal among scientists across cultures. As sure as we are about its correctness, it still is not appropriate to adopt a "This is it!" attitude. Surely there are things we are still missing; whole new areas of knowledge may remain to be discovered. Also, we don't look to our current scientific world view to tell us about the purpose of existence or any possible realms of the supernatural.

The point of this chapter is to launch the "biology part" of this book. We'll start with the origins of cells, simple minds, and selves and with the mechanisms by which they persist and evolve. The chapter introduces some ideas in evolutionary theory, particularly the concept of adaptation. It then looks at how genetically programmed neural circuits of increasing complexity have evolved to drive behaviors that enhance survival and reproduction.

Origins of Sensing and Acting

Where in the last few billion years do we find the origins of our abilities to sense our environment---its chemicals, sounds and lights---and then make appropriate movements, or actions, in response? When did selves and minds appear? The current idea is that the earliest steps toward life involved reactions in a primeval soup that brought together molecules that could make copies of themselves. Evolution then got to work, selecting and designing packets of organic material that had increasing potential for maintaining their own integrity and reproducing. Fascinating experiments have documented the spontaneous formation of simple organic molecules under conditions thought to have existed early in the life of our planet. The study of simple self-replicating molecules, possible precursors of our RNA and DNA, has become a new subdiscipline within chemistry. There is, however, debate over whether these processes all took place on earth. An alternative view is that early life forms appeared on other planets and that the earth was then seeded by organisms or molecules introduced when rock fragments from those worlds struck ours.

An epochal transformation came with the appearance of lipid molecules that could organize themselves into a thin film enclosing these self-replicating molecules, along with other systems that provided energy, preventing their indiscriminate exchange with the external world. These membranes became the boundary at which exchanges of matter, energy, and information could take place. (The evolution of such an enclosure is an example of an emergent property of the sort represented in Figure 1-1: A level of structure appears that feeds back on lower levels of organization to contain and impose boundaries.) The biological cell, which we could call the simplest sort of self, had evolved. Only after this bounded structure appeared was it possible to distinguish "inside" from "outside," ultimately making possible the distinction between "What is happening to me?" and "What is happening out there?" 1



The enclosure of self-replicating molecules by a lipid membrane made possible the first simple cell, a "self" that had an inside distinct from the outside environment.


Why was there a progression from self-replicating molecules to self-replicating cells and thence (much later) to multicellular organisms? What underlying process might have caused the appearance of new and more complex forms? What permitted entities to persist, to reproduce themselves, and to adapt to changing environments? Charles Darwin's central insight was that a very simple mechanism can, in principle, account for the evolution and diversity of all living things. 2 These things, single cells or complex animals, reproduce themselves and, in doing so, make occasional variations or errors that produce slightly different forms. A very tiny fraction of these different forms prove to be better adapted to their environment (perhaps they avoid predators better, use resources more effectively, or are more attractive to the opposite sex) and so are slightly more successful in generating copies of themselves. These more successful forms soon come to dominate the population and are thus poised for further adaptation. This process continues indefinitely, and is sometimes referred to as the Darwin Machine (Figure 2-1). Its cycles of generating, testing, and replicating underlie not only biological evolution but also, as we will see, much of organismal development and behavior.



The idea of the Darwin Machine---with its cycles of generating entities, testing them, and preferentially replicating those that happen to work a little bit better---can be applied to many different self-replicating systems.


Figure 2-1
Cycles of the Darwin Machine. This example of the action of the Darwin Machine starts with a female member of a group of brown rabbits living in an area whose climate is becoming more cold and snowy each year. Most of its newborn are brown, but occasionally a genetic mutation may cause the appearance of an individual that is lighter in color. The darker brown rabbits are more likely to be eaten by predators such as foxes before they reach reproductive age, because they are easier to see against a snowy background (the X marks in the figure indicate their removal). After a number of generations the lighter colored rabbits predominate in the population.

This model was wedded with modern genetics in the early 1900s by Fisher, Haldane, Wright, and others to form neo-Darwinism, which emphasizes the way mutations in individual genes result in the generation of variant adult forms (phenotypes). Phenotypes that are better adapted to their environment reproduce more successfully, leading to an increase in the fraction of the population that exhibits the newer gene combinations (genotypes). The emphasis has shifted, over the past 30 years, from individuals to genes as units of selection, 3 and now it is increasingly believed that selection and adaptation can occur at many levels of organization: genes, groups of genes, individuals, and also families, populations, and larger social groups.

Simple Forms of Behavior

Returning to our simple cell, we can appreciate the origins of sensing and acting in terms of the Darwin Machine. Consider the primitive cells that we have just constructed, the precursors of modern bacteria. 4 Cells that gained the ability to move, perhaps first by amoeboid motion and then by waving small propellers (cilia or flagella) could go to their food rather than depending on its coming. This mobility permitted them to reproduce more effectively, so motile cells came to predominate. The environment of these cells contained good things like food and also bad things like toxic chemicals. Any individual cell that began to develop some means of avoiding the bad things and approaching the good ones would be at an advantage. The eventual result was the appearance of chemotaxis, the ability to sense chemicals in the environment and move toward or away from them (see Figure 2-2). Individuals with this capability were better adapted to their environment than those without, and thus they could survive and reproduce at a higher rate. Over a time scale of billions rather than millions of years, the cells whose modern descendants are bacteria not only invented chemotaxis but also added a nucleus to contain their genetic material. Bacteria able to trap the energy of the sun, and others able to extract energy from complex molecules such as sugars, were incorporated into larger cells and became the power sources of modern plant and animal cells: chloroplasts and mitochondria.

Figure 2-2
In the primordial sea (about 3.5 billion years ago), things to eat and things to avoid. Chemotaxis occurs when a specialized protein molecule called a receptor (one of the dark lines shown in the cell's membrane) binds to a small molecule such as a sugar used for food (the small black dots). The receptor initiates a series of chemical reactions that instruct the cell to tumble about for a while and then take off in a new direction. If this new direction is the right one (the one that takes it toward higher concentrations of sugar), the cell keeps going straight. If it is not, the cell tumbles and tries again. Starting with this simple form of sensing and acting, we can reconstruct a sequence of steps that leads to the complex signaling systems used by our bodies.

Communication Between Cells

The invention of sex permitted the exchange of genes between different mating types in organisms such as yeast, and a new twist to the chemotaxis mechanism appeared. Cells of one type generated small molecules called pheromones to attract their opposite type (see Figure 2-3). Now the signal from the external environment came from another organism, not just from food or things to avoid. Cells were talking to each other.

Figure 2-3
Cells talking to each other (about 1.5 billion years ago). Molecules released by one yeast mating type (the small dots) are recognized by membrane receptors of another mating type, and slightly different molecules (the squares) released by the second type are recognized by the first.

The basic chemical machinery that amoebae, bacteria, yeast, and multicellular animals use for sensing and responding to external stimuli has been preserved throughout evolution and functions in us today. In multicellular animals like ourselves, many of the molecules responsible for sensing and acting that once turned toward the external physical environment now turn toward the environment of other cells. The cells in our bodies use these molecules, which are called hormones, to sense and act on each other (see Figure 2-4).

Figure 2-4
Communication within multicellular organisms (more than 700 million years ago). Hormones (the black dots) released by one cell type, such as a cell in the head that senses food, might tell another kind of cell, such as a digestive cell in the gut, to release compounds that will help break down the food that is soon to appear.

Recall from Figure 1-1 that multicellular animals are composed of stable subsystems: Cells form tissues, tissues form organ systems, organ systems form organisms. Each level of the hierarchy can be said to encapsulate, or contain, lower levels. There is a compelling reason why biological evolution has followed this route. The psychologist and computer scientist H. A. Simon has put it in the form of a parable about two watchmakers, Hora and Tempus. 5 Hora puts together all 100 parts of a watch at once. Tempus puts together five separate stable subunits, or subassemblies, that are then combined to make the final watch. Given that there is a chance of dropping what one is working on and having to start over, who makes more watches? Which watch is easier to repair when broken? This pattern of constructing with stable subassemblies is fundamental in biological organization---and indeed in the construction of any complex entity.

The next step in the story line brings us from hormones to nerve cells and their signaling (see Figure 2-5). A cell, instead of releasing a hormone that diffuses a considerable distance to act on another cell, grows a long, thin process, called an axon, that comes very close to its target cell. An axon can be quite long: Think of the distance between the big toe and the bottom of the spinal cord in your own body---or better yet, in a giraffe. The signaling molecule that it releases, a neurotransmitter, is released from a swelling at the end of the axon (the presynaptic terminal). The neurotransmitter diffuses across a narrow cleft very rapidly to bind to postsynaptic receptors on the target cell. This causes the postsynaptic cell to generate a nerve signal. The whole complex is called a synapse, and the process is referred to as synaptic transmission.

Figure 2-5
Rapid communication at a distance (more than 600 million years ago). The signaling molecules (the black dots), now called neurotransmitters, are released from the end (presynaptic terminal) of a long, thin process (axon) and have only a small distance to move before they can bind to postsynaptic receptor molecules. This makes communication between the cells much more rapid.

Reflexes and Interneurons

The process described in the previous section may seem a bit remote to you, but two-cell systems of this sort are a fundamental part of our own behavioral repertoire. Let the cell on the left in Figure 2-5 become the top cell in Figure 2-6, sensitive to some external form of energy such as the stretch applied to your muscles when a doctor taps your knee with a small hammer. Let the cell on the right in Figure 2-5, to which this cell talks, become a motor neuron in your spinal cord that triggers a compensatory muscle contraction that opposes the stretch, as when your knee jerks forward after the hammer taps it. This is an example of a reflex arc, the simplest multicell circuit found in animal nervous systems.

Figure 2-6
Structure of a simple reflex arc. The top cell is a sensory neuron that is activated by the stretching of tiny processes (dendrites) that thread among fibers of a muscle. An axon connects these dendrites to the cell body of a sensory neuron, which lies just outside the spinal cord. The axon continues on into the spinal cord to activate a motor neuron, which then sends a signal to the muscle that causes a contraction compensating for the stretch. The third cell shown is an interneuron, which can influence the activity of this reflex arc. See the text for an explanation of the other items labeled on this figure.

Two-cell units of this sort are used to construct the large array of reflexes, or harm anticipator--avoiders, that help us (and other animals) ensure a future for ourselves. Essentially the same kinds of circuits are used when a human withdraws a finger from a hot stove, and when a clam slams its shell shut when touched. Reflexes trade accuracy for speed: Better to be safe than sorry. They link sensitivity and responsivity on a rapid time scale, just as in our example of bacterial chemotaxis. They employ receptor cells that evolved to become responsive to external stimuli such as heat, light, sound, taste, touch, and smell. These stimuli became linked to an increasing range of possible behavioral responses carried out by effector cells, such as muscle cells and gland cells. In addition to receptors and effectors responding to and acting on the external world, an array of internal receptors and effectors have evolved to sense and regulate what is happening inside animal bodies---in our case to monitor and control things such as temperature, blood pressure, digestion, and how our joints and muscles are moving. Virtually all of these circuits are hard-wired, which means that they are specified ultimately by our genes and are observed in a fairly stereotyped form in all individuals.

In these simple reflexes we observe the main nerve signals that regulate the mental life of all living creatures, plant and animal. They have maintained the same basic mechanism and form throughout millions of years. These signals are small, brief changes in the voltage across nerve cell membranes: receptor potentials, action potentials, and synaptic potentials. We will not be dealing with the exact forms and shapes of these voltage changes, but their locations are labeled in the drawing of the two-cell stretch reflex shown in Figure 2-6. A stimulus delivered to the receptor cell causes it to generate a local receptor potential, a decrease in membrane voltage that roughly corresponds to the duration and intensity of the stimulus. The cell then converts this to a series of long-distance signals---action potentials, each lasting only a few milliseconds---that travel rapidly (meters per second) along the axon to a presynaptic terminal in the spinal cord. This causes the presynaptic terminal to release neurotransmitter, which diffuses across the synaptic cleft. Interaction of neurotransmitter with postsynaptic receptor molecules of the motor neuron generates synaptic potentials whose magnitude continues to reflect the intensity of the stretch. These synaptic potentials then cause the motor neuron to fire action potentials that travel to its presynaptic terminal with the muscle cell, resulting in activation of the nerve-muscle synapse and contraction of the muscle.



The basic signals of all nervous systems---signals that underlie our mental life---can be illustrated by the simple reflex arc. We can eavesdrop on these signals by placing a tiny sensing electrode inside or near the nerve cells. The small voltage pulses, or action potentials, that travel along the axon and pass by the electrode can be displayed on a computer screen or through a loudspeaker. Because individual action potentials are so fast (only a fraction of a millisecond to a few milliseconds in duration), they are sometimes referred to as spikes. It is an interesting experience to observe an experiment in progress while nerve cell activity is being monitored. You can actually listen to or see action potentials as their voltage signals are converted to clicks on a loudspeaker or the voltage pulses are displayed on a viewing screen. 6


Most reflexes involve more than just receptor and effector cells, and the connections between these two are more complex than the previous discussion indicates. All nervous systems, from the simple nerve nets of a hydra to our own, consist mainly of interneurons. These are cells that intervene to process the information the receptors send to them and then modulate or control the actions that result. Such a cell is included in Figure 2-6. For example, if you are feeling stubborn while in the doctor's exam room, interneurons permit your brain to override the anticipated reflex and inhibit your lower leg from kicking forward when the hammer taps your knee.

The vast majority of nerve cells in our bodies are interneurons. They account for virtually all of our brain cells. In the brain, most of the action is going on constantly in the 1--10 trillion interneurons (the brain and central nervous system) that connect 100 million or so sensory cells to the 1--10 million motor cells. Your brain is mostly sending messages between different parts of itself. Changes in the environment that alter sensory cells result in perturbations of the ongoing activity of interneurons. These interneurons then alter activities in motor cells, which can cause perturbations in the environment. This is how we are structurally coupled with our environment: acting on the environment, sensing the effects of that action on us, and then initiating the next action. We are continuously embedded in an action-reaction loop.

Evolution of the Nervous System

Things were a bit simpler when the most complicated animals around were essentially cavities for ingesting and expelling food, similar to modern hydra and jellyfish. The nervous system was a diffuse fabric of cells (see Figure 2-7). The enteric nervous system that regulates the actions of our gut is derived from this primitive beginning (see "Origins and Structures of the Vertebrate Nervous System" in Chapter 3).

Figure 2-7

A simple nerve net. The nervous system of the hydra, shown on the left, is a uniform and diffuse collection of nerve cells that make multiple contacts with each other to form a net-like structure that can regulate contractions to move food in and out of a digestive cavity. A similar network of neurons, shown on the right, regulates contractions of the wall of our gut.

A next step in evolution was for sensory cells to cluster in the food-intake or head end of simple animals, and bodies began to be designed in segments. The movement of each segment came to be regulated by clusters of nerve cell bodies, or ganglia. A central cord of nerve fibers connected ganglia to each other and to the head (see Figure 2-8). A larger clustering of nerve cell bodies, called a cerebral ganglion, appeared in the head, where a "command central" for integrating sensory input and directing the body to move began to take shape.

Figure 2-8
Invertebrate and vertebrate nervous systems. The central nervous system of an invertebrate consists of paired clusters of nerve cell bodies, or ganglia, running along the belly of the animal underneath the digestive tube (gut), with a larger cluster in the head that deals with sensory input and coordination of the whole body. The central nerve cord of a vertebrate runs along its back and is encased in bone; more of the neurons of the nervous system are clustered in a brain.

Invertebrates and vertebrates share a common segmental body plan, but at some point in the evolutionary line that led to vertebrates, the nervous system turned upside down with respect to the gut so that ventral structures became dorsal, and vice versa (see Figure 2-8). 7 This is why the central nerve cord of a lobster runs along the abdominal side, or ventral, surface of its body, whereas our spinal cord runs along our back, or dorsal, surface. In both vertebrates and invertebrates, increasing numbers of nerve cells in the head region are characteristic of more complicated nervous systems.

The segmental structure of both vertebrate and invertebrate nervous systems is generated by a similar set of genes, called homeotic genes, which are important in determining body pattern. 8 In parallel with the increasing complexity of nervous systems, the number of genes that living organisms carry seems to have undergone three major jumps. The most complex single-cell organisms with a nucleus have fewer than 10,000 genes. Invertebrates have at least 25,000 genes on average, and vertebrates average at least 50,000. 9 The total gene set, or genome, is organized into substructures (chromosomes) that offer highly organized alternatives for selection. 10 During these increases in complexity, some excess baggage appears to have accumulated. Random events and the mechanisms by which component subassemblies of genes are plugged together have led to the accumulation of stretches of "junk DNA" for which no function has been determined. What is even more curious, more than 50% of the genes that code for body proteins can be deactivated by mutation without blocking the development of viable organisms. Some of these genes surely reflect functional redundancy. Evolution has given us backup systems for many functions. Having arrays of genes that code for similar things allows organisms to be more resilient when subjected to environmental insults.



During evolution, different segments of the bodies or nervous systems of invertebrates and vertebrates have been duplicated, mixed, and matched as though they were pieces in a molecular Lego set.


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The bodies and brains of complex animals start their construction from a genetic template provided by their evolutionary history, and their adult forms arise only after a complicated sequence of developmental steps. To change the final product, the building blocks and/or the steps in their construction have to be changed. Thus evolutionary increases in complexity reflect changes in developmental steps. Because developing organisms are modular in organization, novel forms can arise as existing modules are spontaneously duplicated or changed. These forms can spread through the population if the resulting adults form or phenotypes are more likely to pass their genes on to the next generation. Developmental pathways, as we shall see in Chapter 6, are flexible and can generate variations in adult form. Selection then determines which of these variants are then spread and maintained. Thus, evolution is a two step process, with developmentally mediated variation being followed by selection that can result in gene frequency changes. 11

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Think of how different we humans, along with other vertebrates, seem from the vast array of invertebrates on this planet---animals like worms, lobsters, and cockroaches. Our evolutionary line has been diverging from that of the cockroach for at least 500--600 million years, when our common ancestors were probably flatworms with very primitive nervous systems. Some estimates place the divergence of chordates and invertebrates as far as a billion years ago. Since then, both lines have independently invented some advanced features, which include sensory maps, parallel processing, population vector codes, central pattern generators, neuronal plasticity, spatial learning, and memory. (These concepts are covered in later chapters.) We tend to apply the yardstick of our human intelligence to conclude that vertebrates are much "smarter" than invertebrates. However, if we gauge smarts by environmental fitness, humans come out a distant second to the hordes of insects that have so successfully colonized every conceivable ecological niche on earth. Cockroaches will probably still be around if our species manages to do itself in.

An example of a successful invertebrate that, like ourselves, has evolved to become an unspecialized but successful exploiter of complex environments can be found in the octopods. People who study octopus behavior report experiencing these animals as fellow creatures, recognizing different animals and establishing individual relationships with them. Some animals seem timid, others curious and even trusting. It is tempting to treat them as aquatic cats and dogs. They watch you, come to be fed, display simple play behaviors, and will flee with the appearance of fear if you are mean to them. They can be curious about their reflection in a mirror, send out an exploratory arm and then rush in apparent distress back to their hole. 12 In writing such descriptions, observers are implicitly interpreting octopus behavior in human terms. With dogs or cats, there is a certain resonance in doing this because we are all mammals, but the octopus is an alien. Its brain anatomy bears virtually no resemblance to ours, and there is not yet a hint that its motivation would be adjustable in the same way that it is in mammals.

Adaptations and Increasing Complexity

We rationalize increases in the complexity of genomes and nervous systems as adaptations to the environment that made individuals more successful in passing their genes on to future generations. The idea of adaptation is a central one, and it offers an after-the-fact explanation for intricate, apparently purposeful designs that would otherwise seem to require crafting by divine engineers. We can use adaptation to explain why we have image-forming eyes; why birds have hollow, light bones; and why some moth wings have eye markings. The central idea is that each of these complex structures emerged over many generations in incremental stages, each refinement conferring slightly more advantage than its predecessor in fitting the organism to the environment (see the section on "Consciousness and the Evolution of Sensory Organs" below for a more detailed example of this concept). In this sense, an adaptation is a form of knowledge about the environment. 13 Adaptations such as chemotaxis and nervous systems arose through constant interactions of organisms and environments. This relational quality has given them the appearance of being goal-directed, or teleological, but there is no actual goal here---just a drift toward greater complexity because every incremental advantage, however small, might be grabbed and put to use in the next generation.



The concept of adaptation provides an explanation for the appearance of complex, seemingly purposeful designs in the absence of an omniscient designer.


Adaptations are not just responses of passive organisms to implacable forces of the environment. Living creatures also can change the world in which they live in a variety of ways. The composition of the earth's atmosphere is affected as plants and animals generate oxygen and carbon dioxide. Trees cast shadows, dump leaf litter, suck water and nutrients from the soil, and exchange carbon dioxide for oxygen in the air. They are acting on the earth even as the earth is acting on them. Because organisms and their environments shape each other, it makes sense also to view adaptations as relations---as interactions between the two---rather than viewing them only in terms of how they influence the fit of an organism with an unvarying environment. 14 Organisms and the environment in evolution, in organismal development, and during normal behavior form a unity that we often do not appreciate.

Not all adaptations reflect perfect functional solutions to current demands. Structures such as feathers were selected because they served one purpose---temperature regulation---and then recruited to another---flight. Vertebrate jaws may have originated as skeletal arches that enhanced breathing. The bones of our inner ear were once a piece of the jaw. Swim bladders in fish developed from primitive lungs. In these cases a latent potential was recruited for a new use. During evolution there have been frequent reinventions of similar adaptations, and there have been losses, shifts, and even reversals of function. 15 For example, the wings of birds and the wings of bees are analogous traits that perform the same function but arose independently on different branches of the evolutionary tree. In contrast, the limbs of vertebrates descended from a common ancestor and are referred to as homologous traits. The fins of ancestral fish, used for swimming, evolved into the legs of ancestral reptiles, birds, and mammals, used for running or hopping on land. The front legs of certain ancestral mammals and reptile-birds then evolved into the wings of bats and modern birds, respectively, and came to be used for flying. Bird wings and mammalian legs then evolved independently into the flippers of penguins and whales, respectively, thereby reverting to a swimming function and effectively reinventing the fins of fish. At least two groups of fish descendants independently lost their limbs to become snakes and legless lizards.

Evolutionary change is constrained by the building materials on hand. Because genes interact and regulate each other so extensively, there is little room for variability. Phenotypes and genotypes become so complex that they have only limited tolerance for deviation, beyond which they lose their functional integrity. A complex system like a human or a mouse is extremely limited in the direction in which it can change, because it is a hierarchy made up of subcomponents that also have limited room for change, because they themselves are made of further subcomponents. Thus the chance of a mouse evolving into a human is vanishingly small. It would require not only the right sequence of selection pressures, but also the right array of genetic variants and, just as important, the right conditions for driving viable and functional structural change. In the past 500 million years, few new structural types have appeared, and more than 99.9% of all evolutionary lines have become extinct. They were too complicated to change suddenly in response to new demands in the environment. 16 Thus the probability that any present-day life form should exist is extraordinarily low, but it obviously is greater than zero!



Most adaptations are compromises---not optimal solutions but practical, satisfactory ones.


The fossil record appears to have major gaps and does not document intermediate forms in the transitions between major groups. "Punctuated equilibrium" theorists believe that the transformations do not appear because they are essentially instantaneous on an evolutionary time scale. 17 They grant the importance of genetic mechanisms, selection, and adaptation as part of the evolutionary process, but they also believe that speciation and the maintenance of species once they are formed also involve evolutionary processes acting at a higher level than individual organisms. Neo-Darwinians disagree. They maintain that gaps in the fossil record still cover periods of time sufficient for major changes and formation of new species to occur. 18 Recent studies have shown that changes leading to new species can involve only a few genes and can occur over a relatively small number of generations. 19

Classical descriptions of evolution contain such phrases as "evolutionary drive" and "evolutionary progress," which imply an inevitable march toward more complicated forms. More recent studies on the complexity of shape---for example, how mollusk shells or mammalian backbones change over periods of millions of years---suggest, rather, that evolution drifts randomly toward both more and less complexity. With increasing time, this drift causes more complicated forms to appear. At the same time, however, some complicated forms become simpler. 20

Biological Diversity

Biological reality isn't one story line; it is a cacophony of many stories all jumbled together, and there are many possible histories. In an explosion of biological diversity that occurred about 550--570 million years ago, during the Cambrian period, 20 fundamentally different groups of animals appeared. Some have body plans that look different from the forms we know today. 21 It was during this period that complex nervous systems arose, supported by a modular method of construction that employed complicated gene circuits. 22 No new phyla have appeared in the half-billion years since this Cambrian radiation produced the basic set of body plans present today in vertebrates and invertebrates, but each of these body plans has produced an enormous range of variations.

If we had been present 600 million years ago, before the Cambrian explosion, could we have predicted the present diversity of species on this planet? Certainly not. There are too many other scenarios that might have unfolded. We know that sudden changes in patterns of species have been caused by unpredictable geological upheavals such as volcanic action, continental drift, meteor impacts, and climate changes. 23 The survival of our vertebrate predecessor was just one of several possible histories. The appearance of modern humans like ourselves could have been the result of accident or luck that distinguished our line from the many competing hominid lines that existed 500,000 years ago. It also might have been the result of adaptations, such as improved communication or language, that permitted our species to outcompete other hominids for limited resources. We simply don't know which alternative, or what mixture of the two, accounts for our emergence. 24



Survival of the luckiest, as well as survival of the fittest, may have been a factor in the appearance of modern humans.


Origins of Minds, Perceptions, and Affect

The complicated architectures of animals evolved for the same reason that chemotaxis first appeared, to help animals ensure a future for themselves. The purpose of these systems is to generate behaviors that lead to survival and reproduction. Our brains evolved mainly to keep us out of trouble, not to think about themselves. Most of what they do is housekeeping: regulating blood pressure, body temperature, hunger, thirst, digestion, sexual drive, and so on. The brain is an organ, like a kidney or a liver, that plays a specialized role in putting together our complicated machinery and keeping it in one piece. At what point can these nervous systems or brains be said to involve mind, consciousness, or feelings of the sort discussed the Chapter 1? We can approach this question by offering an account of how new functions appeared over time.

The Simplest Form of Mind

In the simplest behavior systems, such as chemotaxis or the reflex arc, a stimulus represents an action. Sensing, whether at the point of stimulation or more centrally in the brain, has little meaning apart from action. An itch is something you scratch, frequently without even thinking about it. In multicellular animals, a stimulus might not only elicit a local reaction but also be relayed to other parts of the body, allowing more complex approach or avoidance movements. In time, animals became more sophisticated, and the sensory side was partially decoupled from the response side. A central site evolved where the action pattern could be held back, in the form of some kind of representation, before being put into effect. (Thus we can suppress an urge to scratch an itch.) This holding, or representational, facility is nervous tissue.

At what point do we say that mind appeared? The definition of mind offered in Chapter 1, mainly in the context of thinking about ourselves, is a global one that offers little guidance on this question. Does a simple stimulus-response system like a bacterium performing chemotaxis have a mind? Does a mechanical thermostat containing a bimetallic element that bends with temperature to activate a switch have a mind?

One approach to answering these questions has been to attribute minds to animals when they first became capable of storing---and possibly recalling and reworking---action-based representation of the effects of environmental stimulation on their own bodies (see Figure 2-9). 25 This function is localized in a ganglion or brain in higher animals. Thus the "center of gravity" of mind is the brain, both in humans and in simple invertebrates, but the brain's activities have little meaning outside of its embodied context.

Figure 2-9
Some have suggested that animal minds originate in the transition from an automatic linking of sensations at the body surface and responding actions (left) to the ability to hold patterns of sensation or action in the brain (right).

With the appearance of a central holding facility, then, sensations can register in some part of the brain without automatically triggering action of the body surface; hence the action can be confined to the brain. It is known that the sensory cortex feeds back on incoming sensory nerves, so an initial stimulus might set up a self-propagating loop. Such a circuit presents us with a possible origin of affect, or emotions. Perhaps to like a stimulus is to respond to it in such a way as to keep up or increase the stimulation, and to dislike it is to respond in such a way as to keep down or reduce it. We can next imagine a multiplication of the circuits for holding representations of sensing and acting and the appearance of superimposed machinery to choose the representatives that are most appropriate.

Consciousness and the Evolution of Sensory Organs

This brings us to the question of when consciousness appeared, particularly the simpler kinds of conscious awareness mentioned in Chapter 1, such as having sensations. The psychologist Nicholas Humphrey suggests that this kind of awareness might correspond to the activity of recurrent feedback loops that can create an extended present outside of physical time. This kind of consciousness can be set apart from the whole range of higher mental functions (perceptions, images, thought, beliefs, and so on). In this view, to be conscious is to have sensations, affect-laden mental representations of something happening to us here and now. Sensations occur in the province of one of the five external senses (sight, sound, touch, smell, and taste) or in the monitoring of our internal bodily states. All other mental activities are outside of consciousness, unfelt and not present to the mind, unless they are accompanied by reminders of sensation, as happens in the case of mental imagery or dreams. This is no less true of conscious thoughts, ideas, and beliefs. Narrative conscious thoughts are typically "heard" as images of voices in the head---and without this recollection of their sensory component they would drop away.

We make a fundamental distinction between our internal sensations or subjective experiences of "What is happening to me?" and our perceptions or analysis of "What is happening out there?" How might we account for the appearance of this distinction during evolution? How did the signals from sensory organs come to be interpreted as signs of something in the external world? A reasonable hypothesis is that two distinct kinds of mental representations developed in simple nervous systems. One eventually led to subjective feelings and first-person knowledge of the self, the other to the objects of cognition and objective knowledge of the external physical world. The evolution of these dual modes of representation might go a long way toward explaining why we now have an apparent standoff between two classes of phenomena: subjective feelings and the phenomena of the material world. 26



More complicated nervous systems began to make a distinction between internal sensation (What is happening to me?) and perception (What is happening out there?).


The evolution of image-forming eyes that has led to our human visual systems is only one of many examples of the development of a distinction between sensation and perception (see Figure 2-10). We can think of the origin of vision in primitive, single-cell organisms as the "taste" or "touch" of light. Molecules in the cell membrane that evolved for reacting to chemical attractants and repellents were further modified to become light-sensitive. This allowed organisms to distinguish "good" from "bad" levels of light. These molecules collected in cells that eventually became the photoreceptor cells (rods and cones) of our own eyes. These photoreceptors (indicated schematically as the vertical bars in Figure 2-10) next clustered together in eyespots sensitive to sudden changes in illumination that might indicate the presence of a predator. The beginnings of a genuine eye came when a patch of photoreceptors was transformed into a cup or depression in the membrane. The edge of the cup cast a shadow when light came from an oblique direction, thus causing a gradient of illumination across the inside of the cup. When the cup was further transformed into a spherical cavity with a narrow aperture at the surface, a pinhole camera was invented. Now the direction of the light was precisely correlated with the position of the image. It was only a small further step to fill in the pinhole with a translucent droplet, producing a full-blown camera with lens. In modern invertebrate and vertebrate eyes, a layer of receptor cells at the back of this camera can report a full image of the external world as signals to the brain.

Figure 2-10
Evolution of the modern image-forming eye. Four stages in the transition from an eye spot to an image-forming lens camera are shown. (a) Eye spot. (b) Eye cup. (c) Pinhole camera. (d) Lens camera.

This sequence provides an illustration of the process of adaptation mentioned earlier in this chapter: A series of small, increasingly useful, or adaptive, changes eventually yields a complicated structure that one might think could have been crafted only by a master designer. Recent studies suggest that image-forming eyes have been independently invented between 40 and 60 times in various invertebrate groups, and the estimated time for the transition from a patch of light-sensitive skin to an eye is under 400,000 generations, a mere eye-blink in geological time! 27

Perceptions and New Behaviors

The invention of the image-forming eye opened up a new world for more sophisticated perceptual analysis. Different objects at different distances cast different shapes on the retina, and stimulation by light of the "seeing skin" had become a source of information about the external world that could be stored by a brain or internal nervous system. By developing a separate channel for visual perception, alongside the already existing channel for visual sensation, animals could take advantage of the defining properties of light (What is happening out there?), while retaining interest in light as an intimate event affecting their own bodies (What is happening to me?).

The option of holding sensations in mind, as well as developing interpretative perceptions of what is happening in the external world, creates a variety of new options for behavior. An early step toward increasing subtlety would be to not just react to the immediate environment, as sensed by touch or taste, but also to develop short-range anticipation over a period of seconds---like the ability to avoid some quickly appearing threat. Humans and very simple invertebrates have circuits, whose hardwiring has been refined over millions of years, for getting out of the way without thinking about it if something suddenly looms and grows larger in our fields of vision.



Our response to light still can contain an affective, or subjective, component. Red light, for example, can sometimes induce physiological symptoms of arousal: Blood pressure rises, and breathing and heart rate speed up. Blue light has the opposite effect. These responses are innate; a 15-day-old human infant is more readily quieted by blue light than by red. Numerous psychological studies show that blue and green reinforce calm, whereas red and yellow can cause agitation. Monkeys show the same affective responses to colored light. On a longer time scale, seasonal affective disorder in humans seems to be caused in part by the lowering of ambient light levels that occurs during the winter months.


Brains evolved to react to features in the environment that were salient to survival and reproduction. Our visual systems, like those of virtually all vertebrates, are most sensitive to patterns with a vertical axis of symmetry, the sort provided by the bodies of other animals. Being informed that other animals are looking at you is frequently important and is often followed by some sort of orienting response. Are you looking at your supper, or are they looking at theirs? A hierarchy of hardwired detectors go into action evaluating "friend, foe, food, or prospective mate." Regions of our brain respond to faces and to whether their attention is directed toward or away from us. One speculation is that orienting responses that began as reactions to alarm signals proved so useful in provoking a generalized update that animals began to go into the orienting mode more and more frequently.

This intermittent regular vigilance may have gradually turned into regular exploration, and a new behavioral strategy began to evolve: the strategy of recruiting information for its own sake in case it might prove useful. If you have ever watched a cat explore, you know that some mammals are hungry for information about the environment. Primates carry this to an extreme. Why did rationality or behaviors arising through learning and intelligence evolve at all? Probably because performing complicated tasks through thought and memory is a more effective and flexible use of our machinery. By generating complex internal models of the world and what to expect in it, we improve considerably on the repertoire that purely instinctive behaviors would permit.


Starting with the simplest kinds of self-replicating entities that we now call cells, we find, several billion years later, a planet whose dominant species is a two-legged animal with a brain that thinks and enables the animal to write about it all. We picture throughout these eons, the relentless cranking of a universal Darwin Machine. Its cycles---of generating entities, testing them, and preferentially replicating those that happen to work a little bit better---underlie not only biological evolution but also, as we will see in later chapters, many aspects of organismal development and behavior. The Darwin Machine, however, is not the only engine of evolutionary change, and an expanded list must include factors such as accident, serendipity, and dumb luck. The origin of our own nervous systems lies in the signaling molecules that free-living single cells seized on to communicate with each other and then retained when they clumped together to become multicellular organisms. In the form of hormones and neurotransmitters, these molecules are intermediaries in organizing the array of electrical nerve signals that are the currency of our mental life.

In even the simplest animals, the vast majority of neurons are not cells that directly sense the environment or command an action on it. Rather, they are the cells called interneurons that intervene, making it possible for the organism to "think about" or hold information, so that sensing and acting can be coupled in less automatic and more varied ways. These cells cluster along a central nerve cord that runs the length of the body, and they also form the brain. More complicated nervous systems began to make a distinction between sensation (What is happening to me?) and perception (What is happening out there?). Varied sensory structures evolved, such as the image-forming eye, along with brains that could interpret sensory input to make a representation of what is happening in the external world. Brain regions appeared that were predisposed to respond to features of the environment crucial to survival and reproduction, such as suddenly appearing objects or faces. In the next chapter, we will consider more specifically the structures of vertebrate brains and nervous systems, designs laid down 400--500 million years ago.

Questions for Thought

1. If you had just 5 minutes to summarize the theory of evolution for a friend, what would you say?

2. This chapter gave an account of how our image-forming eyes might have evolved in a series of very small steps, each step being a change that conferred on the organism some benefit that might render it slightly better at making copies of itself. Can you think of analogous series of steps that might have led to other complicated structures in our bodies?

3. Some schools require that a religious account of human origins be given as much time and credibility as are accorded to the description offered by evolutionary theory. Do you think that such a position can be defended on rational or scientific, rather than religious, grounds? Why or why not?

4. Some scientists believe strongly that humans are the most advanced animal ever to have appeared on this planet and that our presence was foreordained---an inevitable consequence of evolutionary progress. Do you agree or disagree with these views? Why?

Suggestions for Further General Reading

Dawkins, R. 1996. Climbing Mount Improbable. New York: Norton. This is an engaging account of evolutionary processes. Chapter 5 describes the evolution of image-forming eyes.

Dennett, D.C. 1995. Darwin's Dangerous Idea: Evolution and the Meanings of Life. New York: Simon & Schuster. This book describes the adaptationist point of view and provides a summary of the debate on mechanisms of evolutionary change.

Humphrey, N. 1992. A History of the Mind. New York: Simon & Schuster. Speculations in this chapter about the origins of sensation and perception, and about the increasing complexity of nervous systems, are largely taken from this book and from the Dennett (1991) book cited in Chapter 1.

Reading on More Advanced or Specialized Topics

Gerhart, J., & Kirschner, M. 1997. Cells, Embryos, and Evolution. Cambridge, MA: Blackwell Science. This is an informative review of genes, development, and the evolution of animal form.

Keller, E.F., & Lloyd, E.A. 1992. Keywords in Evolutionary Biology. Cambridge, MA: Harvard University Press. This volume provides a series of essays on the key ideas in evolutionary biology and the words used to describe them.

Price, P.W. 1996. Biological Evolution. Orlando, FL: Saunders College Publishing. This volume is a college text that provides a historical view of studies on evolution since Darwin's time.

1. Humphrey, 1992, Ch. 3

2. It also can describe the evolution of many non-living systems. See Kelly, 1994, for review.

3. Dawkins, 1986

4. Humphrey, 1992, pg. 40

5. Simon,. 1969.

6. By giving you such a brief account of the properties of individual nerve cells I don't want to downplay the importance of understanding how individual neurons work. I have spent my professional life studying the underlying molecular details. Complete understanding of cognition, learning, and memory will require intimate knowledge of the molecular changes that occur at synapses during these processes. We know now that very subtle changes in the properties of individual cells can cause networks of cells and whole brains to work in entirely new and different ways. Discussing this material, however, is beyond the scope of this writing. I hope to communicate some central ideas about systems of neurons without requiring this molecular background.

7. Jones and Smith, 1995. The initial option of placing the central nervous system dorsal or ventral to the gut was probably not of major consequence to the primitive flatworm-type bilateral ancestor in which it occurred. However, once the choice was made, the complex architectures that then developed could not be dissociated to form new body plans (Raff,1996).

8. An excellent review of genes, development, and the evolution of animal form is given by Raff, 1996. Depew and Weber, 1996, also discuss evolution and development.

9. Miklos, 1993.

10. Depew and Weber, 1994. See Cossins, 1998, and Rutherford and Lindquist, 1998, for discussion of mutations in fruitflies that provide mechanisms for rapid morphological change.

11. West-Eberhard, 1998. This review is an excellent summary of the new confluence of evolutionary and developmental biology.

12. Cousteau, 1973; Wells, 1978.

13. Plotkin, 1994

14. Lewontin, 1985.

15. Evolutionary convergence, the re-invention by unrelated groups of animals of similar solutions to problems posed by the environment, provides one of the most powerful arguments for the importance of adaptation. Eating termites, for example, is a a very specialized business, and the spiny anteater of Australia and the pangolin of Africa have independently evolved long sticky, worm-like tongues, long snouts, and powerful claws for breaking into termite mounds and extracting their food. Numerous cases of convergence are being documented at the molecular level. For example, very similar antifreeze protein molecules have been evolved by unrelated groups of fish living near the north and south poles

16. Kelly, 1994, Ch.18, 19

17. Kerr, R.A. 1995.

18. Dennett, 1995.

19. Coyne, 1995 described the appearance of new plant species by simple genetic changes; Weiner, 1994 gives a description of evolution occurring in modern times among Darwin's finches on the Galapagos islands. Johnson et al., 1996 and Stiassny and Meyer, 1999, describe the rapid evolution of cichlid fishes in Lake Victoria in Africa, and Losos et al., 1997, describe rapid changes in the morphology of Anolis lizards after their introduction to several Caribbean islands. Trut, 1999, summarizes a breeding program carried out over the past 40 years in Russia that has coverted wild wolves into gentle domestic animals

20. The volume edited by Keller and Lloyd, 1992 covers some of these points and discusses how words such as progress, adaptation, selection, gene, niche, primitive, etc. - mean different things to different people. Gould, 1994, discusses the issue of increasing complexity.

21. Kerr, 1993.

22. Miklos, 1993; Jablonski, 1997.

23. Ager, 1993; Gibbons, 1993a

24. For a recent textbook on biological evolution, see Price, 1996.

25. Many of the ideas in this section are drawn from Humphrey, 1992 and Dennett, 1991.

26. This section is mainly a digest of ideas from Humphrey's (1992) book.

27. Nilsson and Pelger, 1994: Dawkins, 1996.

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