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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.
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DESIGN NOTE: IMPORTANT POINT
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.
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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?"
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DESIGN NOTE: IMPORTANT POINT
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.
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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. 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.
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DESIGN NOTE: IMPORTANT POINT
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, 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. 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. 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.
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DESIGN NOTE: LONG IMPORTANT POINT
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.
********
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). 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. 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. The total gene
set, or genome, is organized into substructures (chromosomes) that offer highly
organized alternatives for selection. 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.
***********
DESIGN NOTE: IMPORTANT POINT
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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******new text*****
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.
******end new text******
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. 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. 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.
***********
DESIGN NOTE: IMPORTANT POINT
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. 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. 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. Thus
the probability that any present-day life form should exist is extraordinarily
low, but it obviously is greater than zero!
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DESIGN NOTE: IMPORTANT POINT
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. 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. 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.
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.
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. It
was during this period that complex nervous systems arose, supported by a modular
method of construction that employed complicated gene circuits. 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. 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.
***********
DESIGN NOTE: IMPORTANT POINT
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). 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.
***********
DESIGN NOTE: IMPORTANT POINT
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!
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.
************
DESIGN NOTE: LONG IMPORTANT POINT
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.
Summary
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.
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