4.2 The Evolution of the Nervous System

The information regarding the evolution of the nervous system derives

from a variety of sources, including the comparison of anatomical features

of fossils with living species thought closely to resemble such fossils. In

this way, biologists and other researchers can study the nervous systems

of living species in order to gain a picture of how the nervous systems of

ancient creatures most likely had functioned. Accordingly, after I speak

about the evolution of the cell, I trace the evolution of the nervous system

in the following progression: euglena →hydra →worm →fi sh →reptile

→mammal →primate. Thankfully, we have fossilized remains of creatures

resembling each of these types of species—including what we take to be

cells and protocells (Forterre, 2005; Pennisi, 2004; Margulis & Dolan, 2002;

Berra, 1990)—making our inferences regarding the evolution of nervous

systems reliable and probable.

Some 3.5 billion years ago, approximately 1 billion years after the earth

was formed and 12 billion or so years after the Big Bang, rudimentary onecelled

life forms—bacteria—appeared here on earth. Since the famous

Stanley Miller experiment in 1953 where four amino acids, urea, and

several fatty acids (all organic molecules) were synthesized from methane,

ammonia, hydrogen, and water, numerous other laboratory experiments, thought experiments, and hypotheses have shown that it surely is possible

to generate organic molecules from inorganic molecules (Forterre, 2005;

Pennisi, 2004; Margulis & Dolan, 2002; Schopf, 2002; Line, 2002; Balter,

2000; Margulis & Sagan, 1986; Dawkins, 1976, 1986, 1996, 2005; DeDuve,

1995, 1996; Ball, 1999; Fry, 2000; Eigen, 1992; Fenchel & Finlay, 1994).

Theorists postulate that amino acid chains, proteins, and then the organelles

of the cell would have been the fi rst things to evolve, in either a primordial

soup or some crystalline formations (Pennisi, 2004; Margulis &

Dolan, 2002; Margulis & Sagan, 1986; Mayr, 2001; Eldredge, 2001; Churchland,

1984). The cell is the fi rst kind of thing that exhibits the four properties

I have spoken of in the previous chapters, namely, internal–hierarchical

data exchange, data selectivity, informational integration, and environmental–

organismic information exchange.

However, added to these properties would be mechanisms of self-replication,

protection, and Humphrey’s (1983, 1992, 1998) distinction between

what is happening out there (hereafter, OT) versus what is happening in here

(hereafter, IH) mechanisms (also see Dawkins, 1996; Churchland, 1984).

The DNA and RNA of cells act as the mechanism of self-replication, the

cell wall acts as the mechanism of protection and encapsulation, and the

processes of the organelles themselves engage in the exchange and control

of information while interacting with environments. In light of the ideas

already presented regarding genetic variation and natural selection, once

RNA and DNA emerge, the mechanisms of evolution can do their work in

cellular processes of organisms. Nervous systems came into being by the

interplay of genetic mutation and environmental fi tness, as generation

after generation of RNA and DNA have replicated in cells over and over

again.

The evolution of the nervous system really begins with microscopic

organisms that developed a light/dark sensitivity area, most likely in primordial

waters, some 3 billion years ago. In the fi rst chapter, I spoke about

the euglena, an algae that has an eyespot and uses its fl agellum to move

toward light in order to get food. This is not yet a nervous system;

however, we witness the information exchange among the processes

within the euglena, as well as the hierarchical organization of such processes

that will be found in nervous systems. We also see the beginnings

of Humphrey’s distinction between OT and IH, since the euglena uses

basic stimulus/response mechanisms to maneuver toward food and away

from danger.

It is important to note that a kind of primitive visual system would

appear to act as the catalyst for the evolutionary development of the

The Evolution of the Visual System and Scenario Visualization 97

nervous system. In fact, light/dark sensitivity emerged among a whole

multitude of organisms in the protist and monera kingdoms (Febvre-

Chevalier, Bilbaut, Febvre, & Bone, 1989). Further refi nements of the light/

dark sensitivity area can be found in animals, either in terms of stimulus/

response mechanisms or various kinds of rudimentary and advanced visual

systems (Cronly-Dillon, 1991; Blake & Truscianko, 1990; Horridge, 1987).

For example, the photosensitive pigment cells in a worm’s skin are responsive

to light, as is the rudimentary pinhole eye of the nautilus. This speaks

to the fact that an organism’s ability to “see” is crucial for its survival,

which makes sense given that the world is bathed in sunlight. Sunlight

interacts in the world in such a way that organisms have taken advantage

of this form of energy so as to optimally engage in forms of feeding, foraging,

fl eeing, fi ghting, and so forth.

The euglena is a one-celled organism. With the arrival of multicellular

organisms having simple nervous systems came the possibility that a

variety of cells could have a variety of functions. The hydra is a cnidarian

that has a simple nervous system called a nerve net. This nerve net forms

an undifferentiated network of sensory neurons, motor neurons, and interneurons

(DeDuve, 1996; Bonner, 2000; Mackie, 1989). This was a crucial

development in the evolution of the nervous system for at least three

reasons. First, the electrochemical processes of the neuron, as well as the

multiple axonal/dendritic neuronal connections, could aid in speeding up

reactions to external stimuli, enabling the animal to be more effi cient at

hunting and/or avoiding being eaten.

Second, more complex and more effi cient forms of communication

within the animal could be achieved with the introduction of sensory

neurons, motor neurons, and interneurons that are specialized in their

functions. Sensory neurons deal with the out there of an animal, since they

are located near the periphery of an animal’s body and are concerned with

relaying external environmental information to it. Motor neurons deal

with the in here of an animal, since they aid in controlling the animal’s

bodily movements, as well as internal responses to external environmental

stimuli. Interneurons connect sensory neurons to motor neurons and act

as a kind of fi lter or buffer for the exchange of information between the

two. Response times could be decreased by specialized neurons’ performing

specifi c tasks in parallel rather than having general-purpose neurons

perform a variety of tasks. A neuron that has emerged to handle only one

kind of problem likely will be able to handle that problem swiftly and

effi ciently because it has to handle only that particular kind of problem and

no other one (also see Arp, 2008b). Further, many specialized neurons working on some problem together minimizes errors and allows systems

to perform more optimally (cf. Culler & Singh, 1999; Bechtel & Abrahamsen,

2002; Feldman & Ballard, 1982). The parallel form of processing

exhibited in the tripartite neuronal network made it through an evolutionary

sieve because of its effi ciency, and such a natural selection comports

with the general evolutionary principle of economy, namely, whatever trait

gives an organism a competitive advantage most likely will be naturally

selected as fi t for that organism and will be passed on to that organism’s

progeny. I will say more about specialized versus general-purpose processing

in the next chapter.

Third, with the introduction of this tripartite neuronal system, Humphrey’s

distinction between IH and OT clearly comes into play. This is

important for more evolved organisms that require part of the nervous

system to be devoted to maintaining homeostasis within the organism (IH)

while another part of the nervous system is attending to external stimuli

(OT). Again, a nervous system having neurons of differing types devoted

to specialized tasks could perform more effi ciently than one having generalpurpose

neurons.

The next step in evolution was for sensory cells to cluster in the heads

of simple animals like the worm. The body began to take on segmentation,

and the movement of each segment came to be regulated by nerve cell

bodies, or ganglia. A central cord of nerve fi bers connected ganglia to each

other and to the head. Also, a larger clustering of nerve cell bodies (a cerebral

ganglion) appeared in the head, where a kind of brain-like command

center for integrating sensory input and directing the body began to take

shape. It makes sense that the clustering of nerve cells together in the head

was selected for, since electrochemical information would not have as far

to travel, thus allowing for conservation of energy and faster communication

between neurons.

Fish evolved from worms, and, at fi rst, there was just a brain-like bulge

on top of the spine. Then, the nerves started to sort themselves into specialized

modules—some became sensitive to certain molecules and formed

smell modules, while others became sensitive to light and formed visual

modules. The major evolutionary advance concerning fi sh has to do with

the clear differentiation of the nervous system into the CNS and the

peripheral nervous system (PNS). The CNS consists of the brain and spinal

cord and is in constant two-way communication with the PNS, which

consists of the somatosensory, afferent somatic, visceral, motor efferent,

automatic (made up of the involuntary sympathetic and parasympathetic

systems), and somatic (voluntary) systems. This is the general layout that

The Evolution of the Visual System and Scenario Visualization 99

is found in reptiles, amphibians, and mammals as well (Audesirk et al.,

2002; Kandel et al., 2000).

The distinction between IH and OT is crucial in order to understand

more complex vertebrate nervous system functioning and behavior. Parts

of the CNS and PNS—like the hypothalamus and parasympathetic systems

in mammals—are devoted to the internal homeostasis of the organism;

these are the IH processes. Other parts of the CNS and PNS—like the LGN

in the brain and the somatic system in mammals—are devoted to reaction

to external stimuli around the organism; these are the OT processes. Ultimately,

as we will see in the evolution of primates, some of the IH processes

formed a kind of feedback loop, generating emotions and conscious

thought. Instead of merely being directed toward the maintenance of OT

and IH processes, the brain evolved abilities to feed back information

within itself that is distinct from, but analogously related to, the processes

of the CNS and PNS. It is arguable that this feedback of information

allowed for the possibility of mental states such as emotion, refl ection, and

consciousness to emerge (Humphrey, 1992, 1998).

After fi sh came into being, the processes and systems of the brains of

subsequent animals evolved at a rate that is disproportionate in relation

to the rest of the body. The evolutionary story from here on out is more

about the increasing size and complexity of the brain rather than that of

the nervous system generally. Presumably, this is the case because the

primary sensory mechanisms converge in the head of most animals, and

the brain has become a kind of command center for both the CNS and

PNS. There is an adaptive payoff to having a locus for information transfer

and exchange. As this locus, the brain can go about its business of processing

information quickly and effi ciently, utilizing neurons that are crossconnected,

working in parallel, and in close proximity to one another.

Given the brain’s ability to process information effi ciently, it has been able

to pass through one environment after another in the evolutionary sieve.

At the same time, more parts and processes have been added to the brain

because, to put it crudely, more parts mean more possibilities for effi cient

information exchange. However, it is not merely the addition of new parts

to the brain that accounts for its effi cient processing, and, later in this

section, I will make some qualifi cations about how to understand the

evolutionary addition of parts to the brain.

The evolutionary development of the brain from fi sh, through reptiles,

to mammals and primates has been explained effectively in the famous

works by MacLean (1967, 1991; cf. Sternberg, 1988; Kaas, 1987, 1993, 1995,

1996). According to MacLean’s model, the primate brain is really a three-part brain having evolved the neocortex but retaining the limbic system

found in mammals and the brainstem core found only in reptiles. The base

of the primate brain is shared with reptiles and consists of the brainstem,

reticular formation, and striate cortex. These areas are where the necessary

command centers for living are located, namely, the control of sleep and

waking, respiration, body temperature, basic automatic movements, and

the primary way stations for sensory input.

Eventually, what MacLean calls the paleomammalian cortex evolved on

top of the reptilian brainstem, allowing for more modules to develop: the

thalamus, allowing sight, smell, and hearing to be used together; the

amygdala and hippocampus, apparatuses for memory and emotions; and

the hypothalamus, making it possible for the organism to react to more

stimuli by refi ning, amending, and coordinating movements. The functioning

of the paleomammalian and reptilian cortices are somewhat analogous

to the functioning of a heart, pancreas, or kidney, since they are

organized less for thought and more for automatic action and response.

This makes sense from an evolutionary perspective. Reptiles, amphibians,

and mammals out in the wild share the common problems of having to

respond quickly to environmental stimuli so as to know whether to fi ght,

fl ee, forage, or procreate in order to survive.

Finally, in the evolutionary history of primates what MacLean calls the

neomammalian cortex (or cerebral cortex) evolved on top of the paleomammalian

and reptilian brains. This area is responsible for the fi ne-tuning of

lower functions, complex multimodular sensory associations, voluntary

motor control, abstract thinking, planning abilities, and responsiveness to

novel challenges. MacLean’s model is powerful because it not only comports

well with the fossil evidence but is consistent with experiments and

studies performed on humans, other primates, mammals, reptiles, and fi sh

(Harvey & Pagel, 1991; Kaas, 1987, 1993, 1995, 1996; Reiner, 1993; Karten

& Shimazu, 1989; Jerison, 1973, 1991, 1997; Wise, 1996; Frith, 1996;

Fuster, 1997; Sternberg, 1988; Northcutt & Kaas, 1995).

We must make at least one qualifi cation regarding MacLean’s model. It

would be an oversimplifi cation to claim simply that structures were added

onto existing structures in the evolution of the brain. It is true in the

evolution of primates that the brain expanded, thereby ultimately allowing

for humans to do such things as invent tools, develop language, and

contemplate Goldbach’s conjecture. However, it is more accurate to say

that with the increase of brain size came the addition of new brain structures

to provide new functions, as well as a reorganization of the connections

of existing brain structures to allow them to serve novel functions,

The Evolution of the Visual System and Scenario Visualization 101

and the expansion of certain structures to augment particular abilities

(Karten, 1998; Karten & Shimazu, 1989; Keverne, Martel, & Nevison,

1996).

For example, Deacon (1990) has put together a convincing case that the

six-layered mammalian neocortex is not homologous to a single structure

in a reptile but instead derives from the merger of the dorsal ventricular

plate (or pallium) and the dorsal ventricular ridge of the reptile. Such a

development would require the addition of new cortical material and the

reorganization of existing cortical material (Rakic & Kornack, 2001; Kaas,

1987, 1993; Reiner, 1993). Also, Gannon, Holloway, Broadfi eld, & Braun

(1998) and Gannon & Kheck (1999) have made a similar case regarding

the evolution of Wernicke’s area in humans as being homologous to the

planum temporale and planum parietale in macaque monkeys and chimpanzees

(cf. the recent research concerning Broca’s area in Petrides et al.,

2005).

The information regarding the evolution of the nervous system derives

from a variety of sources, including the comparison of anatomical features

of fossils with living species thought closely to resemble such fossils. In

this way, biologists and other researchers can study the nervous systems

of living species in order to gain a picture of how the nervous systems of

ancient creatures most likely had functioned. Accordingly, after I speak

about the evolution of the cell, I trace the evolution of the nervous system

in the following progression: euglena →hydra →worm →fi sh →reptile

→mammal →primate. Thankfully, we have fossilized remains of creatures

resembling each of these types of species—including what we take to be

cells and protocells (Forterre, 2005; Pennisi, 2004; Margulis & Dolan, 2002;

Berra, 1990)—making our inferences regarding the evolution of nervous

systems reliable and probable.

Some 3.5 billion years ago, approximately 1 billion years after the earth

was formed and 12 billion or so years after the Big Bang, rudimentary onecelled

life forms—bacteria—appeared here on earth. Since the famous

Stanley Miller experiment in 1953 where four amino acids, urea, and

several fatty acids (all organic molecules) were synthesized from methane,

ammonia, hydrogen, and water, numerous other laboratory experiments, thought experiments, and hypotheses have shown that it surely is possible

to generate organic molecules from inorganic molecules (Forterre, 2005;

Pennisi, 2004; Margulis & Dolan, 2002; Schopf, 2002; Line, 2002; Balter,

2000; Margulis & Sagan, 1986; Dawkins, 1976, 1986, 1996, 2005; DeDuve,

1995, 1996; Ball, 1999; Fry, 2000; Eigen, 1992; Fenchel & Finlay, 1994).

Theorists postulate that amino acid chains, proteins, and then the organelles

of the cell would have been the fi rst things to evolve, in either a primordial

soup or some crystalline formations (Pennisi, 2004; Margulis &

Dolan, 2002; Margulis & Sagan, 1986; Mayr, 2001; Eldredge, 2001; Churchland,

1984). The cell is the fi rst kind of thing that exhibits the four properties

I have spoken of in the previous chapters, namely, internal–hierarchical

data exchange, data selectivity, informational integration, and environmental–

organismic information exchange.

However, added to these properties would be mechanisms of self-replication,

protection, and Humphrey’s (1983, 1992, 1998) distinction between

what is happening out there (hereafter, OT) versus what is happening in here

(hereafter, IH) mechanisms (also see Dawkins, 1996; Churchland, 1984).

The DNA and RNA of cells act as the mechanism of self-replication, the

cell wall acts as the mechanism of protection and encapsulation, and the

processes of the organelles themselves engage in the exchange and control

of information while interacting with environments. In light of the ideas

already presented regarding genetic variation and natural selection, once

RNA and DNA emerge, the mechanisms of evolution can do their work in

cellular processes of organisms. Nervous systems came into being by the

interplay of genetic mutation and environmental fi tness, as generation

after generation of RNA and DNA have replicated in cells over and over

again.

The evolution of the nervous system really begins with microscopic

organisms that developed a light/dark sensitivity area, most likely in primordial

waters, some 3 billion years ago. In the fi rst chapter, I spoke about

the euglena, an algae that has an eyespot and uses its fl agellum to move

toward light in order to get food. This is not yet a nervous system;

however, we witness the information exchange among the processes

within the euglena, as well as the hierarchical organization of such processes

that will be found in nervous systems. We also see the beginnings

of Humphrey’s distinction between OT and IH, since the euglena uses

basic stimulus/response mechanisms to maneuver toward food and away

from danger.

It is important to note that a kind of primitive visual system would

appear to act as the catalyst for the evolutionary development of the

The Evolution of the Visual System and Scenario Visualization 97

nervous system. In fact, light/dark sensitivity emerged among a whole

multitude of organisms in the protist and monera kingdoms (Febvre-

Chevalier, Bilbaut, Febvre, & Bone, 1989). Further refi nements of the light/

dark sensitivity area can be found in animals, either in terms of stimulus/

response mechanisms or various kinds of rudimentary and advanced visual

systems (Cronly-Dillon, 1991; Blake & Truscianko, 1990; Horridge, 1987).

For example, the photosensitive pigment cells in a worm’s skin are responsive

to light, as is the rudimentary pinhole eye of the nautilus. This speaks

to the fact that an organism’s ability to “see” is crucial for its survival,

which makes sense given that the world is bathed in sunlight. Sunlight

interacts in the world in such a way that organisms have taken advantage

of this form of energy so as to optimally engage in forms of feeding, foraging,

fl eeing, fi ghting, and so forth.

The euglena is a one-celled organism. With the arrival of multicellular

organisms having simple nervous systems came the possibility that a

variety of cells could have a variety of functions. The hydra is a cnidarian

that has a simple nervous system called a nerve net. This nerve net forms

an undifferentiated network of sensory neurons, motor neurons, and interneurons

(DeDuve, 1996; Bonner, 2000; Mackie, 1989). This was a crucial

development in the evolution of the nervous system for at least three

reasons. First, the electrochemical processes of the neuron, as well as the

multiple axonal/dendritic neuronal connections, could aid in speeding up

reactions to external stimuli, enabling the animal to be more effi cient at

hunting and/or avoiding being eaten.

Second, more complex and more effi cient forms of communication

within the animal could be achieved with the introduction of sensory

neurons, motor neurons, and interneurons that are specialized in their

functions. Sensory neurons deal with the out there of an animal, since they

are located near the periphery of an animal’s body and are concerned with

relaying external environmental information to it. Motor neurons deal

with the in here of an animal, since they aid in controlling the animal’s

bodily movements, as well as internal responses to external environmental

stimuli. Interneurons connect sensory neurons to motor neurons and act

as a kind of fi lter or buffer for the exchange of information between the

two. Response times could be decreased by specialized neurons’ performing

specifi c tasks in parallel rather than having general-purpose neurons

perform a variety of tasks. A neuron that has emerged to handle only one

kind of problem likely will be able to handle that problem swiftly and

effi ciently because it has to handle only that particular kind of problem and

no other one (also see Arp, 2008b). Further, many specialized neurons working on some problem together minimizes errors and allows systems

to perform more optimally (cf. Culler & Singh, 1999; Bechtel & Abrahamsen,

2002; Feldman & Ballard, 1982). The parallel form of processing

exhibited in the tripartite neuronal network made it through an evolutionary

sieve because of its effi ciency, and such a natural selection comports

with the general evolutionary principle of economy, namely, whatever trait

gives an organism a competitive advantage most likely will be naturally

selected as fi t for that organism and will be passed on to that organism’s

progeny. I will say more about specialized versus general-purpose processing

in the next chapter.

Third, with the introduction of this tripartite neuronal system, Humphrey’s

distinction between IH and OT clearly comes into play. This is

important for more evolved organisms that require part of the nervous

system to be devoted to maintaining homeostasis within the organism (IH)

while another part of the nervous system is attending to external stimuli

(OT). Again, a nervous system having neurons of differing types devoted

to specialized tasks could perform more effi ciently than one having generalpurpose

neurons.

The next step in evolution was for sensory cells to cluster in the heads

of simple animals like the worm. The body began to take on segmentation,

and the movement of each segment came to be regulated by nerve cell

bodies, or ganglia. A central cord of nerve fi bers connected ganglia to each

other and to the head. Also, a larger clustering of nerve cell bodies (a cerebral

ganglion) appeared in the head, where a kind of brain-like command

center for integrating sensory input and directing the body began to take

shape. It makes sense that the clustering of nerve cells together in the head

was selected for, since electrochemical information would not have as far

to travel, thus allowing for conservation of energy and faster communication

between neurons.

Fish evolved from worms, and, at fi rst, there was just a brain-like bulge

on top of the spine. Then, the nerves started to sort themselves into specialized

modules—some became sensitive to certain molecules and formed

smell modules, while others became sensitive to light and formed visual

modules. The major evolutionary advance concerning fi sh has to do with

the clear differentiation of the nervous system into the CNS and the

peripheral nervous system (PNS). The CNS consists of the brain and spinal

cord and is in constant two-way communication with the PNS, which

consists of the somatosensory, afferent somatic, visceral, motor efferent,

automatic (made up of the involuntary sympathetic and parasympathetic

systems), and somatic (voluntary) systems. This is the general layout that

The Evolution of the Visual System and Scenario Visualization 99

is found in reptiles, amphibians, and mammals as well (Audesirk et al.,

2002; Kandel et al., 2000).

The distinction between IH and OT is crucial in order to understand

more complex vertebrate nervous system functioning and behavior. Parts

of the CNS and PNS—like the hypothalamus and parasympathetic systems

in mammals—are devoted to the internal homeostasis of the organism;

these are the IH processes. Other parts of the CNS and PNS—like the LGN

in the brain and the somatic system in mammals—are devoted to reaction

to external stimuli around the organism; these are the OT processes. Ultimately,

as we will see in the evolution of primates, some of the IH processes

formed a kind of feedback loop, generating emotions and conscious

thought. Instead of merely being directed toward the maintenance of OT

and IH processes, the brain evolved abilities to feed back information

within itself that is distinct from, but analogously related to, the processes

of the CNS and PNS. It is arguable that this feedback of information

allowed for the possibility of mental states such as emotion, refl ection, and

consciousness to emerge (Humphrey, 1992, 1998).

After fi sh came into being, the processes and systems of the brains of

subsequent animals evolved at a rate that is disproportionate in relation

to the rest of the body. The evolutionary story from here on out is more

about the increasing size and complexity of the brain rather than that of

the nervous system generally. Presumably, this is the case because the

primary sensory mechanisms converge in the head of most animals, and

the brain has become a kind of command center for both the CNS and

PNS. There is an adaptive payoff to having a locus for information transfer

and exchange. As this locus, the brain can go about its business of processing

information quickly and effi ciently, utilizing neurons that are crossconnected,

working in parallel, and in close proximity to one another.

Given the brain’s ability to process information effi ciently, it has been able

to pass through one environment after another in the evolutionary sieve.

At the same time, more parts and processes have been added to the brain

because, to put it crudely, more parts mean more possibilities for effi cient

information exchange. However, it is not merely the addition of new parts

to the brain that accounts for its effi cient processing, and, later in this

section, I will make some qualifi cations about how to understand the

evolutionary addition of parts to the brain.

The evolutionary development of the brain from fi sh, through reptiles,

to mammals and primates has been explained effectively in the famous

works by MacLean (1967, 1991; cf. Sternberg, 1988; Kaas, 1987, 1993, 1995,

1996). According to MacLean’s model, the primate brain is really a three-part brain having evolved the neocortex but retaining the limbic system

found in mammals and the brainstem core found only in reptiles. The base

of the primate brain is shared with reptiles and consists of the brainstem,

reticular formation, and striate cortex. These areas are where the necessary

command centers for living are located, namely, the control of sleep and

waking, respiration, body temperature, basic automatic movements, and

the primary way stations for sensory input.

Eventually, what MacLean calls the paleomammalian cortex evolved on

top of the reptilian brainstem, allowing for more modules to develop: the

thalamus, allowing sight, smell, and hearing to be used together; the

amygdala and hippocampus, apparatuses for memory and emotions; and

the hypothalamus, making it possible for the organism to react to more

stimuli by refi ning, amending, and coordinating movements. The functioning

of the paleomammalian and reptilian cortices are somewhat analogous

to the functioning of a heart, pancreas, or kidney, since they are

organized less for thought and more for automatic action and response.

This makes sense from an evolutionary perspective. Reptiles, amphibians,

and mammals out in the wild share the common problems of having to

respond quickly to environmental stimuli so as to know whether to fi ght,

fl ee, forage, or procreate in order to survive.

Finally, in the evolutionary history of primates what MacLean calls the

neomammalian cortex (or cerebral cortex) evolved on top of the paleomammalian

and reptilian brains. This area is responsible for the fi ne-tuning of

lower functions, complex multimodular sensory associations, voluntary

motor control, abstract thinking, planning abilities, and responsiveness to

novel challenges. MacLean’s model is powerful because it not only comports

well with the fossil evidence but is consistent with experiments and

studies performed on humans, other primates, mammals, reptiles, and fi sh

(Harvey & Pagel, 1991; Kaas, 1987, 1993, 1995, 1996; Reiner, 1993; Karten

& Shimazu, 1989; Jerison, 1973, 1991, 1997; Wise, 1996; Frith, 1996;

Fuster, 1997; Sternberg, 1988; Northcutt & Kaas, 1995).

We must make at least one qualifi cation regarding MacLean’s model. It

would be an oversimplifi cation to claim simply that structures were added

onto existing structures in the evolution of the brain. It is true in the

evolution of primates that the brain expanded, thereby ultimately allowing

for humans to do such things as invent tools, develop language, and

contemplate Goldbach’s conjecture. However, it is more accurate to say

that with the increase of brain size came the addition of new brain structures

to provide new functions, as well as a reorganization of the connections

of existing brain structures to allow them to serve novel functions,

The Evolution of the Visual System and Scenario Visualization 101

and the expansion of certain structures to augment particular abilities

(Karten, 1998; Karten & Shimazu, 1989; Keverne, Martel, & Nevison,

1996).

For example, Deacon (1990) has put together a convincing case that the

six-layered mammalian neocortex is not homologous to a single structure

in a reptile but instead derives from the merger of the dorsal ventricular

plate (or pallium) and the dorsal ventricular ridge of the reptile. Such a

development would require the addition of new cortical material and the

reorganization of existing cortical material (Rakic & Kornack, 2001; Kaas,

1987, 1993; Reiner, 1993). Also, Gannon, Holloway, Broadfi eld, & Braun

(1998) and Gannon & Kheck (1999) have made a similar case regarding

the evolution of Wernicke’s area in humans as being homologous to the

planum temporale and planum parietale in macaque monkeys and chimpanzees

(cf. the recent research concerning Broca’s area in Petrides et al.,

2005).