Download Chapter 4: Brain evolution

The Role of Genes

What are genes?
› Genes are the blueprints controlling the
morphology and physiology of organisms.
 Genes are composed of DNA sequences
 These DNA sequences code for the
production of amino acids
 Amino acids combine to form proteins

Until recently, phenotypic trait expression was
thought to be the result of one or more genes
working together.
The Human Genome Project

The “human genome project” has
demonstrated that humans possess drastically
less genes than scientists expected
› Scientists originally estimated that the human
genome must consist of hundreds of thousands of
individual genes
› In actuality, researchers found that humans possess
somewhere between 20,000 to 25,000

Scientists originally overestimated the number
of genes due to a misunderstanding of the way
in which genes operate
The Role of Genes

Genes serve more functions than originally
assumed
› Structural genes
 Directly control trait expression through amino-acid
specific coding
› “Re-edit” genes
 Change the sequence of base pairs
› Regulatory genes
 Control the order and timing in which other genes
are activated or deactivated
 E.g., BF-1 – cell divisions in cortical neuron development
The Chimpanzee Genome Project

The chimpanzee genome has revealed a
95-99% genetic similarity between human
and chimpanzee DNA.


So then why are the two phenotypically so different –
in both anatomy and behavior?
The inability for the current understanding of
DNA to explain the clear differences in
phenotypic expression has led to the
development of a new field of research:
› Epigenetics
The Role of Experience

Environment also plays a role in the
development of brains and cognition
› Two developmental periods
 Pre-natal
 Post-natal
Pre-natal Environment

Changes in the uterine environment can affect the
sequence and rate of development
› Fetus hormonal environment
 E.g., Androgens produced during male
development
› Maternal experience
 Diet and toxins
 Level of stress
 Personal experience
› Birth
 Often includes several minutes of oxygen
deprivation
Post-natal Environment

After birth, the brain continues to develop more
neurons and connections between neurons
› Apoptosis
 Programmed cell death
› Pattern determined by experiences of newborn

The brain is a plastic organ that learns in response
to experience
› Synaptic pruning
 Reduction in the number of overproduced or weak
neuronal connections
Post-natal Environment

Consistent and heavily “exercised” portions
of the brain are awarded more neurons
and axons
› Effect of mental exercise is greater in children
than adults

Environment and experience can only
influence brain development within the
limits set by genes
› E.g., language learning in infant bonobo Kanzi
 Developed ability to understand human speech
 Unable to produce syntactical constructions using
more than 4 symbols
The Role of Natural Selection

Natural selection is the primary mechanism
of evolutionary change.
› Differential reproduction
 Individuals who reproduce successfully pass more
genes on to the next generation than those who
do not.

Natural selection increases or decreases
the frequency of genes that already exist.
› It cannot create change without novel material
that provides an evolutionary advantage
Mutation

Changes in the chemical make-up of a
gene (DNA) that produce small effects on
anatomy and physiology
› This is caused by:
 Toxins
 Natural or artificial radiation
 Errors during duplication and cell division

Mutations are only rarely “good”
› Most mutations are either bad or neutral
› They are such a common occurrence that good
results occur often enough by chance
Effects of Mutations

Regulatory genes
› Small mutations in regulatory genes can
produce dramatic changes in anatomy

Structural genes
Gene Duplication
› Gene duplication
 The sequence of base pairs is duplicated, doubling
the DNA length
› Duplicated anatomical structures form that can
serve new functions

If any small change yields a reproductive
advantage its frequency will rapidly
increase.
Heritability Revisited: Epigenetics

A new field of behavioral genetics that
recognizes that not all methods of
heritability involve traditional DNA models
› Previous models were restricted to
 Single dominant/recessive genes
 Single regulatory genes
 Additive genetic patterns

Definition by Bird (2007)
› The structural adaptation of chromosomal
regions so as to register, signal, or perpetuate
altered activity states
Heritability Revisited: Epigenetics

Epigenetics recognizes that phenotype
changes across generations can result
from:
› RNA
› Chromatin
› “Junk” DNA
› Interactions among DNA strands
› Still unrecognized methods of heritability
Heritability Revisited: Epigenetics

Researchers began studying non-DNA
methods of heritability to figure out why
monozygotic twins do not always have
identical vulnerability to genetic illnesses.
› Pembrey et al. (2006)
 Transgenerational epigenetic heritability

The enhanced working memory capacity
that led to the development of modern
executive functions could be the result of a
neural mutation or an epigenetic event.

Evolutionary psychology focuses on the role
that natural selection played in determining
specific features of human cognition.

Human cognition is seen as “massively
modular”
› Consisting of a large number of specific abilities
that have evolved to solve specific evolutionary
problems.
 E.g., Space constancy (Silverman et al., 2000)
 Evolved to solve problems in wayfinding when hunting

Research and theory are based on the
assumption that our minds evolved long
ago in conditions very different from those
in the modern world.

Important features used to study the
evolution of cognitive abilities
› 1. The target ability is narrowly circumscribed
› 2. Natural selection is the only mechanism
invoked
› 3. The evolutionary reasoning is based on
“reverse engineering”

Natural selection can only work on pre-existing
variation
› Because of the incremental nature of this variation,
the existing structures and behaviors of an organism
constrain the possible solutions to an adaptive
problem.

Nature rarely starts from scratch
› It is far easier, and far more cost-effective, to tweak
the extant system.
 E.g., Vertebrate land adaptation

Mutations of older genes that control more
fundamental structures and functions would
not be able to produce a viable offspring.

Exaptation
› When faced with adaptive problems, existing
structures commonly evolve new functions
 E.g., feathers – from thermal regulation to flight

Exaptation has been responsible for many
changes in the evolution of the human
brain and cognition
› Brain exaptation
 E.g., Hippocampus – from spatial orientation and
navigation to declarative memory formation
› Cognitive exaptation
 E.g., Facial recognition – from mate recognition
and assessment to complex social communication

Neural tissue is very “expensive” to maintain
› Requires large numbers of calories
› Every increase in the quantity of neurons will have a
metabolic cost

In order for a brain to evolve in size, the
organism must either:
› Decrease the caloric demands of some other tissue
› Evolve a way to acquire calories more efficiently
 E.g., Trade-off between brain size and digestion

Brains must provide selective advantages to
evolve beyond the minimum size and
organization necessary for the organism’s
continued success.

Within structural and metabolic constraints,
natural selection and mutation have
shaped the vertebrate brain into a
remarkably complex organ.
› We now know that brain and cognitive evolution
need not have been long, slow, gradual
processes.

Natural selection is still the primary agent of
change, but the complex interrelationship
between structural and regulatory genes
yields long-term patterns that are anything
but simple.
Methods for Studying Evolution
› 1. The comparative method
› 2. Paleontology
› 3. Archeology
› 4. Reverse engineering
The Comparative Method

The comparative method uses similarities
and differences between living
organisms to reconstruct the sequences
of divergence in evolutionary history.
› E.g., What is the sequence of evolutionary
divergence of a dog, a lion, a horse, and a
penguin?
Dog
Horse
Lion
Therapsid
Diapsid
Penguin
Ichtheostega
Homologous and Analogous Characteristics

Organisms can resemble one another for two
very different reasons:
› Homologies
 They have a common ancestor from whom they
inherited their similarities
› Analogies (Homoplasies)
 They have adapted over time to doing similar things in
similar environments

It is usually possible to identify differences in
basic anatomy that prevent us from mistaking
analogies for homologies.
› Comparative DNA has become the most important
tool for identifying homologous relationships
Ancestral and Derived Characteristics

Comparison of “ancestral” and “derived”
characteristics can give us a much clearer
understanding of divergence
› Ancestral characteristics
 Shared by all members of a group with a common
ancestor
› Derived characteristics
 Differentiate a sub-group from members of an ancestral
group

These are relative terms, and their reference
varies according to the level of specificity of
the question.
› E.g., Mammals, primates, humans and the neocortex
Why do primates have a neocortex?
…because all primates are mammals.
The neocortex is an “ancestral” characteristic.
The expanded visual cortex is a “derived” characteristic.
Why do humans have an expanded visual cortex?
…because all humans are primates.
The expanded visual cortex is now an “ancestral” characteristic.
The ability of the human hippocampus to form declarative
memories is a “derived” characteristic.
Ancestral and Derived Characteristics

When neuroscientists compare the brains of
closely related forms, there are usually few
obvious differences in gross anatomy
› There are often variations in neural functioning due to
much less visible neuroanatomical differences

The principles of “ancestral” and “derived”
characteristics can also be applied to the
evolved functions of cognitive abilities
› Derived traits
 Explained by circumstances that our ancestors
encountered after the split from apes
› Ancestral traits
 First identify which common ancestor evolved the ability
 Explained by circumstances that our common ancestors
encountered
The Comparative Method

Comparative evidence allows us to frame
evolutionary questions correctly.
› E.g., Why, in an evolutionary sense, can humans
follow the gaze of another individual?
 An ancestral ability of all anthropoids that evolved
as an aid for solving complex social problems.

The comparative method is useful to
identify unique and shared characteristics.
› To better understand how and why these
features evolved, it is important to consider
evidence from other methods of study.
Paleontology

Paleontology is the study of prehistoric
life, including organisms’ evolution and
interaction with each other and their
environments.
› Paleontologists identify and interpret fossils.
› They strive to reconstruct the environment in
which an organism lived through analysis of
sediment, and the identification of other
fossilized animals and plants
Fossils

The fossil record
› Hard body parts are more likely to fossilize
› Soft body parts have usually been
consumed or decayed long before burial

The soft-tissue of brains do not fossilize
very often, but skulls and crania do.
› These fossils are used to measure brain size
 Natural sedimentary casts
 Filling the reconstructed cranium
 Endocasts
Brain Size

The most commonly used measure of brain
difference in evolutionary science is brain
size.
› Animals with large brains exhibit more complex
behaviors than those with small brains
› Brain size is easy to measure using brain weights,
imaging techniques, and endocranial volume.

Brain size in vertebrates is correlated to
body size.
› Meeting the demands of a larger body requires
a larger brain
› Larger brains do not necessarily indicate more
intelligence
Brain Size

When comparing the brain sizes of two
animals, we must also know their body size.
› It is difficult to compare fossils of terrestrial
vertebrates for two reasons:
 Lack of tissue – muscles, organs, etc.
 Finding complete skeletons is rare
› Paleontologists must calculate body size relative
to living organisms

Allometric relationships further complicate
brain measurements.
› Body size increases faster than brain size
› Smaller organisms have larger relative brain sizes
Brain/Body Size Relation

There is a direct and predictable
relationship between brain size and
body size.
› It can be described mathematically or
graphically
 Y = kXa




Y = Brain weight
X = Body weight
k = “Scaling” constant
a = Exponent describes the slope of regression line
The solid regression line represents predicted brain size for body size
 Mammals that fall above the regression line are typically more
encephalized
Mammals fall around a regression line that is farther up on the y-axis.
They tend to be more encephalized than fish, reptiles, and birds.
Mammals experienced an increase in brain power for two reasons:
Internal body temperature regulation requires neurological
resources
Reliance on learned behavior rather than programmed
instinctual responses
Encephalization Quotient

The Encephalization Quotient (EQ) is the
primary way in which brain size is
compared
› EQ tells us the difference between actual and
predicted size
› Any size increase beyond predicted values
should reflect excess capacity not devoted to
regulating basic metabolic functions

EQ is most useful at the general taxonomic
level
› It does not take into account different reasons
for encephalization
 E.g., Humans and dolphins
Endocasts

An endocast is a liquid latex cast of a
reconstructed cranium
› Reflects external brain features impressed on
the cranial bone

Gross features of brain anatomy are usually
preserved.
› E.g., Overall size/shape, lateral fissure, and the
major lobes

Less pronounced features are often
impossible to detect reliably.
› E.g., Gyri and sulci
Evidence from Endocasts

Paleoneurologists focus on two types of
evidence from endocasts:
› 1. The overall shape of the brain
 Height, length, breadth, arcs, and chords
 Reflect an animal’s way of life
 Comparing at the higher taxonomic levels
› 2. Locations of specific surface features of
brain anatomy
 Gyri and sulci location
 Can help trace evolutionary expansion
 Comparing at the lower taxonomic levels
Endocasts can help identify
some important differences
between humans and our
ancestors.
Asymmetries in overall shape
Enlarged left occipital lobe
Expanded right frontal lobe
Locations of specific surface features
Lunate sulcus
Marks boundary between
Parietal lobes
Occipital lobes
Expanded parietal lobes
Brain Size and Endocasts

Brain size and endocasts as evidence for
cognitive evolution:
› Useful at the higher taxonomic levels of
analysis of neurological evolution
› Reveal little information when studying the
evolution of brains within a single subfamily
 E.g., hominins.
Archeology

Archeologists study traces and patterns in
material evidence that exist in the present
in order to reconstruct actions and
behaviors that occurred in the past.
› E.g., Stone tools, pottery, burial goods, etc.

Cognitive archeologists are interested in
the development of cognition on an
evolutionary scale.
› Actions are guided by cognition
› The traces of action preserve something of the
underlying cognition.
Archeology’s Material Bias

Not all actions leave tangible traces, and only
a limited range of traces survive for more than
a brief period of time.

Archeologists tend to rely heavily on those
activities that preserve well, giving archeology a
material bias.
› From garbage and stone tools archeologists
reconstruct subsistence
 Tools - the sequence of actions used to create them
 Garbage - methods of food processing and disposal
› Material evidence also provides insight into symbolic
and social behavior
 Exotic beads/shells indicate long-distance trade
 Cave paintings suggest advances in working memory
Visible Patterns of Cognition

Cognitive science is rich in interpretive
concepts developed to explain features of
human cognition.
› Archeologists must take cognitive concepts and
identify the visible consequences.
 E.g., Working memory and cave paintings
› Archeologists can only identify the minimum
competence necessary to produce the patterns
they study.

Features of modern cognition suggest certain
evolutionary developments and scenarios.
› Without actual evidence from the past these
scenarios remain hypotheses
Reverse Engineering

Analyzes the form and structure of an object in order
to discover its function.

Evolutionary psychologists
› 1. Start with a comprehensive description of the structure
of cognition
› 2. Identify what the cognition is designed to do
› 3. Use this insight to describe how/why it evolved

E.g., Bus (2003) – examined male preference for certain
female body proportions
› 1. Established male preference was not culturally determined
› 2. Human male perceptual system is sensitive to these
proportions
› 3. Perceptual sensitivity functions to detect reproductive
potential
EEA

Environment of Evolutionary Adaptedness
› The early physical and social environment to
which cognitive abilities are adapted

Evolutionary psychologists use experimental
protocols to isolate and describe specific
cognitive abilities.
› These cognitive abilities are best explained by
adaptations to the EEA instead of the modern
world
› Sometimes evolutionary answers from reverse
engineering are proven wrong
 E.g, Bipedal locomotion
Evolution Confounded

Reverse engineering is confounded by
exaptation.
› It is difficult enough to determine which
structures evolved earlier and later
› This is confounded when older structures
adapt to perform new functions
 Some homologies are older than others
 E.g. Opposable thumbs
 Originally adapted to arboreal living
 Extant structure was modified for tool use
Evolutionary Context

For reverse engineering of the human
mind to work we need to know more
about its evolutionary context than we
can reliably recover from the design
itself.
› It is important to learn as much about the
actual context in the EEA as possible.
Multiple Sources of Evidence

Reverse engineering cannot answer
evolutionary questions on its own.
› Whenever possible its conclusions need to
be checked against the actual evidence of
evolution:
 Comparative
 Fossil
 Archeological