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Chapter 4: Brain evolution. Frederick L. Coolidge & Thomas Wynn. How Brains Evolve. The Role of Genes. What are genes? Genes are the blueprints controlling the morphology and physiology of organisms. Genes are composed of DNA sequences

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Chapter 4: Brain evolution

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Chapter 4 brain evolution

Chapter 4: Brain evolution

Frederick L. Coolidge & Thomas Wynn

How brains evolve

How Brains Evolve

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.

  • How brains evolve1

    How Brains Evolve

    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

    How brains evolve2

    How Brains Evolve

    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

    How brains evolve3

    How Brains Evolve

    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

  • How brains evolve4

    How Brains Evolve

    The Role of Experience

    • Environment also plays a role in the development of brains and cognition

      • Two developmental periods

        • Pre-natal

        • Post-natal

    How brains evolve5

    How Brains Evolve

    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

    How brains evolve6

    How Brains Evolve

    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

    How brains evolve7

    How Brains Evolve

    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 bonoboKanzi

        • Developed ability to understand human speech

        • Unable to produce syntactical constructions using more than 4 symbols

    How brains evolve8

    How Brains Evolve

    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

    How brains evolve9

    How Brains Evolve


    • 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

    How brains evolve10

    How Brains Evolve

    Effects of Mutations

    • Regulatory genes

      • Small mutations in regulatory genes can produce dramatic changes in anatomy

    • Structural genes

      • 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.

    Gene Duplication

    How brains evolve11

    How Brains Evolve

    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

    How brains evolve12

    How Brains Evolve

    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

    How brains evolve13

    How Brains Evolve

    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

    Evolutionary Psychology

    • 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

    Evolutionary psychology1

    Evolutionary Psychology

    • 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”

    Chapter 4 brain evolution

    The Role of Constraint

    • 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.

    The role of constraint

    The Role of Constraint

    • 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

    The metabolic trade off

    The Metabolic Trade-off

    • 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 of study

    Methods of Study

    Methods for Studying Evolution

    • 1. The comparative method

    • 2. Paleontology

    • 3. Archeology

    • 4. Reverse engineering

    Methods of study1

    Methods of Study

    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?

    Evolutionary tree of divergence

    Evolutionary Tree of Divergence








    Methods of study2

    Methods of Study

    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

    Methods of study3

    Methods of Study

    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

    Chapter 4 brain evolution

    Why do primates have a neocortex?

    Chapter 4 brain evolution

    …because all primates are mammals.

    The neocortex is an “ancestral” characteristic.

    Chapter 4 brain evolution

    The expanded visual cortex is a “derived” characteristic.

    Chapter 4 brain evolution

    Why do humans have an expanded visual cortex?

    Chapter 4 brain evolution

    …because all humans are primates.

    The expanded visual cortex is now an “ancestral” characteristic.

    Chapter 4 brain evolution

    The ability of the human hippocampus to form declarative memories is a “derived” characteristic.

    Methods of study4

    Methods of Study

    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

    Methods of study5

    Methods of Study

    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.

    Methods of study6

    Methods of Study


    • 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

    Methods of study7

    Methods of Study


    • 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

    Methods of study8

    Methods of Study

    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

    Methods of study9

    Methods of Study

    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

    Methods of study10

    Methods of Study

    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

    Brain body size relationship mammals

    Brain/Body Size Relationship:Mammals

    • The solid regression line represents predicted brain size for body size

      • Mammals that fall above the regression line are typically more encephalized

    Brain body size relationship vertebrates

    Brain/Body Size Relationship:Vertebrates

    • 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

    Methods of study11

    Methods of Study

    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

    Methods of study12

    Methods of Study


    • 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

    Methods of study13

    Methods of Study

    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

    Methods of study14

    Methods of Study

    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.

    Methods of study15

    Methods of Study


    • 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.

    Methods of study16

    Methods of Study

    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

    Methods of study17

    Methods of Study

    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

    Methods of study18

    Methods of Study

    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

    Methods of study19

    Methods of Study


    • 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

    Methods of study20

    Methods of Study

    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

    Methods of study21

    Methods of Study

    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.

    Methods of study22

    Methods of Study

    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

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