evolution and the brain jana vukovic human neurobiology 217 jvukovic@anhb.uwa.edu.au

Evolution and the Brain

Jana Vukovic

Human Neurobiology 217

jvukovic@anhb.uwa.edu.au

Key points

• Classifying the human

• Vertebrate vs. invertebrate nervous system

• Less to more complex vertebrate nervous system

• Brain size and evolution

Origin of Brain Cells and Brains

Despite the age of the Earth (4 500 million years), brain cells and brains are quite recent adaptations

– First forms of life: 3 500 million years ago

– First brain cells: 700 million years ago

– First brain: 250 million years ago

– First human-like brain: 3 - 4 million years ago

– Modern brain: 0.1-0.2 million years ago

Classification of Life

• up to 100 million species of life on Earth• only described about 1.5% • Taxonomy:

– Branch of biology concerned with naming and classifying species– Groups organisms with common characteristics

To classify the modern human

Five different kingdoms:

• Monera (bacteria)

• Protista (single cells)

• Plantae (plants)

• Fungi (fungi)

• Animalia (animals)

The only ones with brain cells and brains.

Animal phyla

Different Phyla

Chordates: Invertebrate vs.vertebrate

Complexity of movement

• Nerve net:

– Sensory and motor neurons

Anemone

• Segmented nerve trunk:– Divided into a number of parts

– Bilaterally symmetrical

• Ganglia:– Collection of nerve cells that

function somewhat like a brain

AscidianC. elegans

Invertebrate vs. Vertebrate Nervous System

Invertebrate- Nerve nets, segmented nerve trunk,

ganglia- Stimulus/response, receptor/effector- Reflexes, conditioned responses

Vertebrate– Brain and spinal cord encased in

cartilage/boneEarthworm – invertebrate

– Crossed organization: Each hemisphere receives information from and controls the opposite side of the body– Spinal cord is dorsal at the back of the heart and gut

Evolution of Vertebrate Nervous System

• Behavioral complexity is correlated with the evolution of cerebral hemispheres and cerebellum:

– Cerebellum: involved in the coordination of motor and possibly other mental processes

– Increased size and folding (to fit more tissue into the skull)

Less to more complex vertebrates

Less to more complex vertebratesCortical specilasation

Sensory/motor cortex Frontal cortex

Less to more complex vertebrates

• Changes in representation of the body on the sensory cortex and locomotion

Less to more complex vertebrates (i.e. rat to human)

• Changes in size vascularity

variety of non-neuronal cells• Complexity of circuits• Changes in time course of development• Changes in relative size of different parts of the brain• Many conservative features – eg. neurochemistry

Chordate – mammals – primates

Features common to primates:• Excellent color vision• Eyes in front of face: enhance depth perception• Females: Usually only one infant per pregnancy;

infants require more care• Larger brains

Primates hominidae • Hominid:– Primates that walk upright; includes all forms of humans, living and

extinct• Australopithecus– Austral: “southern”; pithekos: “ape”– Our distant ancestor• Gave rise to the genus Homo, or human– 4 million years ago

Brains were 1/3 the size of oursBrains were 1/3 the size of ours• Homo habilis (“handy human”)– 2 million years ago; in Africa– Made simple stone tools• Homo erectus (“upright human”)– Migrated to Europe and Asia– 1.6 million years ago– Larger brain than H. habilis– More sophisticated tools than H. habilis

• Homo sapiens (“wise human”)– Africa and Asia: 200,000 years ago– Europe: 100,000 years ago– Larger brain that H. erectus– Coexisted with other hominids• Example: Neanderthals, who had comparable or even

larger brains than H. sapiens• Exact reason why we replaced Neanderthals is unknown

Brain size in mammals

Encephalization Quotient

Encephalization Quotient (EQ):– Measure of brain size obtained from the ratio of actual brain size to the expected brain size for an animal of a particular body size• H. sapiens have the largest EQ

Why is the Hominid Brain Enlarged ?1. The Primate Lifestyle• Eating behavior of primates is more complex than

other animals– Finding fruit is more difficult than eating grass or other vegetation on the ground• Need good sensory, spatial, and memory skills– Fruit eaters have larger brains

Why the Hominid Brain Enlarged

2. Changes in Hominid PhysiologyRadiator Hypothesis (Falk, 1990)– The more active the brain is, the more heat it generates• Increased Blood Circulation• Improved Brain Cooling• Enabled Size of Hominid Brains to Increase

Stedman and colleagues (2004)• Genetic Mutation • Smaller Facial Muscles & Bones• Change in Diet• Increased Brain Size

Why the Hominid Brain Enlarged

3. Neoteny• Rate of maturation is slowed– Allows more brain cells to be produced• Adults retain some infant characteristics• Newly evolved species resemble the young of

their common ancestors– Human heads look more like the heads of

juvenile chimpanzees than adult chimpanzees

Modern HumansAre people with larger brains more intelligent?

Problems with answering this question:• How does one measure intelligence?• How does one measure brain size?– Control for skull thickness– Volume versus weight– Control for body weight• Can fluctuate within an individual over time– Effect of age, physical health, brain damage?

Modern Humans

Are people with larger brains more intelligent?

• Brain size and intelligence do not seem tobe particularly related

– Research has shown that many smart people (e.g., Einstein) have average size brains– Women’s brains weigh about 10% less than

men, but the two sexes do not differ in measures of average intelligence

Can you name few differences between the invertebrate and vertebrate nervous system?

Invertebrate- Nerve nets, ganglia- Stimulus/response, receptor/effector- Reflexes, conditioned responses

Vertebrate– Brain and spinal cord encased in cartilage/bone– Crossed organization: Each hemisphere receives information from and controls the opposite side of the body– Spinal cord is dorsal at the back of the heart and gut

What are the changes that occurred between less to more complex

vertebrates?• Changes in size • Changes in relative size of different parts of the

brain–cerebral cortex ,cerebellum• Changes in representation of the body on the

sensory cortex and locomotion• Cortical specialistion (association cortex)• Complexity of circuits• Vascularity, variety of non-neuronal cells• Changes in time course of development

Thank you...