Evolution and the Brain

1. Evolution and the Brain Jana Vukovic Human Neurobiology 217 jvukovic@anhb.uwa.edu.au

2. Key points • Classifying the human • Vertebrate vs. invertebrate nervous system • Less to more complex vertebrate nervous system • Brain size and evolution

3. 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

4. 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

5. To classify the modern human

6. Five different kingdoms: • Monera (bacteria) • Protista (single cells) • Plantae (plants) • Fungi (fungi) • Animalia (animals) The only ones with brain cells and brains.

7. Animal phyla Different Phyla

8. Chordates: Invertebrate vs.vertebrate Complexity of movement

9. • Nerve net: – Sensory and motor neurons Anemone

10. Segmented nerve trunk: – Divided into a number of parts – Bilaterally symmetrical • Ganglia: – Collection of nerve cells that function somewhat like a brain C. elegans Ascidian

11. 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/bone Earthworm – 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

12. 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)

13. Less to more complex vertebrates

14. Less to more complex vertebratesCortical specilasation Sensory/motor cortex Frontal cortex

15. Less to more complex vertebrates • Changes in representation of the body on the • sensory cortex and locomotion

16. 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

17. 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

18. 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 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

19. • 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

20. Brain size in mammals

21. 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

22. 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

23. Why the Hominid Brain Enlarged 2. Changes in Hominid Physiology Radiator 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

24. 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

25. Modern Humans Are 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?

26. Modern Humans Are people with larger brains more intelligent? • Brain size and intelligence do not seem to be 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

27. 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

28. 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...