Chapter 4: Brain evolution

1. Chapter 4: Brain evolution Frederick L. Coolidge & Thomas Wynn

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. How Brains Evolve 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

12. 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

13. 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

14. 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

15. 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.

16. 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

17. 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”

18. 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.

19. 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

20. 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.

21. Summary • 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.

22. Methods of Study Methods for Studying Evolution • 1. The comparative method • 2. Paleontology • 3. Archeology • 4. Reverse engineering

23. 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?

24. Evolutionary Tree of Divergence Dog Lion Horse Therapsid Diapsid Penguin Ichtheostega

25. 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

26. 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

27. Why do primates have a neocortex?

28. …because all primates are mammals. The neocortex is an “ancestral” characteristic.

29. The expanded visual cortex is a “derived” characteristic.

30. Why do humans have an expanded visual cortex?

31. …because all humans are primates. The expanded visual cortex is now an “ancestral” characteristic.

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

33. 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

34. 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.

35. 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

36. Methods of Study 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. Methods of Study 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

44. 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

45. Endocasts • 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

46. 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.

47. Methods of Study 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.

48. 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

49. 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

50. 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