2020-10-13 09:19:04
Prelude
Prelude
Today’s Topics
- Exam 2 this Thursday (40 questions)
- Evolution and U.S. public attitudes
- The evolution of the human brain
Evolution and U.S. public attitudes
Public acceptance of evolution
Principles of evolution
- Life forms existing in the Earth’s past differed from those living today
- New generations of life forms inherit properties from their predecessors
- New life forms evolved as a result of mutations, selection pressures, and geological events
- Greater survival/reproductive success for some, not others
Evidence for evolution
- Fossil
- Fossil dating (radiometric)
- Geological
- Where fossils are found relative to one another (relative dating)
- How long it takes to form layers
Types of evidence
- Molecular
- Similarities between vastly different species (e.g., in neurotransmitters, receptors, metabolic pathways, etc.)
- Genetic
- Rates of mutation
- Developmental patterns of gene expression
- Anatomical
Nothing in Biology Makes Sense Except in the Light of Evolution
“Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science. Without that light, it becomes a pile of sundry facts some of them interesting or curious, but making no meaningful picture as a whole.”
Why Gilmore thinks it’s controversial (in the U.S.)
- Contradicts verbatim/non-metaphorical reading of some religious texts
- Makes humans seem less special
- Time scales involved beyond human experience
- Scientific method vs. other ways of knowing
- Found in nature ≠ good for human society
Why Gilmore thinks it’s controversial (in the U.S.)
- Few negative consequences of ‘disbelief’
- U.S. culture individualistic, skeptical of experts & expertise
- Lower levels of religious belief among U.S. scientists
- Politics
Evolution of the human brain
The dawn of time
History of life on Earth
Time scales
Cambrian Explosion
What sparked the explosion? (Fox, 2016)
- Behavior requires movement through space
- Behavior requires coordinating perception with action
- Behavior requires perception at a distance
- Behavior requires energy
How nervous systems differ
- Body symmetry
- radial
- bilateral
- Segmentation
- Centralized vs. distributed function
- Cephalization: sense organs & nervous system concentrated in anterior
- Encasement in bone (vertebrates)
(Arendt, Tosches, & Marlow, 2016)
Cellular/molecular mechanisms
- Similarities in patterns of early nervous system development
- across vastly different species
- with very distant (in time) common ancestors
- limited number of successful ways to build nervous systems that successfully regulate behavior
(Arendt et al., 2016)
Comparing brain structures
Comparing brain structures
- Vertebrates have similar brain plans
- Species differ in relative size of parts
Brain sizes differ by vertebrate groups
But have similar “slopes”
- Brain sizes scale with body size
Comparing brain sizes
Brain sizes across vertebrate groups
- Brain size scales with body size (more or less)
- Mammals and birds have big brains
- Some animals have big brains for their bodies
- Humans
- Crows
- Porpoises
- Bigger than expect brains (relative to average) = high ‘encephalization factor’
Cortical size within groups
Evolutionary trends in cortical size
| Structural measure | Non-human comparison | Human |
|---|---|---|
| Cortical gray matter %/tot brain vol | insectivores 25% | 50% |
| Cortical gray + white | mice 40% | 80% |
| Cerebellar mass | primates, mammals 10-15% | 10-15% |
- Cerebral cortex larger in humans
Evolutionary trends in cortical size
Evolutionary trends in primate brain size
Evolutionary trends
- In primates, including humans
- Smaller olfactory bulbs
- Cerebellum comparable to other mammals
- Large cerebral cortex
Selection pressures
- Natural and sexual selection for
- Traits that improve reproductive success
- Physical AND psychological traits
- Hardware and software
Samsung Galaxy S8
Apple iPhone X
Virtues of big phones/brains
- More storage
- More processing capacity
- Better sensors
- Better output
- More, better apps
- Do more, faster
Costs of big brains
- Long time to build
- Lots of energy to nourish/maintain
- Long time to program/train/educate
- Head/neck must be strong enough to carry
- How to connect brain/body parts widely, but process info quickly
Does size matter? Maybe not so much.
A new view (Herculano-Houzel, 2016)
- Number of neurons in cerebral cortex makes humans “special”
| Species | # cortical neurons | cortical mass (g) |
|---|---|---|
| Human | 16 B | 1233 |
| Chimpanzee | 6 B | 286 |
| Elephant | 5.6 B | 2800 |
| Baboon | 2.9 B | 120.2 |
A new view (Herculano-Houzel, 2016)
| Species | # cortical neurons | cortical mass (g) |
|---|---|---|
| Giraffe | 1.7 B | 398.8 |
| Rhesus | 1.7 B | 69.8 |
| Pig | 303 M | 42.2 |
| Rabbit | 71 M | 4.4 |
But humans follow typical scaling rules
# of cortical (or its equivalent) neurons predicts “cognition”?
How did the human brain get this way?
- Build upon mammalian/primate norms
- More efficient energy intake
- calories/hr foraging vs.
- cooking?
- Prolonged (in time) pattern of development
- Specialized pattern of development
- Significant time post-natal/pre-reproductive (childhood)
Take homes
- Life forms on Earth have evolved over billions of years
- Nervous systems emerged ~500-600 million years ago
- Centralized nervous systems have similarities in organization
- Human brains similar to closely related species, but more cortical neurons
References
Arendt, D., Tosches, M. A., & Marlow, H. (2016). From nerve net to nerve ring, nerve cord and brain — evolution of the nervous system. Nature Reviews Neuroscience, 17(1), 61–72. https://doi.org/10.1038/nrn.2015.15
Dobzhansky, T. (1973). Nothing in biology makes sense except in the light of evolution. The American Biology Teacher, 35(3), pp. 125–129. Retrieved from http://www.jstor.org/stable/4444260
Fox, D. (2016). What sparked the Cambrian explosion? Nature, 530(7590), 268–270. https://doi.org/10.1038/530268a
Herculano-Houzel, S. (2012). The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost. Proc. Natl. Acad. Sci. U. S. A., 109 Suppl 1, 10661–10668. https://doi.org/10.1073/pnas.1201895109
Herculano-Houzel, S. (2016). The human advantage: A new understanding of how our brain became remarkable. MIT Press. Retrieved from https://market.android.com/details?id=book-DMqpCwAAQBAJ
Herculano-Houzel, S. (2017). Numbers of neurons as biological correlates of cognitive capability. Current Opinion in Behavioral Sciences, 16(Supplement C), 1–7. https://doi.org/10.1016/j.cobeha.2017.02.004
Hofman, M. A. (2014). Evolution of the human brain: When bigger is better. Frontiers in Neuroanatomy, 8. https://doi.org/10.3389/fnana.2014.00015
Miller, J. D., Scott, E. C., & Okamoto, S. (2006). Public acceptance of evolution. SCIENCE-NEW YORK THEN WASHINGTON-, 313(5788), 765. https://doi.org/10.1126/science.1126746
Northcutt, R. G. (2002). Understanding Vertebrate Brain Evolution. Integrative and Comparative Biology, 42(4), 743–756. https://doi.org/10.1093/icb/42.4.743
Rakic, P. (2009). Evolution of the neocortex: A perspective from developmental biology. Nature Reviews Neuroscience, 10(10), 724–735. https://doi.org/10.1038/nrn2719