2022-02-24 07:39:15

Prelude

Prelude

Announcements

  • Exam 2 next Tuesday (no class meeting)

Today’s Topics

  • Evolution and U.S. public attitudes
  • The evolution of the human brain

Evolution and U.S. public attitudes

Public acceptance of evolution

2021 data

“A structural equation model indicates that increasing enrollment in baccalaureate-level programs, exposure to college-level science courses, a declining level of religious fundamentalism, and a rising level of civic scientific literacy are responsible for the increased level of public acceptance.”

(Miller et al., 2021)

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 reproductive success (more offspring) 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

Homology vertebrates-en.svg
By Волков Владислав Петрович - Own work, CC BY-SA 4.0, Link

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.”

(Dobzhansky, 1973)

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

Cambrian Explosion

  • Complex multicellular lifeforms emerged ~541 million years ago
  • “Explosion” in geological terms: lasted ~13-25 million years

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 fast & specific communication systems
  • Behavior requires energy

Tree of life

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 ways to build nervous systems that successfully regulate behavior

(Arendt et al., 2016)

Comparing vertebrate 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

Evolutionary trends in cortical size

Evolutionary trends in primate brain size

Evolutionary trends

Selection pressures

  • Natural and sexual selection for
    • Traits that improve reproductive success
  • Physical AND psychological traits
    • Hardware and software

Samsung Galaxy S21+

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?

  • Builds upon mammalian/primate norms
  • More efficient energy intake
    • calories/hr foraging vs.
    • cooking?
  • Specialized pattern of development
    • Significant time post-natal/pre-reproductive (childhood)

Take homes

  • Life forms on Earth have evolved over billions of years
  • Complex organisims with nervous systems emerged ~500-600 million years ago
  • Centralized nervous systems have similarities in organization
  • Human brains similar to closely related species, but have more neurons in cerebral cortex

Next time

  • Exam 2 on Canvas (start time 3:05 PM)

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., Ackerman, M. S., Laspra, B., Branch, G., Polino, C., & Huffaker, J. S. (2021). Public acceptance of evolution in the united states, 1985-2020. Public Understanding of Science, 9636625211035919. https://doi.org/10.1177/09636625211035919

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.