Evolution

2025-10-14

Rick Gilmore

Department of Psychology

Prelude

– twoodburn (2008)

Today’s topics

• Quiz 2
• Evolution of the nervous system

Evolution of the nervous system

Principles of evolution

• Life on Earth has existed for ~3.5 billion years
• Life forms existing in the Earth’s past differed from those living today
• New generations of life forms inherit properties from their predecessors

Principles of evolution

• New life forms evolved as a result of mutations, selection pressures, and geological events
• Greater reproductive success (more offspring) for some individuals, not others
• Characteristics of those who have more offspring increase in relative frequency in the population

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

Evidence for evolution

• Chemical
  • Similarities between vastly different species (e.g., in neurotransmitters, hormones, receptors, metabolic pathways, etc.)
• Genetic
  • Rates of mutation
  • Developmental patterns of gene expression

Evidence for evolution

• Anatomical

Homology among vertibrates: Wikipedia

Tree of life

https://www.evogeneao.com/en/explore/tree-of-life-explorer

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)

Our story

– Scale: (2023)

Looking back to the dawn of time

Hubble deep field: Wikipedia

Looking back to the dawn of time

JWST NASA

Timeline of life on Earth

Cambrian Explosion

– The Economist (2015)

Cambrian Explosion

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

Did behavior spark the explosion? (Fox, 2016)

• Behavior requires movement through space
• Behavior requires perception at a distance
• Behavior requires coordinating perception with action

Did behavior spark the explosion? (Fox, 2016)

• Behavior requires fast & specific communication systems
• Behavior requires energy

Requirements of behavior

• Input
  • From external world
  • Internal states
• Processing
• Output
  • Move body
  • Change physiological state

How nervous systems differ…

• Body symmetry
  • radial
  • bilateral
• Segmentation
• Centralized vs. distributed function

– Park (2009)

How nervous systems differ

• Cephalization: sense organs & nervous system concentrated in anterior
• Encasement in bone (vertebrates)

Arendt, Tosches, & Marlow (2016)

Nervous systems are similar…

• in patterns of early nervous system development
  • across vastly different species
  • with very distant (in time) common ancestors
• Why?
  • Later forms evolved from earlier ones
  • Limited number of ways to build nervous systems that work well

Arendt et al. (2016)

Vertebrates have similar brain plans

• Species differ in relative size of parts

Northcutt (2002)

Vertebrate brain sizes scale with body

Northcutt (2002)

Some vertebrates have bigger brains than others

Jerison from brainmuseum.org

Some vertebrates have bigger brains than others

• Mammals and birds have big brains
• Some specific mammals and birds have big brains for their bodies
  • Humans
  • Crows
  • Porpoises
• Bigger than expected brains (relative to average) = high ‘encephalization factor’

Cerebral cortex size differs among mammals

Hofman (2014)

Primates have large cerebral cortex

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%

Primates have large cerebral cortex

Rakic (2009)

Selection pressures shaping brain evolution

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

Virtues of big brains

• More memory
• More & faster processing capacity
• Better sensors
• Better output
• Do more, faster

Costs of bigness

• Long time to build
• Long time to program/train/educate
• Lots of energy to nourish/maintain
• Head/neck must be strong enough to carry
  • How to connect brain/body parts widely, but process info quickly

A new view (Herculano-Houzel, 2016)

• Number of neurons in cerebral cortex makes humans “special”, not overall mass

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
Giraffe	1.7 B	398.8
Rhesus	1.7 B	69.8
Pig	303 M	42.2
Rabbit	71 M	4.4

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
Giraffe	1.7 B	398.8
Rhesus	1.7 B	69.8
Pig	303 M	42.2
Rabbit	71 M	4.4

How did the human brain get this way?

• Evolved from mammalian/primate norms
• More efficient energy intake
  • calories/hr foraging vs.
  • cooking? (Wrangham, 2009)

How did the human brain get this way?

• Specialized patterns of development
  • Significant time post-natal/pre-reproductive (childhood)
  • Central role of language & culture

Wrap-up

Main points

• Life forms on Earth have evolved over billions of years
• Complex multicellular organisms with nervous systems emerged ~500-600 million years ago
• Centralized nervous systems have similarities in organization
• Brain sizes scale with body size, but species differences

Main points

• Human brains similar to closely related primate species, but have more neurons in cerebral cortex
• Cerebral cortex in humans may have developmental mechanisms not found in other animals (Vanderhaeghen & Polleux, 2023)

Next time

Resources

About

This talk was produced using Quarto, using the RStudio Integrated Development Environment (IDE), version 2025.5.1.513.

The source files are in R and R Markdown, then rendered to HTML using the revealJS framework. The HTML slides are hosted in a GitHub repo and served by GitHub pages: https://psu-psychology.github.io/psych-260-2025-fall/

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. (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

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

Northcutt, R. G. (2002). Understanding vertebrate brain evolution. Integr. Comp. Biol., 42(4), 743–756. https://doi.org/10.1093/icb/42.4.743

Park, S. (2009). Hydra movement. Youtube. Retrieved from https://www.youtube.com/watch?v=-UI531GMRTM

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

Scale:, T. (2023). To scale: TIME. Youtube. Retrieved from https://www.youtube.com/watch?v=nOVvEbH2GC0

The Economist. (2015). What caused the cambrian explosion? Youtube. Retrieved from https://www.youtube.com/watch?v=qNtQwUO9ff8

twoodburn. (2008). "Gentle Arms of Eden", by Dave Carter and Tracy Grammer. Youtube. Retrieved from https://www.youtube.com/watch?v=bAGMATHlSK4&list=RDbAGMATHlSK4

Vanderhaeghen, P., & Polleux, F. (2023). Developmental mechanisms underlying the evolution of human cortical circuits. Nature Reviews. Neuroscience. https://doi.org/10.1038/s41583-023-00675-z

Wrangham, R. (2009). Catching fire: How cooking made us human. Basic Books. Retrieved from https://market.android.com/details?id=book-ebEOupKz-rMC