Brain Size

Certainty Style Key
Hover over keys for definitions:
True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Absolute Difference
MOCA Domain: 
MOCA Topic Authors: 

Humans have the largest brains of any primate in absolute terms, as well as relative to body size. Brain size varies with body size - larger bodied species tend to have larger brains. The evolutionary increase in brain size in the hominin lineage, subsequent to the split between humans and chimpanzees, is well documented in the fossil record, with the greatest increase in the last 2 million years with the emergence of the genus Homo.


Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are currently defined as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25000 thousand (25 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6000 thousand (6 million) years ago
  • the emergence of Homo ergaster was 2000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 400 thousand years ago
  • the common ancestor of modern humans was 100 thousand years ago

Possible Appearance: 
6,000 Thousand Years
Probable Appearance: 
2,500 Thousand Years
Definite Appearance: 
2,000 Thousand Years
Occurrence in Other Animals: 

 Encephalization—increases in brain size corrected for changes in body size—occurred independently in many vertebrate and mammalian groups (Jerison, 1973; Striedter, 2005).  Among mammals, some of the greatest increases in relative brain size occurred in the cetaceans—i.e., dolphins, porpoises, and other toothed whales (Marino et al., 2004).


  1. Primate brain size is predicted by diet but not sociality, DeCasien, Alex R., Williams Scott A., and Higham James P. , Nature Ecology & Evolution, 2017/03/27, Volume 1, p.0112 - , (2017)
  2. A single splice site mutation in human-specific ARHGAP11B causes basal progenitor amplification, Florio, Marta, Namba Takashi, Pääbo Svante, Hiller Michael, and Huttner Wieland B. , Science Advances, 2016/12/07, Volume 2, Issue 12, (2016)
  3. Evolution of Osteocrin as an activity-regulated factor in the primate brain, Ataman, Bulent, Boulting Gabriella L., Harmin David A., Yang Marty G., Baker-Salisbury Mollie, Yap Ee-Lynn, Malik Athar N., Mei Kevin, Rubin Alex A., Spiegel Ivo, et al. , Nature, 2016/11/10, Volume 539, Issue 7628, p.242 - 247, (2016)
  4. Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution, Seymour, Roger S., Bosiocic Vanya, and Snelling Edward P. , Royal Society Open Science, 2016/08/31, Volume 3, Issue 8, (2016)
  5. The Molecular Basis of Human Brain Evolution, Enard, Wolfgang , Current Biology, 2016/10/24, Volume 26, Issue 20, p.R1109 - R1117, (2016)
  6. A dual comparative approach: integrating lines of evidence from human evolutionary neuroanatomy and neurodevelopmental disorders., Hanson, Kari L., Hrvoj-Mihic Branka, and Semendeferi Katerina , Brain Behav Evol, 2014, Volume 84, Issue 2, p.135-55, (2014)
  7. Rapid evolution of the cerebellum in humans and other great apes., Barton, Robert A., and Venditti Chris , Curr Biol, 2014 Oct 20, Volume 24, Issue 20, p.2440-4, (2014)
  8. The human brain in numbers: a linearly scaled-up primate brain., Herculano-Houzel, Suzana , Front Hum Neurosci, 2009, Volume 3, p.31, (2009)
  9. Principles of Brain Evolution, Striedter, G. , Sunderland, Mass. , p.363, (2005)
  10. Origin and evolution of large brains in toothed whales, Marino, Lori, McShea Daniel W., and Uhen Mark D. , The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology, Volume 281A, p.1247–1255, (2004)
  11. Evolution of the Brain and Intelligence, Jerison, Harry J. , New York, p.482, (1973)