Brain Size

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
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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 defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 
6,000 thousand years ago
Probable Appearance: 
2,500 thousand years ago
Definite Appearance: 
2,000 thousand years ago
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).


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