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Cited by 726Primate Behavior and EcologyNeurogenesis and neuroplasticity mechanismsNeural dynamics and brain function

The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost

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Herculano-Houzel, Suzana·2012·Proceedings of the National Academy of Sciences
DOI 10.1073/pnas.1201895109PMC PMC3386878
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Abstract

Neuroscientists have become used to a number of “facts” about the human brain: It has 100 billion neurons and 10- to 50-fold more glial cells; it is the largest-than-expected for its body among primates and mammals in general, and therefore the most cognitively able; it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of an increased metabolic need of its neurons; and it is endowed with an overdeveloped cerebral cortex, the largest compared with brain size. These facts led to the widespread notion that the human brain is literally extraordinary: an outlier among mammalian brains, defying evolutionary rules that apply to other species, with a uniqueness seemingly necessary to justify the superior cognitive abilities of humans over mammals with even larger brains. These facts, with deep implications for neurophysiology and evolutionary biology, are not grounded on solid evidence or sound assumptions, however. Our recent development of a method that allows rapid and reliable quantification of the numbers of cells that compose the whole brain has provided a means to verify these facts. Here, I review this recent evidence and argue that, with 86 billion neurons and just as many nonneuronal cells, the human brain is a scaled-up primate brain in its cellular composition and metabolic cost, with a relatively enlarged cerebral cortex that does not have a relatively larger number of brain neurons yet is remarkable in its cognitive abilities and metabolism simply because of its extremely large number of neurons.

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  • Neural network parameters represent a computational analog to biological neural capacity, directly reflecting studies showing strong correlations between brain volume and total neuron count across species, humans, and mice brains.

    “Our neural network parameters represent a computational analog to biological neural capacity, directly reflecting the studies showing strong correlations between brain volume and total neuron count across species, humans, and mice brains (68–70).”

    From What if eye...? Computationally recreating vision evolution (2025) · cites this paper as [70]

  • During much of evolutionary history, mechanisms evolved to minimize muscle size and thus decrease non-essential energy expenditure.

    “During much of evolutionary history, when food scarcity and caloric deprivation was a major evolutionary pressure, mechanisms evolved to minimize muscle size and thus decrease non-essential energy expenditure42,44,55.”

    From GDF8 and activin A blockade protects against GLP-1–induced muscle loss while enhancing fat loss in obese male mice and non-human primates (2025) · cites this paper as [42]

    The “Cost of Being Human” section discusses the energy costs of larger bodies and brains, but does not address evolutionary mechanisms that minimize muscle size. Searches for muscle size, muscle mass, and food scarcity found no passage supporting that specific claim; page numbers were not available in our copy. (LLM, conf=0.75)

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  • Minimizing muscle mass during caloric restriction can substantially reduce energy expenditure, thus sparing energy for the maintenance of other more critical organ systems.

    “minimizing muscle mass during caloric restriction can substantially reduce energy expenditure, thus sparing energy for the maintenance of other more critical organ systems42–44.”

    From GDF8 and activin A blockade protects against GLP-1–induced muscle loss while enhancing fat loss in obese male mice and non-human primates (2025) · cites this paper as [42]

    The ‘Cost of Being Human’ section says larger bodies have greater total metabolic requirements, but it does not address reducing muscle mass during caloric restriction or whether doing so spares energy for other organs. A search for muscle mass and caloric restriction found no passage making that claim; page numbers were not available in our copy. (LLM, conf=0.76)

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  • A large part of phenotype variability across patients with the same ABCD1 genotype may be due to stochasticity.

    “Beyond identifiable causes, a large part of phenotype variability across patients with the same ABCD1 genotype may “simply” be due to stochasticity, a factor hitherto considered “noise” by Cartesian researchers.”

    From Revisiting the Pathogenesis of X-Linked Adrenoleukodystrophy (2025) · cites this paper as [135]

    The ‘Not All Brains Are Made the Same: Neuronal Scaling Rules’ and ‘Shared Scaling Rules: Nonneuronal Cells’ sections discuss variation in brain-cell scaling across mammalian species, not variation among patients. Searches for ABCD1, same-genotype phenotype variability, and stochasticity found no passage addressing the claim. (LLM, conf=0.78)

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  • Brain evolution is often driven by adaptive changes in cellular composition or molecular expression within conserved structures.

    “Brain evolution may often be driven by adaptive changes in cellular composition or molecular expression within conserved structures4–6.”

    From Innovations present in the primate interneuron repertoire (2020) · cites this paper as [5]

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Research works printed but not linked: 3

  1. [14]14. Herculano-Houzel PNAS | June 26, 2012 | vol. 109 | suppl. 1 | 10667
  2. [31]31. Jerison HJ (2007) How can fossils tell us about the evolution of the neocortex? Evolution of Nervous Systems: A Comprehensive Reference, ed Kaas J (Elsevier, Ox- ford), Vol 3, pp 500-508.
  3. [91]91. Owen-Smith NR (1988) Megaherbivores (Cambridge Univ Press, Cambridge).

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Authors
Herculano-Houzel, S.
Affiliations
Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, 21941-902, Rio de Janeiro, Brazil; and Instituto Nacional de Neurociência Translacional, Instituto Nacional de Ciência e Tecnologia/Ministério de Ciência e Tecnologia, 04023-900, Sao Paulo, Brazil
Venue
Proceedings of the National Academy of Sciences
Publisher
National Academy of Sciences
Published
2012-06-22
Volume
109
Issue
supplement_1
Pages
10661-10668
ISSN
  • 0027-8424 (ISSN-L)
  • 1091-6490
DOI
10.1073/pnas.1201895109
PMC
PMC3386878
OpenAlex
W1966084915
Semantic Scholar
65f9d025887298541bab310b071635e4588d2b68
Cited by
726
References
100
Citations by year
2026: 37 · 2025: 69 · 2024: 93 · 2023: 73 · 2022: 58 · 2021: 60 · 2020: 89 · 2019: 51 · 2018: 36 · 2017: 30 · 2016: 38 · 2015: 38 · 2014: 30 · 2013: 19 · 2012: 4
FWCI
4.12
Type
journal article
Language
en
Open access
Locations
  • Proceedings of the National Academy of Sciences (publishedVersion)
  • Europe PMC (PubMed Central) (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
Topics
  • Social Sciences › Psychology › Social Psychology › Primate Behavior and Ecology
  • Life Sciences › Neuroscience › Developmental Neuroscience › Neurogenesis and neuroplasticity mechanisms
  • Life Sciences › Neuroscience › Cognitive Neuroscience › Neural dynamics and brain function
Keywords
human brain, primate brain, brain scaling, brain size, brain energy metabolism, brain energy consumption, cerebral cortex, neuron number, glial cells, brain evolution, cognitive ability
MeSH terms
Animals, Brain, Biological Evolution, Humans, Nerve Net, Neuroglia, Neurons, Primates

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