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Knowing What Counts: Unbiased Stereology in the Non-human Primate Brain
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Evolutionary modeling and correcting for observation error support a 3/5 brain-body allometry for primates.

Mark Grabowski1, Kjetil L Voje2, Thomas F Hansen2

  • 1Division of Anthropology, American Museum of Natural History, New York, 10024, USA; Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biosciences, University of Oslo, 0316 Oslo, Norway; Center for the Advanced Study of Human Paleobiology, Department of Anthropology, The George Washington University, 800 22(nd) St., NW Suite 6000 Washington, DC 20052, USA.

Journal of Human Evolution
|May 15, 2016
PubMed
Summary

Researchers discovered a new 3/5 scaling exponent for primate brain-body evolution, suggesting brain size is constrained by body size. This finding indicates early hominins were more encephalized than previously believed.

Keywords:
Brain-size evolutionEncephalization quotientHuman evolutionMacroevolutionPhylogenetic comparative methodsPrimate evolution

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Area of Science:

  • Evolutionary biology
  • Comparative anatomy
  • Neuroscience

Background:

  • Brain-body allometry is crucial for understanding brain evolution.
  • Previous studies often used 2/3 or 3/4 scaling exponents, influenced by statistical and evolutionary assumptions.
  • The influence of these assumptions on findings requires careful consideration.

Purpose of the Study:

  • To quantify primate brain-body allometry using advanced phylogenetic comparative methods.
  • To investigate evolutionary models of brain size, specifically adaptive vs. constrained evolution.
  • To estimate and account for observational error in both brain and body size measurements.

Main Methods:

  • Phylogenetic comparative methods were employed.
  • Models of both adaptive and constrained evolution were tested.
  • Observational error in response and predictor variables was estimated and controlled for.

Main Results:

  • Results supported a constrained evolutionary model where brain size evolves in unison with body size.
  • Controlling for phylogeny and observational error significantly impacted the analysis.
  • A novel 3/5 scaling exponent for primate brain-body evolutionary allometry was determined.

Conclusions:

  • The study provides a more accurate exponent for primate brain-body allometry.
  • New encephalization quotients reveal early hominins were significantly more encephalized than previously estimated.
  • Findings refine our understanding of human brain evolution and early hominin cognitive capabilities.