Morphometrics of the Amygdala

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True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Relative Difference
Human Universality: 
Individual Universal (All Individuals Everywhere)
MOCA Domain: 

 Recent data indicate that the human amygdala is distinct from that of our closest living relatives, the great apes, in both size and neuron number (Barger, et al., 2007; 2012). The absolute size of the amygdala is approximately four times greater than that of great apes, but does not exceed allometric predictions, while human neuron numbers fall within the great ape range.

However, evidence for changes in the intrinsic organization of the amygdala over the course of human evolution, i.e., evolutionary reorganization, indicate that it may have become specialized to process a heightened amount of neocortical information. Specifically, in allometric analyses, the nucleus of the amygdala which receives the greatest amount of input from the neocortex, the lateral nucleus, is nearly 40% larger than predicted for a great ape of human brain size and contains 69% more neurons than predicted for a primate with a similar number of total amygdala neurons. In contrast, the nucleus which has the greatest influence on brainstem activity, is over 3 time smaller than predicted (but contains only around 10% fewer neurons than predicted).

Timing

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 (Lineage Separation Event): 
Probable Appearance (Lineage Separation Event): 
Definite Appearance (Lineage Separation Event): 
Background Information: 

The amygdala is a subcortical structure which has long been highlighted as a major constituent of the neural circuits processing emotional and social behavior (Maclean, 1949; Adolphs, 2010). Anatomically and functionally, it is composed of 13 distinct, interconnected nuclei, some which receive  with brainstem and hypothalamic structures that regulate emotional and "fight or flight" behaviors (Freese and Amaral, 2009).  As such, the amygdala is situated in a position to modulate emotional responses to external stimuli and has been characterized as a salience (Adolphs, 2010), value (Morrison, 2010), and threat (Amaral, 2003) detector. 

The Human Difference: 

The comparative evidence (Barger, et al., 2007;2012) suggests that the human amygdala has undergone evolutionary reorganization of its component parts, especially emphasizing the lateral nucleus, the primary gateway for neocortical information arriving in the amygdala. This distinguishes the human amygdala even from that of great apes, pointing to potential evolutionary changes in the manner in which emotion processing may occur in human brains. 

Universality in Human Populations: 

Universal

Mechanisms Responsible for the Difference: 

The proximate mechanisms responsible for the difference are not explicitly known. Variation in amygdala size has been linked to social behavior, but the influence of ontogenetic change and neural plasticy on these differences in unclear. Little information on the development of individual amygdaloid nuclei is available for any species of primate other than the rhesus macaque. The lateral nucleus has been indicated to be influenced in 2 neurodevelopmental disorders associated with social behavior (Schumann and Amaral, 2006; Galaburda and Bellugi, 2001), but, again, these studies have focused largely on adults and have not addressed developmental change. 

Possible Selection Processes Responsible for the Difference: 

The size of the amygdala appears to be most linked to measures of social complexity. Within species, increased amygdala volume has been linked to measures of social group size within species (Bickart, et al., 2011; Kanai and Bahrami, 2012; Sallet, et al., 2011). Across primate species, increases in social group size are positvely related to increases in the size of the portion of the amygdala most connected with neocortex (Barton, 2000).  

Implications for Understanding Modern Humans: 

Several researchers have pointed to the behavioral importance of changes in emotional behaviors to human-specific social adaptations and have theorized that these changes would be reflected in human brains (e.g., Hare, 2007; Vilensky, et al., 1982). The neural data suggest that the human amygdala distributes more neural resources to nuclei that process neocortical input and evaluate information early in the processing scheme than those that directly interact with brainstem nuclei to initiate autonomic responses to stimuli.  

Occurrence in Other Animals: 

 The amygdala is present in all mammals. Homologues are present in birds and reptiles (Jarvis, 2009). 

Related MOCA Topics
Referenced By:
Title Certainty
Limbic Thalamic Nuclei Size Speculative

References

  1. Online social network size is reflected in human brain structure., Kanai, R, Bahrami B, Roylance R, and Rees G , Proc Biol Sci, 04/2012, Volume 279, Issue 1732, p.1327-34, (2012)
  2. Amygdala volume and social network size in humans., Bickart, Kevin C., Wright Christopher I., Dautoff Rebecca J., Dickerson Bradford C., and Barrett Lisa Feldman , Nat Neurosci, 02/2011, Volume 14, Issue 2, p.163-4, (2011)
  3. Social network size affects neural circuits in macaques., Sallet, J, Mars R B., Noonan M P., Andersson J L., O'Reilly J X., Jbabdi S, Croxson P L., Jenkinson M, Miller K L., and Rushworth M F. S. , Science, 11/2011, Volume 334, Issue 6056, p.697-700, (2011)
  4. Re-valuing the amygdala., Morrison, S. E., and Salzman C. D. , Curr Opin Neurobiol, 04/2010, Volume 20, Issue 2, p.221-30, (2010)
  5. What does the amygdala contribute to social cognition?, Adolphs, Ralph , Ann N Y Acad Sci, 2010 Mar, Volume 1191, p.42-61, (2010)
  6. Evolution of the Pallium in Birds and Reptiles, Jarvis, Erich D. , Encyclopedia of Neuroscience, Berlin, Heidelberg, p.1390 - 1400, (2009)
  7. From Nonhuman to Human Mind: What Changed and Why?, Hare, B. , Current Directions in Psychological Science, Volume 16, p.60-64, (2007)
  8. Stereological analysis of amygdala neuron number in autism., Schumann, C. M., and Amaral D. G. , J Neurosci, 07/2006, Volume 26, Issue 29, p.7674-9, (2006)
  9. The amygdala: is it an essential component of the neural network for social cognition?, Amaral, D. G., Capitanio J. P., Jourdain M., Mason W. A., Mendoza S. P., and Prather M. , Neuropsychologia, Volume 41, Issue 2, p.235-40, (2003)
  10. Primate evolution and the amygdala, Barton, R., and Aggleton J. P. , The amygdala: A functional analysis, New York, NY, p.480-508, (2000)
  11. V. Multi-level analysis of cortical neuroanatomy in Williams syndrome., Galaburda, A M., and Bellugi U , J Cogn Neurosci, Volume 12 Suppl 1, p.74-88, (2000)
  12. The limbic system and human evolution, Vilensky, J.A., Van Hoesen G.W., and Damasio A.R. , Journal of Human Evolution, Volume 11, p.447 - 460, (1982)
  13. Psychosomatic disease and the visceral brain; recent developments bearing on the Papez theory of emotion., MacLean, P D. , Psychosom Med, 12/1949, Volume 11, Issue 6, p.338-53, (1949)