Friday, July 27, 2007

Homo amygdala?   posted by Razib @ 7/27/2007 04:54:00 PM

A Mind for Sociability:
The amygdala, a small, almond-shaped area deep within our brains, appears to be essential in helping us read the emotions of others. Research shows that the structure is crucial for detecting fear, but scientists have also found evidence that it can help spot a wide variety of mental states...scientists noted that the amygdalas of patients with autism, which is characterized by decreased social interaction and an inability to understanding the feelings of others, have fewer nerve cells, especially in a subdivision called the lateral nucleus.
In humans, however, the lateral nucleus occupied a bigger fraction of the amygdala, and was larger compared to overall brain size, than in the other species, the team reports online today in the American Journal of Physical Anthropology. Although the functions of the amygdala's subunits are unclear, the lateral nucleus makes more direct connections with the brain's temporal lobe--which is involved in social behavior and the processing of emotions--than other parts of the amygdala make, the researchers note.

In Grooming, Gossip, and the Evolution of Language Robin Dunbar argued for the critical selective pressure of social groups in driving up the size and complexity of the human brain (and obviously, the emergence of language). This might explain the gradual increase in brain size over the past few million years until about 200,000 years B.P., but what about the Great Leap Forward & expansion out of Africa ~50,000 years ago? Remember, behaviorally modern humans postdated anatomically modern humans (e.g., a form of H. sapien which was gracile, high cranial vault, etc., was extant in Africa before expanding to the rest of the world) by 150,000 years. In The Dawn of Human Culture Richard Klein suggests that there was a biological change, a reorganization of the brain (Dunbar offers this idea as well). Greg has suggested that Neandertal introgression & hybrid vigor might have been at work; remember that Neandertals had the largest cranial volume of any Homo species. In The Prehistory of the Mind: The Cognitive Origins of Art, Religion and Science Steven Mithen suggests that the breakdown of separation between domain specific intelligennces (e.g., social intelligence, theory of mind, intuitive physics, folkbiology, etc.) was the critical factor in triggering the cultural revolution which lead to modernity. Mithen argues that the use of analogy to map across the various domains, and apply insights from each domain to the others, might have resulted in a massive increase in cognitive flexibility and creativity. A neurobiological implication that our species' amygdala is more "hooked in" with our "higher cognitive functions" seems to lend some credence to that viewpoint.

Update: Kambiz has more.