Humans are highly social, but we don't get pally with just anybody. Before forming relationships with other people, we normally size them up to see how trustworthy they are. A new study suggests that this behavior stems from an evolutionary reorganization in a part of the brain responsible for detecting other people's emotions.
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 (Science NOW, 7 April 2006). Last year, for example, 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.
To see how the amygdala varies in different primate species, a team led by anthropologist Katerina Semendeferi of the University of California, San Diego, measured brain area in autopsy material from 12 ape and human specimens. The researchers found that although the human amygdala was much larger than those of the apes, it was actually the smallest when compared to overall brain size.
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.
The team concludes that the amygdala's lateral nucleus has enlarged relative to the rest of the structure since the human line split from the apes, and that this enlargement might reflect the "social pressures" of living in large groups. For example, Semendeferi and her colleagues note that the orangutan, which has a relatively smaller amygdala and lateral nucleus than those of the other species, is solitary.
The study is "outstanding" for the number of ape specimens it included, says James Rilling, an anthropologist at Emory University in Atlanta, Georgia, but more samples of each species will be necessary to draw firm conclusions. If further studies confirm these results, Rilling says, it is a reasonable hypothesis that the prominence of the lateral nucleus is related to social cognition. The next step, he adds, would be to use brain-imaging techniques to see whether the human lateral nucleus really does make more connections to the temporal lobe than that of other apes.