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. 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 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.