"He got friendly holding my hand/She got friendly down in the sand". Yeah, humans are the most social monkeys. But we do not establish friendships with anybody, we generally try to find out how trustworthy and compatible with us certain persons are.

A new research made by a team led by anthropologist Katerina Semendeferi of the University of California, San Diego, shows that during our evolution we reshaped the amygdala, the brain nucleus involved in detecting other people's emotions, to improve this skill.

The amygdala, a small, almond-shaped tiny center deep within our brains ("amygdala" in Latin means "almond") has been proven for long to be involved in detecting fear, but recent evidence shows that it is involved in a wide array of emotions.
A 2006 research showed that patients with autism display low social interaction and an inability to assess the feelings of others and the amygdala has less neurons, especially in the lateral nucleus.

The research team measured amygdala and other brain nuclei in autopsy material from 12 ape and human specimens. The human amygdala appeared to be much larger than those of the apes, even if it was the smallest compared to overall brain size. Still, the lateral nucleus was much larger in humans compared to overall size of the amygdala and the brain, than in the other primates.

The lateral nucleus has more direct connections with the brain's temporal lobe, related to social behavior and the processing of emotions, than other areas of the amygdala.

"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." signaled the researchers.
The team noticed that the orangutan, a solitary species, has a relatively smaller amygdala and lateral nucleus than other primates.

"The study is "outstanding" for the number of ape specimens it included, but more samples of each species will be necessary to draw firm conclusions. If further studies confirm these results, it is a reasonable hypothesis that the prominence of the lateral nucleus is related to social cognition. The next step 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," said James Rilling, an anthropologist at Emory University in Atlanta, Georgia.