Jesse Bering does the heavy lifting:
In a study published earlier this year in the journal Psychophysiology, University of Würzburg psychologist Matthias Wieser and his colleagues tried to make some evolutionary sense of why public speaking is so many people’s biggest fear. One of the reasons, they argue, is that the state of social anxiety generated by being the center of attention has an unfortunate adaptive effectwe become super-alert to the presence of angry faces. This limbically-driven attentional bias in which angry or unhappy faces are processed especially rapidly by a socially anxious brain would have been evolutionarily adaptive (and probably continues to be so) because it helped our ancestors to avert dangerous social uprisings that had the potential to ruin their reputations, reproduction and very survival.