Thousands of years ago, when humans roamed the African savannah, it was in our best interests to live in tribes. Being part of a tribe allowed us to sleep soundly knowing that others were looking out for man-eating saber-toothed tigers. If we became too sick to hunt, we knew our fellow tribe members would help us out. We could trust that other people would look out for us and they could trust that we would look after them. A solitary human would never have survived that hostile environment.