In On Friendship, science journalist Lydia Denworth goes in search of the biological, psychological, and evolutionary foundations of friendship. She and I visit a monkey sanctuary in Puerto Rico and a baboon colony in Kenya to explore the social bonds of apes that provide insight into our own. The author shows that friendship arose at the dawn of humanity: the desire to establish close connections also exists among primates. Lydia Denworth also meets scientists working at the cutting edge of brain and genetics research and discovers that friendship is reflected in a person's brain waves, genomes, and cardiovascular and immune systems, while loneliness can cause significant harm to health and increases the risk of death . The author concludes that social connections are critical to health and longevity, and encourages us to pay special attention to our friendships and nurture our friendships.