 I'm a biologist. The one thing that fascinates me more than any other is the human condition. What does it mean to be human? What sets us apart from other animals? In order to understand how humans are different, we first have to understand how we're the same. Over the years we've seen the traits that we believe made us unique fall one by one. Other animals use tools. Other animals have culture. Today I'd like to add another item to this list, friendship. We study monkeys with the aim of understanding their social lives. I'd like to convince you not only that monkeys and other animals have friends, but that by understanding these friendships and other animals we can gain a greater understanding of ourselves. So I think it's apparent now more than ever in our history that we live in a social world. We have social media sites that make us acutely aware of how connected we are to others and we have social scientists telling us that the more integrated we are in society, the healthier and happier that we'll be. But we aren't alone in our friendly dispositions. So monkeys like these rhesus macaques live in groups with many individuals spending their entire lives, which can be upwards of 25 years together. Each of these individuals is friends with a small number of their group mates. These friends spend time together, they eat together, they groom each other, and they support each other in fights. Social bonds like these have been documented in other primates, so in chimpanzees, in baboons, and in other types of animals like hyenas and dolphins. Now what of these relationships in animals taught us about ourselves? Most importantly, they've shown us that friendship is a product of natural selection. So friendship evolved. It's not a learned behavior or a cultural fabrication. And we know this because other primates have friends, which tells us that our common ancestor more likely than not had friends too. In addition, we know something evolved if it influences an individual's ability to survive and to reproduce. This is because those who are best at surviving and reproducing are selected by nature for those winning characteristics that they have to give rise to future generations. Just so happens when you compare individual monkeys, the ones with the strongest friendships live the longest and have the most babies. So friendship helps monkeys survive and reproduce. We can infer from this that friendship helps humans survive and reproduce too. Human friendship evolved. This leaves us with a very big question. Friendship evolved, but why? What purpose does it serve? To begin to answer this question, I ask you to consider the following. An acquaintance leads you $5. I think most people agree they feel the urgency to pay this kind of debt back pretty quickly. Now, instead of an acquaintance, imagine the money comes from a friend of yours. The urgency you feel to repay them is reduced and your friend might suggest you don't pay them back at all. Why might this be? I suggest this is because that while the purpose of friendship is about providing each other help and support, that the currency on which this is based and the timescale on which it occurs is grander than some small $5-sized favors. It might seem a bit cold and calculated to think about our friends in this way, but remember what animals have taught us. Other animals have shown us that friendship is about survival. Our friends help us to survive. And the flip side of that is that we help them to survive too. It's recognizing that it's deeply rooted in our evolutionary history for friends to help each other leave healthy, successful lives isn't a cold sentiment at all, but it should instead make us appreciate and nurture our friendships all the more. Thank you.