Friendship isn't just something we humans do - anyone who has seen kittens and puppies cuddling or chimpanzees enjoying a good nit-picking session can attest that friendship occurs across different species.
According to the Evolutionary Origins of Friendship, when animals form friendships, "male allies have superior competitive ability and improved reproductive success; females with the strongest, most enduring friendships experience less stress, higher infant survival, and live longer."
But while the paleo version of friendships, back when we were cavemen, may have been largely linked to survival, the core benefits of friendship have not changed much.
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"We used to need friends for survival," says the popular philosopher Alain de Botton talking to HuffPost UK Lifestyle, "now we have the police and the state for survival. So what are friends for? They are there to support us in our commitments, to guide us gently away from risks, and to help us to develop our thoughts and insights. The job has turned from physical to psychological."
It can be easy, sometimes, to lose the idea of what friendship is or should be about.
But essentially, says Karin Sieger, psychotherapist and HuffPost UK blogger, true friendships "are based on unconditional concern for the other. We do things for the other out of friendship not in order to gain anything. Friendships can provide grounding, safety, comfort, the experience of trust and respect, of being understood and valued."
So what does that mean in a 21st century setting? A lot of us - especially women - follow a pattern of friendship, where you have lots of friends at school, even more friends when you go on to university and then one by one, our friends seem to drop off.
When we go through challenging times too or if we disagree with friends, it can be easy to shut them out. Or to think that you don't need friends now that you're married/have children.
But psychotherapist Dr Sheri Jacobson says: "Although many of us like to think we can 'go it alone', we are social, tribal creatures by nature. Closeness helps us develop important skills like communication and commitment. If we grow up without intimacy and don't foster intimacy in our adult friendships, we might be less likely, to, say, commit to a job long-term and work well with others.
"Friendship can make us feel like life has meaning, and conversely, when we push friends and family away, we can feel isolated and lonely. This depletes our self-esteem and can lead to low mood. Without intimacy through friendships, we can lose the motivation and inspiration to strive for what we desire. And if we distance ourselves from intimacy, we can start cutting off in other parts of our lives too. If we can't be close with family and friends, it's less likely, for example, that we are able to ask for help from colleagues at work when we need it."
Whereas friendships once were about survival, immigration and the Industrial Revolution impacted the role friends began to play in the absence of family and living in the same community you were born into.
Karin adds: "With the later increase in disposable income, leisure time and suburbia men could develop new foundations and outlets for friendships."
Think clubbing, hanging out in the pub, kitty parties, society balls, dinners, football games, car pooling and play dates.
So what does a 21st century friendship look like? Dr Robert Holden, director of The Happiness Project and Happiness NOW! says: "21st century friendships are soul friendships. They are about supporting each other to live a life that is full of purpose, courage and creativity."
However there are challenges - in this digital world of ours, it's harder to maintain or make meaningful relationships. We are more independent than ever, and the only predators we really need help with are rubbish dates.
"If you go to the heart of our western culture," says Dr David Hamilton, author of Why Kindness Is Good For You, "you'll find that we are a very independent culture. We think in terms of "I" and see ourselves in competition with others.
"Some other cultures (particularly Eastern) think more in terms of "We" and they see themselves as in relationship with each other. It's called, 'Inter-dependent'. My advice is to think less "I" and start thinking and acting more "We". When we do this we start to feel that we are part of something. We tend to feel more connected and therefore less lonely. My experience is that it subtly affects our behaviour so that we are more likely to move into social circles."
Robert agrees. He says: "One of the biggest blocks to happiness is what I call dysfunctional independence, which is about trying to do life all by yourself. You can’t be completely independent and enjoy intimacy. True friendship means that sometimes you have to be vulnerable, ask for help, and let the love in."
When you're going through a tough time, that can be easier said than done. Especially if it's depression, or a situation you don't want to talk to your friends about.
But Sheri advises: "I have seen many clients who come to therapy for help with depression, working such long hours that they don’t get as much contact with friends (and family) as they need. Clients are often surprised, down the road, when they spend quality time connecting with friends, that this can directly elevate their mood.
"It’s also noteworthy that a common regret when individuals are on their death-bed is 'not staying in touch with friends’. Which goes to show how valuable friendships are to us."
But pick your friends wisely.
Dr Tom Stevens, consultant psychiatrist at the London Bridge Hospital says: "From a purely psychiatric perspective an important issue here is how your friend will respond when you are in trouble. In many ways practical and emotional support during crises are undoubtedly beneficial. However when exposed to life difficulties being let down by a friend has been shown to make matters worse than having no friend at all."
It appears that whereas once we used to rely on our friends to stay alive, now we rely on them for something equally important: navigating through the psychological parts of life.
As Sheri says: "Physical survival is not generally difficult in the Western world today, so you could ask why friendships are still important? I’d say that the need for friendships is an evolutionary imprint from a previous age, but that it still serves a vital purpose today - namely providing emotional support which ultimately contributes to good mental health.