According to the six degrees of separation theory, everyone on Earth can be linked in just a small number of logical steps. hanna lindon takes a look at the science behind the theory and delves into some of the most fascinating connections, ending with the link that unites us all…

What connects you to the Queen, Barack Obama, an Argentine cattle herder or a Bostonian stockbroker? According to one extremely-popular theory, the answer is only six degrees of separation. It’s popularly believed that we are all six introductions or less away from any other person in the world, however remotely linked they might seem to be to us at first – but where did this theory come from, and is it really as accurate as we all think?

You might have assumed that it was science buffs who originally dreamed up the six degrees of separation concept, but in fact it was a fiction writer called Frigyes Karinthy. Back in 1929, the Hungarian author wrote a short story called Chains, in which his characters postulated that any two individuals could be connected through a maximum of five acquaintances. The idea was based on globalisation and the ever-increasing connectedness between the world’s people in the wake of the First World War.

MYTH?

Abe

The molecules we breathe have once passed through the lungs of Abraham Lincoln/Julius Caesar/Jesus

True! Physicists have calculated that, every time we breathe, we are highly likely to be inhaling at least some of the molecules exhaled by one of these famous historical figures

It may have begun as fiction, but the concept rapidly caught the imagination both of the public and of the academic community. A series of experiments carried out by famous social psychologist Stanley Milgram and other researchers in the mid 20th century showed that people in the United States were seemingly connected by approximately three friendship links, and in 2003 Columbia University’s ‘Small World Project’ appeared to back up these findings. The university’s researchers assigned each of the 60,000 participants a random ‘target’, asking them to link to this person via email by creating a human chain. Not all of the chains were completed, but of those that were, Columbia professor Duncan Watts says the average number of links was six.

Other slightly-less-scientific investigations have been undertaken into the six degrees of separation theory. Back in 2008, Microsoft researchers announced that they had trawled around 30 billion electronic messages and found that any two strangers are, on average, connected by 6.6 degrees of separation. Twitter users appear to be even more closely connected – according to a study of 5.2 billion Twitter relationships, everyone is five steps or less away from any other Tweeter. Facebook’s data team announced in 2011 that there is an average distance of 4.74 ‘friends’ between every member of their social network.

Some connections, though, are more improbable and just more fascinating than others – and finding these unlikely links may simply be a question of taking a closer look at your family tree.

Keeping it in the family

The advent of the internet made it increasingly easy to collate and research family history. Go back far enough, and you’re almost certain to find an ancestor in common with one of today’s most celebrated personalities. Some famous figures themselves are linked in the most unlikely of ways. Barack Obama, for instance, is a tenth cousin once removed of George W Bush – the pair are connected by a Samuel Hinkley of Cape Cod, who died back in 1662. Obama’s other remote family connections include Brad Pitt and ex US Vice President Dick Cheney, while fellow Democrat Hillary Clinton is distant cousins with an exotic array of A-listers including Angelina Jolie, Celine Dion and Camilla, Duchess of Cornwall.

Delve into the ancestry of Hollywood stars, and you’ll find more connections with the British royal family. According to Nick Barratt, a genealogist on the BBC’s Who Do You Think You Are?, Johnny Depp is the 20th cousin of Queen Elizabeth II and both are descendants of Edward III.

It’s all in the DNA

Two hundred years ago, Darwin was ridiculed for postulating a familial connection between humans and apes, but we’re now pretty sure that the two species share a common ancestor. And it isn’t just apes – humans are ultimately linked to every other species on the planet. The common factor is DNA. Even fruit flies have 60% of their DNA in common with humans, while 75% of mouse genes have human equivalents and we are 90% genetically similar to cats. Genetic variations between humans are even smaller. In fact, we differ from one another by just 0.5%. Connections don’t get much closer than that.

By looking at DNA, we can make links between communities from around the world. The Scotland DNA Project recently found that 1% of Scotsmen – that’s around 26,000 individuals – can trace their lineage back to the Berber and Tuareg tribesmen of the Sahara.

MYTH?

Mood

The phases of the moon are linked to human mood

False! This is an old one, with a full moon historically linked to everything from ‘lunacy’ (the clue is in the word) to werewolves. Most scientific studies, however, debunk this myth, showing no connection between the phases of the moon and human behaviour.

Continental connections

It isn’t just human populations who can be geographically linked to one another. The amazing geological similarities between rocks in different parts of the world prompted scientists to come up with the idea that the continents were once arranged in a very different pattern to the one we live with today. The most remarkable likenesses can be seen between the facing shorelines of Africa and South America, which essentially fit together like a geographical jigsaw. Both were formed two billion years ago when the two landmasses were, the theory goes, part of one giant supercontinent.

The fossil records of the two continents are also interconnected. Remains of a freshwater crocodile-like reptile called Mesosaurus dating back around 258 million years have been found in southern Africa, eastern South America and nowhere else. It would have been impossible for Mesosaurus to swim beneath the continents, suggesting that South America and Africa were, indeed, once joined together.

Strange science

Geology and evolutionary theory both throw up intriguing links – but it’s physics that has postulated the most profound and universal connections that we can conceive of. Quantum theory is vastly complex, but in its most simple form it states that everything is connected to everything else. Professor Brian Cox explains: “This means that the subatomic constituents of your body are constantly shifting, albeit absolutely imperceptibly, in response to events happening an arbitrarily-large distance away; for the sake of argument, let’s say on the other side of the universe.”

In other words, everything – no matter how distant it might be – is connected to everything else. And that’s a comforting thought.