One possibility is that the people in the sample set are all distantly related. The genomes in question all come from the Framingham Heart Study, a well-known dataset associated with people on the east coast of the US who are generally white and of European ancestry. Perhaps the genetic links are simply a reflection of this common background.

Not so, say Christakis and Fowler. The correlation they have found exists only between friends but not between strangers. If this was a reflection of their common ancestry, then the genomes of strangers should be correlated just as strongly. “Pairs of (strictly unrelated) friends generally tend to be more genetically homophilic than pairs of strangers from the same population,” they point out.

There are certainly other processes that could lead to friends having similar genomes. One idea that dates back some 30 years is that a person’s genes causes them to seek out circumstances that are compatible with their phenotype. If that’s the case, then people with similar genes should end up in similar environments.

That makes sense. “If one individual builds a fire because he feels cold in the same circumstances as the other, both benefit,” say Christakis and Fowler.

What’s more, this idea may not only apply to the physical environment but also to the social environment. So people with similar genes may end up in similar social groups too.

Both of these factors make it more likely that your friends will have similar genes. But nobody has gathered evidence for this until now.

There may be another mechanism at work. One idea is that humans can somehow identify people with similar genetic make up, perhaps with some kind of pheromone detector. Indeed, Christakis and Fowler say that some of the genes they found in common are related to olfaction, a discovery they describe as “intriguing and supportive”.

Whatever the cause, the discovery that our friends are genetically similar to us has significant implications. “The subtle process of genetic sorting in human social relationships might have an important effect on a number of other biological and social processes,” say Christakis and Fowler.

For example, germs, viruses and even information may spread more (or less) easily amongst groups that share a particular genetic background.

More significantly, this may be the first evidence that our social environment is an evolutionary force that can influence our genetic make up. And if so, we may have evolved a predeliction to choose friends who are similar to us once we started to interact socially with other people who are unrelated.

That’s an important process that would especially speed up the evolution of phenotypes that cooperate well, say Christakis and Fowler.

There are caveats of course. This research will be controversial and others will want to be sure that Christakis and Fowler have not misinterpreted their data.

The research will be hard to reproduce. The Framingham Heart Study is unique in that it is a large database that includes both genetic data and friendship links. Christakis and Fowler acknowledge that there is no other like it.

But if the result is widely accepted it will be hugely influential.

One of the big mysteries associated with the development of human capabilities is that our evolution appears to be accelerating. After billions of years of relatively slow change, it created the marvel that is the human brain and consciousness, in the metaphorical blink of an eye.

Christakis and Fowler end their paper with the tantalising speculation their discovery may shed light on this mysterious process of acceleration that has produced the wonder of complex reasoning and consciousness.

Ref: arxiv.org/abs/1308.5257: Friendship and Natural Selection