The key to a hidden kingdom

It’s a modern pastime to hypothesise about what makes a good relationship. One evening not long ago in a Berlin bar, I listened to a friend diagnose how things were going with his partner: ‘We might have become a bit too symbiotic.’ Surprisingly earnest perhaps, but that’s what you get when a sociologist dates a psychoanalyst. On the way home, I wondered why symbiosis, apart from the obvious dangers of parasitism, might not be that desirable coexistence all our theories point toward. After all, the OED recommends that this ‘interaction between two different organisms living in close physical association’ is ‘typically to the advantage of both’. The darker side of symbiosis, I suppose, is the risk of losing yourself.

Yet the richest, riskiest symbioses are playing out inside you. The human body is home to 100 trillion microbes in mobile constellation (a mere 100 million stars, by comparison, make up the Milky Way). On our skin, encircling our orifices, throughout our guts and even within our cells, their genes outnumber ours by 500 to one. The changing census of these microbial presences (both tourists and residents) is our individualised microbiome. It is shaped by the food we eat, the company we keep, whether we were born vaginally or cut out of our mothers, fed from the breast or the bottle. A rough-and-ready measure of a healthy symbiosis is if our bacterial communities are not seen, smelt or felt. We ignore their mainly reliable, diligent and beneficial labour: they are our silent majority.

Recently scientists have started to make some noise about these bacteria who do more good than harm. On the final page of The Origin of Species, Darwin paid homage to the ‘grandeur’ of evolution as a ‘war of nature’. But Ed Yong is here to tell us that this is less than half the story.