What happens in the gut doesn’t stay in the gut – it sometimes affects the brain. Animal studies have started to show that the microbiome, from its staging ground in the bowel, can influence the development of its host’s brain.

Rochellys Diaz Heijtz found that germ-free mice, without any microbiome, were more active, less anxious and less risk-averse than usual. Their brains differed in the activity of over a hundred genes that provide cells with energy, influence chemical communications in the brain and strengthen the connection between nerve cells. Heijtz could even shift her germ-free mice towards “normal” behaviour and genetic activity by giving them a microbiome transplant, but this only worked early in their lives.

But later, Javier Bravo at University College Cork managed to change the behaviour of normal adult mice by feeding them with a probiotic bacterium called Lactobacillus rhamnosus, often found in yoghurts and dairy products. The bacterial menu changed the levels of signalling chemicals in the rodents’ brains, and reduced behaviours associated with stress, anxiety and depression.

Meanwhile, Gil Sharon found that gut bacteria can shape the sexual choices of flies. Flies that are raised on diets of starch prefer to mate with other “starch flies” while those raised on maltose prefer “maltose flies”. When Sharon dosed the flies with antibiotics, she killed both their gut bacteria and their sexual preferences. If she inoculated the sterile flies with the microbiome of their peers, their preferences reappeared instantly. It’s possible that the bacteria influence the levels of sex pheromones that affect the fly’s attractiveness.

These studies show that you can’t understand an animal’s evolution simply by considering the evolutionary pressures that act on its genome. You also have to consider the genes of the bacteria and other passengers that live inside it. We’re each like a superorganism – a unified alliance between the genes of several different species, only one of which is human.