With just shy of 21,000 genes, the human genome is hardly bigger than that of The Worm (C. elegans). It is half the size of the rice plant, and even the humble water flea outstrips it, with 31,000 genes. None of these species can talk, create, or think intelligent thoughts. You might think, as the scientists entering the Genesweep pool did, that

After the small intestine has digested and absorbed as much as it can from what we’ve eaten, the leftovers move into the large intestine, where most of our microbes live. Here, they function like factory workers, each breaking down its own preferred molecules and absorbing what it can. The rest is left in a simple enough form for us to absorb

I used a citizen-science programme, the American Gut Project, based at the laboratory of Professor Rob Knight at the University of Colorado, Boulder. Available to anyone around the world for a donation, the AGP sequences samples of microbes from the human body to learn more about the species we harbour and their impact on our health. By sending a

Stool, far from being the remains of our food, is mostly bacteria, some dead, some alive. Around 75 per cent of the wet weight of faeces is bacteria; plant fibre makes up about 17 per cent.

You are just 10 per cent human. For every one of the cells that make up the vessel that you call your body, there are nine impostor cells hitching a ride. You are not just flesh and blood, muscle and bone, brain and skin, but also bacteria and fungi. You are more ‘them’ than you are ‘you’. Your gut alone hosts 100 trillion of them, like a coral

Metabolic rates do vary from person to person, but it is actually overweight people who have the faster metabolisms, not lean people. It simply takes more energy to run a big body than a small one.

Our own cells, though far larger in volume and weight, are outnumbered ten to one by the cells of the microbes that live in and on us. These 100 trillion microbes – known as the microbiota – are mostly bacteria: microscopic beings made of just a single cell each. Alongside the bacteria are other microbes – viruses, fungi and archaea.

Together, the microbes living on the human body contain 4.4 million genes – this is the microbiome: the collective genomes of the microbiota. These genes collaborate in running our bodies alongside our 21,000 human genes. By that count, you are just half a per cent human.

In 2010, a huge study was conducted by a team of hundreds of scientists who hunted through the genes of a quarter of a million people in the hope of finding some that were associated with weight. Astonishingly, they discovered just 32 genes in our 21,000-strong genome that seemed to play a role in weight gain. The average difference in weight