If I were to say that part of you is not you, your response would be disbelief. I am not talking about artificial hips or other implants, but the fact that a part of all humans is not human. We are, like so many other animals and plants, in a symbiotic relationship – a partnership with other life forms. And furthermore, we have evolved to enhance this partnership, as it is of mutual benefit.

The non-human part of us comprises the bacteria which live in the colon. This population of billions, comprising many different species, live in harmony with us, and the human colon has evolved to take advantage of their presence. So why are they there and how do we gain from their activities?

Many other mammals have ‘biomes’ (the recent trendy term for resident populations of other organisms), and perhaps the most well-known is that which populates the stomach of cows and other ruminants. And the bovine example can suggest why these biomes exist. In the case of ruminants, their function is to break down the grasses and other plants which cows eat. You might ask: which came first the herbivorous cow or its biome. We can only assume they – the cow with its specialised stomach, and the organisms which live there, evolved together.

So back to we humans and our bacterial friends living in the colon. The latter have a similar function to those in ruminants – they break down substances which we have not evolved to digest. In this case, the hard coverings of seeds and nuts, the fibres in plant foods and grains and otherwise indigestible starches. Our association with the colon bacteria evolved during the long time when our ancestors were hunter-gatherers and relied for food mainly on plants, fruits and wild cereals and less on the occasional animal they killed.

This association between us and our colon biome is of great importance to human health but, unfortunately, because it has been regarded as just waste management, has been little studied. The implications of our intimate relationship with the contents of our colons will be covered in subsequent posts. In the meantime, you could start to pay some attention to the dietary needs of your internal friends.