The idea of “holobionts” represents a paradigm shift in biology

“How many cells are there in a human being?” It sounds like a question from a nerdy pub quiz. It is also a profound philosophical inquiry. One answer is around 37trn. This is the number, in a typical adult weighing 70kg, that trace their descent from the fertilised egg which brought that human into existence.Look at it another way, though, and you arrive at a figure roughly twice as large. That adds in the archaean, bacterial, fungal and protist cells which occupy the mouth, gut, skin, lungs and almost every other surface, nook and cranny of the human body. These cells contribute only about 0.3% to a person’s body weight. But being, on the whole, much smaller than “proper” human cells, they are almost equally numerous.