Newly discovered life forms inside our bodies profoundly affect our health – and provide a glimpse of the vast and mysterious biological "dark matter" within us

Eiko Ojala

ERIC BAPTESTE is on a hunt for life, but not as we know it. He doesn’t think we have to sift through Martian soils or trawl lunar oceans to find these entities. His hunting ground is far closer to home: the human body.

“Biology is full of surprises,” says Bapteste, an evolutionary biologist at the Pierre and Marie Curie University in Paris. “Since we have not yet exhaustively sampled all the DNA in the world, there is still room for finding rare, strange creatures.”

A realist might say that Bapteste’s mission is doomed to fail. After all, we are in the 2010s, not the 1710s. It is unthinkable that biologists can unearth new divisions of life on Earth – let alone make those discoveries in the intimately familiar environment of the human body.

They would be wrong. Recent research shows that our bodies are home to microbes unlike anything science has encountered before – some so alien that they are rewriting the tree of life. What’s more, this microbial “dark matter” could be having a profound effect on our health, for better and worse.

The body is home to some 39 trillion microbes, which outnumber our 30 trillion human cells. Our skin has 10 million bacteria per square centimetre. Earlier this year, a study found that as many as 2000 different species can thrive in the human gut – although a smaller subset of these live in or on any one individual. For years we assumed these microbes were harmful, but we now know that many of them are actually our allies, closely linked with our health and well-being (see “The …