There are more living organisms on the skin of a single human being than there are human beings on the surface of the earth.
We are not alone. No matter how clean we are or how healthy we feel, we carry around on our bodies billions of microbes—microscopic one-celled organisms called bacteria and viruses. Although they cannot be seen, microbes hide under fingernails, lurk between teeth, and live in hair. More than six hundred thousand bacteria are living on just one square inch of skin, and an average person has about a quarter of a pound of bacteria in and on his or her body at any given time. There are more microbes on a person's body than there are humans on Earth.
Viruses and bacteria are responsible for some of the deadliest diseases in history, such as AIDS, the plague, and the flu. And yet bacteria perform the most important roles in maintaining life on this planet. "They [bacteria] protect us and feed us," says Abigail Salyers, former president of the American Society for Microbiology. "All life on Earth depends on their activities."
Microbes have been found in almost every type of environment. Some thrive in subzero Arctic ice, while others live in boiling undersea volcanoes. Bacteria have been found inside oil-drilling cores pulled from more than a thousand feet down in the earth's crust, and it has been estimated that there may be as much as 100 trillion tons of bacteria deep beneath the surface of the earth. If all the subterranean microbes were brought to the surface, they would cover the planet with a layer five feet deep. Microbes have been discovered six miles beneath the Pacific Ocean, where the pressure is equivalent to being squashed by fifty jumbo jets, as well as nineteen miles out in space.
Professor Adrian Gibbs of the Australian National University asserts, "You can increase the probability of finding new things by looking in interesting places, like deep-sea vents or thermal pools, but you can also find them in your own backyard."
There is more unseen life than seen. The mass of all microbes on the planet is twenty-five times more than the mass of all other animals combined. The human race may believe it is at the top of the food chain, but microbes are the food chain.