“The body is often likened to a machine, but it is so much more than that. It works 24 hours a day for decades without (for the most part) needing regular servicing or the installation of spare parts, runs on water and a few organic compounds, is soft and rather lovely, is accommodatingly mobile and pliant, reproduces itself with enthusiasm, makes jokes, feels affection, appreciates a red sunset and a cooling breeze. How many machines do you know that can do any of that? There is no question about it. You are truly a wonder.”

Human bodies are almost unfathomably complex, built out of proteins specified in genetic instructions at the center of each of the body’s trillions of cells. The body functions continuously, sometimes for decades, without any one part being in charge, executing elaborate behaviors and giving rise to the ongoing panoply of conscious experience. Daunting to scientists in its complexities, the body contains mysteries that continue to be unfathomable.

“It may be slightly surprising to think it, but our skin is our largest organ, and possibly the most versatile. It keeps our insides in and bad things out. It cushions blows. It gives us our sense of touch, bringing us pleasure and warmth and pain and nearly everything else that makes us vital. It produces melanin to shield us from the sun’s rays. It repairs itself when we abuse it. It accounts for such beauty as we can muster. It looks after us.”

The skin is a shield and cushion that also helps regulate body temperature, assists the immune system in fighting pathogens, alerts its owner to changing conditions in the environment, and communicates with others through blushing, sweating, and aromas. The skin thus serves a high number of functions, coordinates them smoothly, and does so largely in the background.

“Make no mistake. This is a planet of microbes. We are here at their pleasure. They don’t need us at all. We’d be dead in a day without them.”

Without microbes, much of our food would go undigested; without them pulling vital nitrogen from the air and putting it into edible plants, most organisms, including humans, would quickly die off. Though individual microbes are too tiny to see without a microscope, all of them together outweigh all the visible plants and animals 25 times over.