Thus the microbial community in the soil, like in the human biome, provides "invasion resistance" services to its symbiotic partner. We disturb this association at our peril. As Michael Pollan recently noted, "Some researchers believe that the alarming increase in autoimmune diseases in the West may owe to a disruption in the ancient relationship between our bodies and their 'old friends' -- the microbial symbionts with whom we coevolved."

Not only do soil microorganisms nourish and protect plants, they play a crucial role in providing many "ecosystem services" that are absolutely critical to human survival. By many calculations, the living soil is the Earth's most valuable ecosystem, providing ecological services such as climate regulation, mitigation of drought and floods, soil erosion prevention, and water filtration, worth trillions of dollars each year. Those who study the human microbiome have now begun to borrow the term "ecosystem services" to describe critical functions played by microorganisms in human health.

With regard to stabilizing our increasingly unruly climate, soil microorganisms have been sequestering carbon for hundreds of millions of years through the mycorrizal filaments, which are coated in a sticky protein called "glomalin." Microbiologists are now working to gain a fuller understanding of its chemical nature and mapping its gene sequence. As much as 30 to 40 percent of the glomalin molecule is carbon. Glomalin may account for as much as one-third of the world's soil carbon -- and the soil contains more carbon than all plants and the atmosphere combined.

We are now at a point where microbes that thrive in healthy soil have been largely rendered inactive or eliminated in most commercial agricultural lands; they are unable to do what they have done for hundreds of millions of years, to access, conserve, and cycle nutrients and water for plants and regulate the climate. Half of the earth's habitable lands are farmed and we are losing soil and organic matter at an alarming rate. Studies show steady global soil depletion over time, and a serious stagnation in crop yields.

So, not only have we hindered natural processes that nourish crops and sequester carbon in cultivated land, but modern agriculture has become one of the biggest causes of climate instability. Our current global food system, from clearing forests to growing food, to fertilizer manufacturing, to food storage and packaging, is responsible for up to one-third of all human-caused greenhouse-gas emissions. This is more than all the cars and trucks in the transportation sector, which accounts for about one-fifth of all green house gases globally.

The single greatest leverage point for a sustainable and healthy future for the seven billion people on the planet is thus arguably immediately underfoot: the living soil, where we grow our food. Overall soil ecology still holds many mysteries. What Leonardo Da Vinci said five hundred years ago is probably still true today: "We know more about the movement of celestial bodies than about the soil underfoot." Though you never see them, ninety percent of all organisms on the seven continents live underground. In addition to bacteria and fungi, the soil is also filled with protozoa, nematodes, mites, and microarthropods. There can be 10,000 to 50,000 species in less than a teaspoon of soil. In that same teaspoon of soil, there are more microbes than there are people on the earth. In a handful of healthy soil, there is more biodiversity in just the bacterial community than you will find in all the animals of the Amazon basin.

An electron micrograph of a mycorrhiza with radiating mycorrhizal fungal filaments

We hear about many endangered animals in the Amazon and now all around the world. We all know about the chainsaw-wielding workers cutting trees in the rainforest. But we hear relatively little about the destruction of the habitat of kingdoms of life beyond plant and animal -- that of bacteria and fungi. Some microbiologists are now warning us that we must stop the destruction of the human microbiome, and that important species of microorganisms may have already gone extinct, some which might possibly play a key role in our health.