instances of cooperation and synergy.For example,if a Bacteroides bacterium in the gut can detoxify a chemical in the environment that inhibits E.coli,then E.coli benefits.A one-way helpful relationship like this is called commensal. Interactions are even more powerful when the benefits are mutual.Imagine that E.coli's main waste product turns out to be a good food source for Bacteroides.In this case,the two species will tend to congregate in the same environments.Each is doing nothing more than following its own program,but ultimately they help each other:this is svmbiosis Under other conditions,many different bacteria help each other.Perhaps in a fast-moving stream,bacterium A eats the waste products of bacterium B and also sticks to the edges of rocks.Meanwhile bacterium C,which cannot stick,can adhere to bacterium A to avoid being swept away and helps anchor A in place.And B produces a compound nutritious to C.Now we have a situation where bacteria A, B,and C tend to cluster together,to the mutual benefit of all three. Over the more than 4 billion years of bacterial evolution,with bacteria dividing and new cells coming along as often as every twelve minutes,and astronomical numbers of individual bacteria,there has been nearly infinite variation.From this endless process,individual bacteria have arisen that have populated all available niches on Earth Sometimes bacteria can live stably together,forming a consortium.These cooperative groups abound in the environment-in soil,in streams,on decaying logs.in hot springs -nearly everywhere there is life.The earliest unequivocal proof of ancient life is the existence of 3.5-billion-year-old fossilized"microbial mats"found in Australia,consortia that arranged themselves into large,layered sheets forming whole miniature ecosystems.In all likelihood,some layers performed photosynthesis,some breathed oxygen,some performed fermentation, and some ate unusual inorganic compounds.One species'meat is another species' poison;by settling into layers and combining their abilities,their concerted efforts lead to the benefit of all There are microbes that can form gelatin-like layers surrounding themselves These thick gels are called biofilms.Their composition varies,but biofilms can protect the bacteria from drying out,or from excessive heat,or from the onslaught of immunity.The presence of biofilms helps explain bacterial persistence in harsh circumstances Microbes also form consortia and vast webs of cooperative functions not only in soils,oceans,and rocky surfaces but in animals as well.Such organisms in the human body are the central characters in my tale of"missing microbes."The great biologist Stephen Jay Gould provided a frame of reference for all of terrestrial biology when he wrote: we live in the Age of Bacteria (as it was in the beginning is now,and ever shall be,until the world ends)..."This is the context for human life,background and foreground. 22instances of cooperation and synergy. For example, if a Bacteroides bacterium in the gut can detoxify a chemical in the environment that inhibits E. coli, then E. coli benefits. A one-way helpful relationship like this is called commensal. Interactions are even more powerful when the benefits are mutual. Imagine that E. coli’s main waste product turns out to be a good food source for Bacteroides. In this case, the two species will tend to congregate in the same environments. Each is doing nothing more than following its own program, but ultimately they help each other; this is symbiosis. Under other conditions, many different bacteria help each other. Perhaps in a fast-moving stream, bacterium A eats the waste products of bacterium B and also sticks to the edges of rocks. Meanwhile bacterium C, which cannot stick, can adhere to bacterium A to avoid being swept away and helps anchor A in place. And B produces a compound nutritious to C. Now we have a situation where bacteria A, B, and C tend to cluster together, to the mutual benefit of all three. Over the more than 4 billion years of bacterial evolution, with bacteria dividing and new cells coming along as often as every twelve minutes, and astronomical numbers of individual bacteria, there has been nearly infinite variation. From this endless process, individual bacteria have arisen that have populated all available niches on Earth. Sometimes bacteria can live stably together, forming a consortium. These cooperative groups abound in the environment—in soil, in streams, on decaying logs, in hot springs—nearly everywhere there is life. The earliest unequivocal proof of ancient life is the existence of 3.5-billion-year-old fossilized “microbial mats” found in Australia, consortia that arranged themselves into large, layered sheets forming whole miniature ecosystems. In all likelihood, some layers performed photosynthesis, some breathed oxygen, some performed fermentation, and some ate unusual inorganic compounds. One species’ meat is another species’ poison; by settling into layers and combining their abilities, their concerted efforts lead to the benefit of all. There are microbes that can form gelatin-like layers surrounding themselves. These thick gels are called biofilms. Their composition varies, but biofilms can protect the bacteria from drying out, or from excessive heat, or from the onslaught of immunity. The presence of biofilms helps explain bacterial persistence in harsh circumstances. Microbes also form consortia and vast webs of cooperative functions not only in soils, oceans, and rocky surfaces but in animals as well. Such organisms in the human body are the central characters in my tale of “missing microbes.” The great biologist Stephen Jay Gould provided a frame of reference for all of terrestrial biology when he wrote: “… we live in the Age of Bacteria (as it was in the beginning, is now, and ever shall be, until the world ends)…” This is the context for human life, background and foreground. 22