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Humans like to think that they rule the world but, compared with bacteria, we are clearly in a minor position. Bacteria first evolved some 3.5 billion years ago, 2 billion years before the first eukaryotes appeared (some evidence suggests bacteria evolved even earlier). Today, bacteria are found in every conceivable environment, including boiling springs, highly saline lakes, and beneath more than 2 miles of ice in Antarctica. They are found at the top of Mt. Everest and at the bottoms of the deepest oceans. They are also present on and in us—in alarming numbers! Within the average human gut, there are approximately 10 trillion bacteria, ten times the total number of cells in the entire human body. No one knows how many bacteria populate the world, but an analysis conducted by scientists in 1998 estimated that the total number of living bacteria on Earth exceeded 5 million trillion trillion (5 × 10³⁰).

Not only are bacteria numerically vast, they also constitute the majority of life’s diversity. The total number of described species of bacteria is less than 10,000, compared with about 1.4 million plants, animals, fungi, and single-celled eukaryotes. But the number of described species of bacteria falls far short of the true microbial diversity.

Species of bacteria are typically described only after they have been cultivated and studied in the laboratory. Because only a few species are amenable to laboratory culture, for many years, it was impossible to identify and study most bacteria. Then, in the 1970s, molecular techniques for analyzing DNA became available and opened up a whole new vista on microbial diversity. These techniques revealed several important facts about bacteria. First, many of the relations among bacteria that microbiologists had worked out on the basis of physical and biochemical traits turned out to be incorrect. Bacteria once thought to be related were in fact genetically quite different. Second, molecular analysis showed that members of one group of microbes—now called the archaea—were as different from other bacteria (the eubacteria) as they are from eukaryotes. Third, molecular analysis revealed that the number of different types of bacteria is astounding.

In 2007, Luiz Roesch and his colleagues set out to determine exactly how many types of bacteria exist in a gram of soil. They obtained soil samples from four locations: Brazil, Florida, Illinois, and Canada. From the soil samples, they extracted and purified bacterial DNA. From this DNA, they determined the sequences of a gene present in all bacteria, the 16S rRNA gene. Each different species of bacteria has a unique 16S rRNA gene sequence, so they could determine how many species of bacteria existed in each soil sample by counting the number of different DNA sequences.

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Roesch’s results were amazing. The number of different eubacterial species in each gram of soil ranged from 26,140 for samples from Brazil to 53,533 for Canadian samples. Many unusual bacteria were detected that appeared dissimilar to all previously described groups of bacteria. Another interesting finding was that soil from agricultural fields harbored considerably fewer species than did soil from forests.

This study and others demonstrate that bacterial diversity far exceeds that of multicellular organisms and, undoubtedly, numerous groups of bacteria have yet to be discovered. Like it or not, we truly live in a bacterial world.