Bacteria were among the first forms of life to appear on Earth. They inhabit every habitat on the planet, from the frozen arctic to hydrothermal vents belching carbon dioxide superheated to several hundreds of degrees. They thrive in radioactive waste, at the deepest part of the ocean and even the vacuum of space. There are more bacteria living in and on your body than you have human cells--by a factor of 10. As Neil deGrasse Tyson is fond of saying, more bacteria live and work in just one centimeter of your lower colon than the sum of all humans who have ever lived. They are, by just about any definition, the dominant form of life on this planet. And yet, they are so small (about two microns, or two millionths of a meter) that half a million can rest comfortably on the head of a pin. If an average bacteria were the size of a quarter, the average human would be 15.5 miles high.

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