Bacteria are everywhere, in numbers we can scarcely imagine.
We may think of them as agents of disease and filth, the cause of cholera, syphilis, anthrax, leprosy, bubonic plague, and tuberculosis. But we can’t live without them.
Our debt to bacteria goes back to a time when Earth’s early atmosphere had no oxygen, but consisted of nitrogen, carbon dioxide, and methane. Without oxygen, multicellular life wasn’t possible. The exact form of the very earliest life remains a puzzle, but the earliest fossils of life, dating back 3.5 billion years, are colonies of bacteria. “Microbes ruled the world,” says Jennifer Macalady ’91, an associate professor of geosciences at Penn State University. These bacteria took energy from waterborne chemicals, such as sulfide and iron. Sometime later, bacteria learned how to manufacture their own food through photosynthesis, probably increasing the mass of Earth’s biosphere by orders of magnitude. Cyanobacteria—photosynthetic bacteria that expel oxygen as waste—appeared more than 2.7 billion years ago, and slowly oxygen began accumulating in the atmosphere.