From the standpoint of biological complexity, there are two main categories of organisms: multicellular ones (macrobes) and unicellular ones (microbes). A closer look at the microbes reveals an even deeper evolutionary cleft, one of the most dramatic sorts and one that has puzzled evolutionary biologists for as long as the divide has been known. At the most basic level, there are only two kinds of cells in the living world: eukaryotes, typically large complex cells that always have a nucleus, and prokaryotes, typically small and more simply structured cells that always lack a nucleus.

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