The Cambrian Explosion
The Cambrian “explosion” was an event of rapid increase (in geological terms) of multicellular life. Trilobites like the one shown below are the most representative creatures of the time. With the Cambrian explosion, the Phanerozoic eon (the current eon) began. Its name was derived from the Ancient Greek words φανερός (phanerós) and ζωή (zo̱í̱), meaning visible life. The first Era of the Panerozoic is the Paleozoic Era, from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life". It is subdivided into six geologic periods (from oldest to youngest): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
Source: Carl Sagan, Cosmos (New York: Random House, 1980)
Image by Charlie Sanabria under CC BY 4.0 license
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!