The Cambrian Period was a geological time period 55 million years ago that's well known for having the evolutionary "explosion" that gave us vertebrates, and it's directly preceded by the Precambrian Eon, the earliest part of Earth's history. The term was first used in 1836 by English scientist Adam Sedgwick, who named it after the historical name for Wales, Cambria, in reference to a system of rocks from the Cambrian Period exposed in the area. That's the Latinized form of Cymru, the Welsh self-appellation, and eventually traces to Proto-Celtic mrogis, meaning "country", and Proto-Indo-European morg, "border". Usage of the phrase Cambrian Period has steadily been increasing over the past few centuries, especially being popularized by palaeontologist Stephen Jay Gould's 1989 book Wonderful Life.
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