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Pre-cambrian (or Precambrian) is a term used to describe the eons before the Cambrian. It lasted from about 4.567 billion years BC to about 542 million years BC. It starts with the beginning of the Earth, about 4.567 billion years BC. When it ends, there were many hard-shelled animals around. It is named after the Roman name for Cymru / Wales, because rocks thought to be from this time were first studied there.