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Illustration showing the Earth's geological timescale. Starting from the bottom (oldest), time units marked include eons (hadean, archean and proterozoic), periods (Devonian to Cretaceous) and epochs (paleocene to miocene). Unmarked uppermost layers include the pliocene, pleistocene and holocene. It has been proposed that a new epoch, called the anthropocene, should be included to represent the impact of humans on the planet. The markers on the left facing side of the timeline represent: climatic events (triangle), magnetic events (cross), biological events (circle) and isotopic events (square). Question marks are unknown events