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Douglas Harper's Etymology Dictionary

word-forming element, used from mid-19c. (first in Eocene) in compound words formed by earth-scientists, and meaning "characterized by the earliest appearance of," from Greek eos "dawn, morning, daybreak," also the name of the goddess of the morning, from PIE *aus- (1) "to shine," especially of the dawn (see aurora). Piltdown Man, before exposed as a fraud, was known as Eoanthropus.