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 ēōs "dawn, morning, daybreak," also the name of the goddess of the morning, from PIE root *aus- (1) "to shine," especially of the dawn. Piltdown Man, before exposed as a fraud, was known as Eoanthropus.
Entries linking to eo-
Eocene adj.
in reference to the second epoch of the Tertiary Period, 1831, from eo- "earliest" + Latinized form of Greek kainos "new" (see -cene). Coined in English (along with Miocene and Pliocene) by the Rev. William Whewell, English polymath, and meant as "the dawn of the recent." As a noun from 1851.
It has occurred to me that [kainos] is a better word than [neos], and I propose for your terms, 1 acene, 2 eocene, 3 miocene, 4 pliocene. ... For eocene you might say spaniocene, but I like your eo better. Is not this shortest and best? [Whewell, letter to Lyell, Jan. 31, 1831]
*aus- 1
Proto-Indo-European root meaning "to shine," especially of the dawn. It forms all or part of: austral; Australia; Austria; Austro-; Aurora; east; Easter; eastern; eo-; Ostrogoth.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usah "dawn;" Greek ēōs "dawn;" Latin Aurora "goddess of dawn," auster "south wind;" Lithuanian aušra "dawn;" Old English east "east."