Is it ensure, insure, or assure?
before vowels pale- word-forming element used in scientific combinations (mostly since c.1870) meaning "ancient, early, prehistoric, primitive," from Greek palaio-, comb. form of palaios "old, ancient," from palai "long ago, far back," related to palin "again, backwards," tele- "far off, at a distance," from PIE root *kwel- "to turn, move about," also "far" (in space and time); see cycle (n.).
paleo- or pale-
Ancient; prehistoric; old: paleopathology.
Early; primitive: paleokinetic.
A prefix that means "prehistoric" (as in paleontology) or "early or primitive" (as in Paleolithic).