1640s, "one sprung from the soil he inhabits" (plural autochthones), from Latinized form of Greek autokhthon "aborigines, natives, primitive inhabitants," literally "sprung from the land itself," used of the Athenians and others who claimed descent from the Pelasgians, from autos "self" (see auto-) + khthōn "land, earth, soil" (from PIE root *dhghem- "earth").
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
<a href="https://www.etymonline.com/word/autochthonous">Etymology of autochthonous by etymonline</a>
Harper, D. (n.d.). Etymology of autochthonous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/autochthonous