de

De covers both possession ("of") and origin ("from"), so it does the work of the English apostrophe-s as well: la mano de la homo. Like every Esperanto preposition of place it is followed by the plain nominative, never the accusative. Watch the split with its two neighbours: a part taken out of a whole uses el (unu el ni, "one of us"), and a quantity uses da (multe da tempo, "much time").