late Middle English (as an adjective in the sense 'descending'): from French, present participle of descendre 'to descend' (see descend). The noun dates from the early 17th century
The correct spelling for the noun meaning ‘person descended from a particular ancestor’ is descendant, ending with the suffix -ant, not -ent (as in she claims to be a descendant of Paul Revere). The word descendent is an adjective, now used almost exclusively in scientific contexts, meaning ‘descending from an ancestor’ (as in extinct species are replaced by descendent species). Almost 15 percent of the citations for the noun in the Oxford English Corpus use the wrong spelling.