c. 1300, "genealogical extraction from an original or progenitor," from Old French descente "descent, descendance, lineage," formed from descendre "to come down" (see descend) on analogy of French nouns such as attente from attendre "to expect," vente "sale" from vendre "to sell," pente "slope" from pendre "to hang" (the etymological English word from Latin would be *descence).
Meanings "action of descending" (on); "act of passing from a higher to a lower place" in any way are from late 14c.; that of "a downward slope" is from 1590s. From c. 1600 as "a sudden invasion or attack." Biological sense "evolution" is from 1859 in Darwin, though there are uses which suggest essentially the same thing going back to 1630s.