In its simplest sense, descend means to go or move downward, but there are a couple of subtle variations on this theme. Yes, a bird can descend from the sky, but also, humans are descended from prehistoric ancestors.
If you've ever heard a pilot say "prepare for our descent", then you know that only one letter separates the noun descent from the verb descend. They both derive from the same Latin components de, meaning "down," and scandere, "to climb." Similarly, if someone condescends to you, they are said, derogatorily, to be coming down to your level. All Americans are descended from peoples from other continents, such as Europe, Africa, and Asia. We've all "come down" from our ancestors.