When you materialize, you show up suddenly after being missing, unborn, or unseen. Think of Harry Potter removing his invisibility cloak. He materializes.
Living things aren't the only things that materialize. Car keys and reading glasses materialize when you've been searching for them for a while and then they are suddenly there on the table. If you're lucky an unexpected business deal will materialize. Like material, the word is related to the Latin word meaning "matter." Matter, is, of course, all the stuff of this earth, anything and everything that takes form. So to materialize is to take form.