Primaeval is simply the British spelling of primeval. Same word, same meaning: very old, from the earliest times, before cities and writing — forests, fear, the first ages of the world. You may see it in older British books. Americans almost always write primeval. Though actually, even in Britain, primeval is now the more common spelling; primaeval looks a little old-fashioned, the way mediaeval looks next to medieval. The ae is the old Latin-style vowel, not a different word and not a different meaning. If you are learning one form to use, learn primeval; if you meet primaeval on the page, read it as the same.

原始的、太古的（英式拼法，即 primeval）