Other forms: primevally
Use primeval when you want to suggest something has existed from the beginning. The Big Bang? Dinosaur bones? Ancient trees? Yep — primeval.
The adjective primeval is from the Latin root words primus, meaning "first," and aevum, meaning "age." You might have an irrational, primeval fear of monsters — meaning that your monster fear has been there for as long as you can remember.