Just when you thought it was safe to assume that we now know all we are ever likely to know about the past, someone digs another hole and unearths (literally or figuratively) some ancient  one day it is a fragile scroll, found in a cave near the Dead Sea, that turns out to be pre-Biblical; the next day it is an entire terracotta army of Chinese  the next it is a skull, excavated from the Olduvai Gorge, that compels anthropologists (once again) to revise their guesses about the earliest stages of Homo sapiens sapiens vs hominids.
We never really know everything there is to know about our past because every day someone will dig out treasures from the past, keeping anthropologists on their toes and guessing. 