Of course that it's a metaphor. You can trace "the fall" in history -- homo sapiens is at least 210K years old, but it wasn’t until approximately 70 thousand years ago that people started to wear clothes and at around that time also (undisputed) symbolic art (70–75 kya) and intentional burials (80–100 kya) appeared for the first time. So, for some 100K years, even though they had a bit more body hair than we have today, our ancestors were already what we could consider a “naked ape.” And, whatever happened in the heads of our predecessors 100 kya, it was obviously the era when people became painfully aware of their mortality. I would say that that was the time when symbolic thinking evolved.