Glad you brought up the Ice Age, if BF exists then it definitely evolved to work around all those creatures that perished over 10,000 yrs ago. In time/evolution that's a blip, almost nothing so I'm not sure that's enough time for an entire species to let go of its habits that formed from millions of years of coexistence with Saber-cats, a variety of Wolves, large LARGE Bears, a variety of other Cats including American Lions and Hyenas (the latter being a Mustelid). That's why I brought it up in another thread how BFs might have some form of species-wide PTSD of humans and other predators that they avoid us like the plague because that's what kept them alive while the rest perished. Keep your family close and guarded.
I'm speculating of course on all of this, but maybe the individuals that seem to lack the fear of people are those who are simply getting on with the times.
But as pointed out earlier, yes Orangs are the one exception and it's for a very good reason. Perhaps BF is this way too, forming communities but are still rather introverted and solitary MOST of the time.