I think he makes two points: early man left only the very slighest traces, and two, the gap between man and the other primates is unbridgeable except at a material level and even that by an exercise of human thought. It is a bit like trying to trace the personal history of some derelict by looking at his bones a hundred years after he is buried in a pauper’s grave. Some things are unknowable. “Dust thou art and to dust shalt thou return. “
“...unbridgeable except at a material level and even that by an exercise of human thought...”
Wow. Sounds like science.