You have to remember this is rediscovering the past in ways that previous cultures only had mythology around. The fact that this paper is basically “Stone Age people aren’t less sophisticated” is a relatively new idea since levi strauss reinvented anthropology in the 1950s and 1960s

Hindu, then Greek then confuscian theologian-philosophers laid the foundations for the idea that their group had left behind simply being “animals” and sought out to distinguish human form (in their specific form) from all other forms of life.

Humans also approach things linearly and it fights intuition that regression is not just possible but the norm.