21:27 - We are just as smart now as we were in the ice age
Our neurophysiology has not changed much since the ice age. In other words, were an ice age descendent were transported by a time machine and were born in our current era, (s)he would have the same cognitive capacity as a modern human.
Peter mentions that we came out of our caves and begun agriculture. There is an interesting research paper that hypothesizes that over a period of the last 1.5 million years, human hunters in the Southern Levant successively extirpated the largest species by overshooting hunting over many generations, until the wild fauna population could no longer support human populations, at which point, humans may have turned to agriculture out of necessity. If true, this would support the idea that nonsustainable practices have been with us for a long time and we were out of balance long before Adam Smith wrote about it.