I think it’s disingenuous to talk as though you need 5 billion people to start impacting the land. One household destroys plenty. The majority of North American land was being cultivated in some fashion at the time of Spanish discovery, and the population of the continent was probably around 3 million people, 5% of modern day.

This study is about the Neolithic era, which is a period of huge earthworks. You may have heard of some of them. The idea that they were rotating crops to manage soil chemistry and practicing agroforestry 4-6000 years ago in a society known specifically for making giant fucking tombs is pretty neat when today with all the progress we’ve made we’re clear cutting the Amazon rainforest for cattle and similarly out of place monocrops.