What I was taught growing up: Wild edible plants and animals were just so naturally abundant that the indigenous people of my area, namely western Washington state, didn't have to develop agriculture and could just easily forage/hunt for all their needs.
The first pebble in what would become a landslide: Native peoples practiced intentional fire, which kept the trees from growing over the camas praire.
The next: PNW native peoples intentionally planted and cultivated forest gardens, and we can still see the increase in biodiversity where these gardens were today.
The next: We have an oak prairie savanna ecosystem that was intentionally maintained via intentional fire (which they were banned from doing for like, 100 years and we're just now starting to do again), and this ecosystem is disappearing as Douglas firs spread, invasive species take over, and land is turned into European-style agricultural systems.
The Land Slide: Actually, the native peoples had a complex agricultural and food processing system that allowed them to meet all their needs throughout the year, including storing food for the long, wet, dark winter. They collected a wide variety of plant foods (along with the salmon, deer, and other animals they hunted), from seaweeds to roots to berries, and they also managed these food systems via not only burning, but pruning, weeding, planting, digging/tilling, selectively harvesting root crops so that smaller ones were left behind to grow and the biggest were left to reseed, and careful harvesting at particular times for each species that both ensured their perennial (!) crops would continue thriving and that harvest occurred at the best time for the best quality food. American settlers were willfully ignorant of the complex agricultural system, because being thus allowed them to claim the land wasn't being used. Native peoples were actively managing the ecosystem to produce their food, in a sustainable manner that increased biodiversity, thus benefiting not only themselves but other species as well.
So that's cool. If you want to read more, I suggest "Ancient Pathways, Ancestral Knowledge: Ethnobotany and Ecological Wisdom of Indigenous Peoples of Northwestern North America" by Nancy J. Turner
