The basic issue is that we as humans have evolved to eat cooked food. This allows our teeth, jaws, and digestive system to become smaller, and our brain to grow larger. There are various exhibits about concentration of energy, mostly in French (type words into Google translate). The issue with humans’ big brains is that we are forced to either use renewable energy sources in an unsustainable way, or use non-renewable energy resources. We cannot get enough concentrated energy resources just from using renewable resources in a renewable way. This is why economies collapse, over and over again.
This is an exhibit showing how we have evolved toward ever-more concentrated energy use. - Gail Tverberg