i have also noted that purslane can increase yields due to its mulching capability. A friend in sandy soil had two non-irrigated corn plots (he is a vegan), in a dry year only the one “infested” with purslane gave a crop. and all domestic animals love it.
It is true that we have always eaten insects, but if your diet gives you enough of a nutrient, eventually you lose the ability to manufacture it. it is the same with vitamin C. Although we have been eating insects far longer than we have eaten megafauna, insects were never a dominant part of the diet. But homo sapiens was often subsisting entirely on megafauna.
The other thing I did not mention: to elongate, say from 18 to 20 or 22, you need a high concentration of 2 and 4 fat molecules available for the reaction. That is acetic acid and butyric acid. Where do they come from? all from fermentable fiber, fermenting in the gut (for you and me, beans, potatoes, and the like). We have about ten times less by concentration, and therefore a smaller chance of doing the conversion, because we have a smaller gut compared to wild and domestic animals.