We almost never see hunter-gatherer groups pursue leafy or cruciferous veggies as a source of significant calories.
Eh, but they still ate some, and ate plenty of starchy stems and root vegetables. Also, in modern first-world diets, I'd say the biggest thing vegetables provide is fiber, which was also provided in hunter-gatherer diets by un-domesticated grains or seeds.
Every hunter gatherer culture was/is different, but I don't think any consume large quantities of grains or seeds- that was mostly only made practical as a major source of calories by the development of agriculture.
It's not clear that humans actually need much insoluble fiber (what's mostly found in grains and seeds) for good health. Starchy tubers (which make up as much as 70% of calories in many equatorial hunter gatherer societies such as the Kitvavans) are a good source of soluble fiber, which acts as a substrate for gut bacteria.
Compared to agricultur...
Dying or becoming severely physically/mentally ill is very likely going to significantly lower the output of your utility function, so it would probably be a very bad idea to ignore the low-hanging resources which can significantly extend the time for which you are alive and well. I have attempted to search LessWrong for a list of such resources, and haven't been able to find one.
Are there any books, websites, or posts that contain significantly low-hanging fruit in this area? If so, please list them in the comments below.