Well in the times of cavemen we also didn't have 26 people owning as much as the poorest half of the world. So while it might not have been the issue then, that doesn't mean it can't be now. Things can have multiple causes, and causes can change over time.
I haven't read the linked paper so I don't know how they'd define poor in it, but in my opinion in times of cavemen everyone was free to catch any prey to feed their families, everyone was free to pick any fruits from any trees and bushes, everyone was free to move in to any caves on any land.
I'd say they had everything they needed. Nowadays I wouldn't call that poor as masses of people working several shifts to be able to pay the rent and feed their families while they are feeding others to obesity higher up in the chain.
