"Geographical luck and circumstance." This is Jareld Diamonds thesis. His response to Yalli's question which was, "Why is that you white men have so much cargo and we New Guineans have so little cargo?" It seems like a simple question, but there is alot behind it. It can go all the way back their ancestors. 

About 13,000 years ago people gathered in the middle east (Mesopatamia) in a spot they call the Fertile creasent. Which is in present day Iraq. Before the last ice age it was a perfect spot for agriculture, because it was fertile. I agree with Diamond. I think that the reason why New uineans have to eat sago tree and not get the nutrition (and also the reason behind Yalli's question) is because they have to plant things that don't need alot of sun, because they are in the forest. If you try to plant somthing that needs sun light, in the middle of a rain forest it's going to work.