The answer to this can be found in Chapter 8. There, Diamond argues that New Guinea was disadvantaged in a number of ways when compared to the Fertile Crescent. Those ways included:
- None of the world's grass species with large seeds is native to New Guinea. This means they had no cereal crop to plant.
- No large gomesticable animals are native to New Guinea. This means New Guineans could not get much protein.
- The root crops were not very well suited for agriculture either. This was mainly because they did not grow well at higher elevations, limiting the area of the island that could be farmed.
For these reasons, people in New Guinea were not able to have really productive agriculture like people could in the Fertile Crescent.