In the final section of Guns, Germs, and Steel, Diamond reviews the histories of five geographic areas, exploring the question of why they developed to the points they did—and no further—by 1500 CE. He begins with Australia, the continent that had the least complex technology, the lowest population density, and perhaps the most distinctive societies of all.

Australia was inhabited by modern humans very early, perhaps even before Europe. Its people had some of the earliest stone tools, the earliest hafted blades, and the earliest boats. However, Australia’s people did not develop as much technology from that point as one might expect. Even more perplexing, Australia’s society did not develop the food production, bows and arrows, or other trappings of modern society that their neighbors, the New Guineans, did.

Some people assume that Aboriginal Australians are somehow genetically inferior to their New Guinean neighbors. However, Aboriginal Australians and New Guineans are genetically separated by only about 10,000 years of history. Diamond thinks their environments rather than their genetics determined their ability to achieve—or not achieve—technological advancement.

New Guinea has a good climate, good soil, and adequate rainfall. Early New Guineans were able to domesticate many crops. Their pigs, chickens, and sweet potatoes were imported from Asia, but the rest of their crop package is native. The transition to food production apparently sparked a population explosion that led to an increase in technological advancement and political complexity.

Still, New Guinea did not advance to the technological level that Eurasia did. This is because the New Guinean crop package was lacking in protein and because the island’s available area was so limited. New Guinea’s population never surpassed one million before European arrival, and the rugged terrain kept its people politically divided. The other world centers of agriculture all had populations in the tens of millions before they developed writing, metallurgy, and statehood.

Australia is the driest, least fertile, and least ecologically diverse continent. Shortly after humans arrived, its large native mammals went extinct. This left the continent without domesticable mammals. The ecology offered few plants worth domesticating in the relatively infertile soil. Frequent, severe droughts make agriculture risky there even today. Given the climate and conditions of the environment, hunting and gathering made the most sense.

However, Australian Aborigines did modify and manage their land. In the wettest, most productive parts of the continent, populations rose greatly within the past 5,000 years. Aborigines developed new gathering methods. They dug canals to improve eel fisheries, and they began harvesting a variety of wild millet. Diamond believes that the tools and methods they were developing would eventually have developed into agriculture.

Australians and their neighbors, the Tasmanians, seem to have developed some forms of technology that they later abandoned. Similarly, they seem to have been slow to adopt new technologies from elsewhere. They traded with Indonesian and New Guinean neighbors, but this did not substantially change the Aborigines’ way of life. Diamond hypothesizes that a lack of population density prevented technology from spreading and in some cases may have caused it to die out. He also points out that the Aborigines’ trade with New Guineans happened through intermediaries, not through the New Guineans themselves, so the differences in technology between the two groups would have been less obvious.

When Europeans arrived in New Guinea, they had difficulty conquering the area because of malaria. New Guineans struggled with European diseases but not as much as Native Americans did. This was partly because the colonization happened later, when healthcare was better, and partly because trade with Indonesia had already brought many Eurasian diseases to New Guinea. European crops and livestock did poorly in New Guinea, so Europeans never did particularly well there.

In Australia, however, Europeans were able to take over virtually all of the land with their guns, germs, and steel. They converted it to a food-producing economy by importing crops and livestock from other continents.