Diamond begins his book with a question asked by his native New Guinean friend why it was Europeans who conquered the world. Certainly one answer has been commonly promulgated: Europeans are inherently disposed to create a better culture. Diamond disputes this and spends the next four hundred pages showing that, ultimately, geography is the main determiner of how human culture develops.

The first priority of any society is, of course, how to feed itself. Initially this was accomplished by hunting and gathering. If there are any particularly nourishing plants available these soon become a staple of the local diet. Selection by these humans eventually lead to the plants being domesticated, that is, genetically changed so that they put most of their resources into producing seed easily harvested by humans. However, not all continents have an equal distributions of plants that are domesticatable. Eurasia had the most, particularly confluent in the Fertile Crescent, but also in eastern China. Asia also had a fair amount, but the Americas had very few and they were domesticated only after millenia had elapsed from the domestication of plants in Eurasia. Australia had no domesticatable plants. This is not from lack of effort by inhabitants of less well-endowed continents—modern man has not succeeded in domesticating plants passed over by our ancestors.

Similarly, not all continents possed the same number of domesticatable animals, needed for their protein content. Although animals of most species have been tamed, most are not suitable for domestication, which requires that animals be large, subordinate to humans, and be able to live peaceably in dense groups. It turns out that Eurasia had by far the most, 13, compared to the America’s tenuous one (llamas), with none in Australia and Africa, having all been exterminated in the initial immigration. This meant that not only did Eurasia have an abundant protein supply, but it also had increased muscle-power for tilling soil and heavy loads, as well as a formidable military weapon (cavalry).

The process of domestication led to a shift to farming, as hunter-gatherers discovered that that their needs could be met locally instead of by roaming. This permitted larger population densities, which gave a distinct numerical advantage over hunter-gatherer bands in war, as they are never more than widely spaced bands of a few dozen, since hunting and gathering cannot support larger numbers. It also permitted specialization, since not everyone needed to produce food, leading to larger opportunities for technological innovation. Furthermore, the close proximity to animals developed immunity to additional diseases and the larger population densities caused epidemics which conferred additional genetic immunity to those groups. Thus farming populations inevitable displaced or destroyed hunter-gathering bands in locations suitable for their crops.

Eurasia is oriented east to west, which means that it can be traversed at the same lattitude, permitting the same crops to be grown over the length of the continent. As a result, domesticated crops from the Fertile Crescent and from China quickly diffused over the entire continent, placing all humans in Eurasia at the same level of food production ability. In contrast, Asia and the Americas are oriented from north to south, which means that although crops in northern North America are well suited to southern South America, they cannot grow in the middle Americas, which will severely retard their spread. Although corn and beans were eventually domesticated in middle America, the change in lattitude so retarded their spread that it took a millienium to reach North America (where they caused a flowering of Mississippian civilization). Furthermore, the Americas are plagued by a desert where in Mexico, the narrow ithmus of Panama, and the Andes mountains, which meant that cultural communication was virtually non-existent.

Technology, a large reason for the success of Europeans, is engendered by large populations with easy communication internally and externally. Large populations produce more potential inventors. China, which had relatively few barriers to unification, was (and is) the largest population base and so was long the technological innovator. However, China had little contact with outsiders and its unification also worked against it because a single Emperor or small ruling group could, and often did, outlaw a technology for political reasons. Europe was a set of chronically divided states with easy communication between them, so although a particular state could reject a technology, if it was useful, other states would adopt it and the state would be forced to adopt it (either as a defensive technique or as a result of defeat).

Diamond gives plenty of examples to show that these are indeed the causes of the course of civilization. An examination of the farming South Asians who colonized the Pacific (the Austroneasians) shows that whenever they migrated to an area that could not support their crops they reverted to hunting and gathering societies. The larger the island, the more developed the civilization (due to increased ability to generate food). An extreme case of isolation left a island that could support only a population of 4,000 hunter-gatherers and who, because of their extreme isolation, had lost virtually all of their technology, including items like fish-hooks and pottery. That domesticable species are domesticated whenver possible is illustrated by the idomestications of wheat and barley in the Fertile Crescent, millet and rice in China, and by the independent domestications of American plants. Japan, which embraced guns when brought by the Portugese until the samurai upper class realized that a weapon that could be used by any unskilled peasant would mean their end and banned their production, provides an example of isolation causing technological loss. A contrary example is that of the northern New Zealand Maori tribes, some of which acquired guns from the Europeans. Because of their proximity to each other, tribes that also acquired guns survived while those that did not were exterminated.

In 1492, on the eve of European colonial expansion, the world stood as follows. Europe had large population densities and societal structures that could muster large armies. It had the horse, a formidable military asset until the invention of the tank. It carried immunity (and the germs themselves) to many quite deadly diseases. It had a geo-political system that enforced adoption of technology. China was much similar except that it had abandoned several very useful technologies. Africa was short on large animals and disease resistance, although it did have steel much earlier than Eurasia. The Americas, which had eventually domesticated a somewhat daunting percursor to corn, was still limited to foot power (and infantry), had no resistance to Eurasian animal and epidemic diseases, with isolated civilizations, and none of whose populations were literate, which barrier to communication limited the pace of invention. Australia was limited to hunting and gathering (except in one tribe on the eastern coast which had eel farms) due to a complete lack of domesticable plants and animals.

It is small wonder then, that the Spanish cavalry with steel swords (and ineffective but loud muskets) massacred the Incan and Aztec infantry, or that the smallpox vectors on Columbus’ ship wiped out 90% of the North American native population, clearing the continent for settlement by Europe’s colonies. Or that the China was conquered by the technology it gave up. And it was practically inevitable that Europeans would displace the hunting and gathering populations like the Aboriginal Australians. Africa fared somewhat better, with central Africa’s malaria, native states, and lack of suitability for Eurasian crops retarding, but ultimately not preventing, European conquest.

Diamond gives a very thorough answer to his friend Yali’s question. He conducts a number of “natural experiments” between peoples of similar initial conditions to ultimately arrive at the conclusion that societies are driving by food production. Those peoples who had the fortune to be farmers early, whether through natural abundance of domesticable species or through cultural diffusion, got a head start on the development of technology, which ultimately decided what cultures won and lost. He provides the reader with much anthropological background, traces human migration by means of linguistic analysis, and frequently refers to the archaeological record. However, the logical layout is sometimes not as clear as could be desired and the final sections describing how the continents developed, while interesting, often repeated the information already rather thoroughly discussed in previous chapters. That notwithstanding, Guns, Germs, and Steel is a well written, comprehensive fusion of anthropology, archaeology, sociology, biology, liguistics, and history that definitively answers the question put forth.