The entire book looks to answer one question that was posed to the author by Yali, a native New Guinean: "Why is it that you white people developed so much cargo (their term for the material goods brought by Europeans such as cloths, medicine, and matches) and brought it to New Guinea but we black people had little cargo of our own?" It seems a simple question at first, and you can easily give answers like, "because Europeans developed centralized governments and ocean-going ships while New Guineans did not." However, this book goes much further and explains the ultimate causation of why all societies developed differently.
Most people that are asked this question would automatically assume the racist answer that some people developed differently simply because they're inferior to those who formed complex societies. Jared Diamond on the other hand, completely convinced me that this is untrue. He instead puts the blame solely on the environments in which societies arose. More specifically, he blames four main differences: the number of suitable plants and animals for domestication, how easily societies and their innovations can diffuse through a continent, how easily they can diffuse between continents, and the differences in total area and population size within different continents.
All complex societies need a surplus of food in order to support their specialized workers, and the best way to achieve this is to domesticate wild plants and animals, but as it turns out, very few species of wild plants and animals are suitable for domestication. It also turns out that a disproportionate number of these wild ancestors originated in Eurasia (Europe and Asia); 33 of 56 large-seeded grass species (things like wheat and barley, the main carbohydrate sources of ancient civilizations) as well as 13 of the 14 mammalian candidates for domestication (both aboriginal Americans and Australians effectively wiped out all large mammals on their respective continents shortly after arriving) originated in Eurasia. Also, many plants suitable for domestication were much harder to domesticate than those of Eurasia. For instance, the wild ancestor of modern corn was about an inch long and was covered with a rock-hard skin, and it took thousands of years for it to become a desirable crop. This gave Eurasians, especially the Fertile Crescent and China, the biggest head start towards becoming complex societies.
There is some debate about domestication, but it is sure that no more than nine locations (Fertile Crescent, China, Mesoamerica, Andes/Amazonia, West Africa and Sahel, India, Ethiopia, Eastern United States, and New Guinea) were home to independent domestication. Everywhere else either copied or was inspired by the idea from elsewhere. Unfortunately the idea was not spread equally amongst the continents, and therefore, ecological barriers played a major role in the development of complex societies. Eurasia is the only continent (besides Antarctica) whose major axis goes from East to West rather than North to South. This may seem trivial, but an East-West axis means that the continent stays at the same latitude and therefore has similar climates throughout. This was crucial for the spread of crops which are dependent on certain climates to grow. For instance, North Africa was abundant with European crops suitable for the moderate climate and winter rains, and it is often clustered within the term of Eurasia, but those crops never crossed further south into Africa until modern times even though southern Africa was also suitable for European crops, because they couldn't grow in the intermediate span of a differing climate and summer rains. Other more straightforward obstacles like mountains and arid deserts also blocked the spread of societies. This was most prevalent in the Americas where three major societies (the Incas of the Andes, and Aztecs of Mesoamerica, and the Mississipians) developed but never came into contact with each other. This was due to the Andes mountains themselves, the extreme bottleneck of Panama, and the deserts surrounding Mesoamerica.
Equally important as the barriers within continents are the barriers between continents. Obviously, some continents were harder to reach than others simply because of wide expanses of oceans and the lack of ocean-going ships. As a result, the Americas were colonized much later than Eurasia, and Australia was very secluded with very little contact from other continents after the initial colonization until Europeans came in modern times and nearly wiped out the aboriginal population (only those living in places unsuitable for European crops survived). Partly because of this long-time seclusion, aboriginal Australians remain the most "backwards" people in our eyes, and they still use stone tools even today.
Lastly, area and population size of the different continents has unequivocally played a role in our history. More area and higher populations leads to more innovators available to produce technology as well as more societies in which to compete with each other. This constant pressure to either gain the latest technology or be swallowed by those societies more technologically advanced has kept Eurasia at the forefront of technology development for most of human history. Alternatively in the America's where the societies weren't in high competition with each other, Inca, a major, complex society and one of the nine societies to develop a agriculture, never even produced it's own writing system.
All of these ultimate factors in human history have led to the proximate factors of guns, germs, and steel which were the tools of conquest Europeans used to spread all around the world, and they're why the world is the way it is today.
Despite that this may seem like a history text book, Jared Diamond presents everything in a way that is enjoyable, and the vast amount of interesting information kept me reading even when I could've been out enjoying the summer (or more likely shut inside playing video games). I highly recommend this book for any nerds like me interested in gaining a little knowledge.