Ecological model for history

The ecological model for history comprises ecological variables that Jared Diamond identified as important variables in shaping the course of history. Diamond published these variables in his book "Guns, germs and steel" in 1997².

Diamond argues that thirteen variables can explain the overall progression of history in the world (namely, a history that ended up favoring European and Asian societies until recently. The thirteen variables in the model are the following:

Geographical axes and ecological barriers
Plant biodiversity	Animal biodiversity
Crop cultivation	Animal husbandry
	Epidemics
Sedentarism
Societal complexity
Specialists
Writing	Technology
	Military technology

The continents' geographical axes played an important role in the chain of events that constitute human history because plants and animals adapt much easily to Earth's longitudinal conditions -e.g. similar day length, similar temperature, similar rainfall pattern, etc- than to latitudinal features -i.e. different day length, different temperatures, different rainfall pattern, etc. The longer longitudinal east-west axis of Eurasia predisposed this region to the spread of crops, animals and people because they ended up populating similar ecological niches. The similarity of these niches required little adaptation favoring not only the spread of species, but, later on, the spread of trade routes and the interchange of technologies and inventions that, overall, exploited similar needs and opportunities.
Ecological barriers also played a similar role than geographical axes in allowing (by their absence) or preventing (by their presence) the spread of food production and interchange of inventions. Although some ecological barriers are independent of geographical axes (e.g. the Himalayas), most ecological barriers impose latitudinal limits (e.g. desserts) than longitudinal ones. Therefore, ecological barriers further favored Eurasia over other societies.
Longitudinal geographical axes with few ecological barriers normally imply a greater range of territory. The larger the territory, the greater the chances of encountering weather variability and geographical barriers along these axes (although not as extreme as along latitudinal axes). This variability in weather and geography allows for greater plant and animal biodiversity to exploit the different ecological conditions. In the case of Eurasia, this biodiversity allowed for having a greater number of usable crops and animals in comparison with other regions.
Crop cultivation was thus favored by the geographical and ecological features of Eurasia, as well as by its biodiversity. Although food production started independently in different regions of the world, the greater biodiversity of Eurasia allowed it to exploit most modern crops from the beginning, while its longitudinal axis allowed for cultivation of those crops to spread quickly to other points of Eurasia.
Animal husbandry was also favored by the same conditions that favored agriculture.
A sedentary lifestyle occurred in time with the increase in food production from agriculture and animal husbandry. Sedentarism allowed for further increases in food production, as it is easier to accumulate surplus being sedentary than being nomadic.
The consequent increase in population, linked to sedentarism and food production, allowed for the appearance of epidemics (transmitted from animals), while easier trade routes allowed for transmission of those epidemics along different human populations. However, the increase in population also ensured that some people became immune to the epidemic and that part of the population would survive, in time both increasing the immunity of the population to similar epidemics, but also the survivability of viruses among people (because given enough people, the virus could survive in other parts of the population when one part of it became immune).
Sedentarism also allowed for increased levels of social complexity (from tribes to chiefdoms to states to empires).
Increased levels of social complexity allowed for the appearance of specialized people, such as traders, inventors, political elites, soldiers, etc. They are considered specialized in the sense that they don't have to produce their own food but gain it by creating something that the food producer needed (e.g. inventors) or by simply taxing it and redistributing the taxed food among the specialists (e.g. the political elite, soldiers and priests).
Among the most important inventions are writing, technology and military technology. Each further allowed for greater social complexity and wars of conquest.