The theory outlined
Before human beings developed agriculture, they lived as hunter-gatherers, as some still do. Diamond argues that Eurasian civilization is not so much a product of ingenuity, but of opportunity and necessity. That is, civilization is not created out of sheer will or intelligence, but is more like a house of cards, each level dependent upon the levels below it. Specifically, the key to civilization is agriculture. The keys to agriculture are domesticable plant and animal species for food and work. The demands for domesticability of an animal species are particularly stringent. Diamond identifies six criteria including the animal being sufficiently docile, gregarious, willing to breed in captivity and having a social dominance hierarchy. Though not stated explicitly, his theories are similar to that of cultural ecology.