Thousands of years ago, humans domesticated every possible large wild mammal species fulfilling all those criteria and worth domesticating, with the result that there have been no valuable additions of domestic animals in recent times, despite the efforts of modern science.

Diamond is arguing that the roster of domesticated large mammals is not an accident of modern ingenuity but the outcome of deep prehistory and biological constraint. In his framework, only a small subset of big mammals meet practical criteria for domestication (e.g., suitable diet, growth rate, temperament, breeding behavior, and social structure). Those few species were domesticated early by ancient peoples, leaving modern societies with little “low-hanging fruit” to add—hence the lack of major new domesticates even with scientific breeding. The point supports his broader thesis that geographic and ecological luck, more than inherent cultural superiority, shaped long-term differences in human development.