It is unclear why hunter-gatherers in different parts of the world independently developed agriculture at roughly the same time. A. One obstacle to the transition from a nomadic lifestyle to the sedentary lifestyle required by agriculture was that hunter-gatherers had not developed storage techniques. B. It seems unlikely that agriculture emerged in response to a food shortage brought on by a worldwide population crisis that developed once the whole world was occupied. C. The origins of agriculture maybe linked to climate change at the end of the last ice age, but this does not explain why earlier climatic instability had not led to agriculture. D. The only available means of understanding the social organization and technical abilities of ancient hunter-gatherer societies is the study of contemporary hunter-gatherers. E. One recent theory suggests that the invention of agriculture was made possible by the integration of various mental capacities in the human mind. F. Little is known about why only some societies that adopted agriculture rapidly progressed to using metal tools, becoming literate, and developing a state.