The historical field of world history, which looks at common themes occurring across multiple cultures and regions, has enjoyed extensive development since the 1980s. However, World History research has tended to focus on early modern globalization (beginning around 1500) and subsequent developments, and views post-classical history as mainly pertaining to Afro-Eurasia. Historians recognize the difficulties of creating a periodization and identifying common themes that include not only this region but also, for example, the Americas, since they had little contact with Afro-Eurasia before the Columbian Exchange. Thus recent research has emphasised that "a global history of the period between 500 and 1500 is still wanting" and that "historians have only just begun to embark on a global history of the Middle Ages".