Final answer:
Around 900 AD, the form of government that emerged is known as feudalism, which was a system that organized society around land ownership and military service. It developed after the Roman Empire's collapse and involved a hierarchy of lords and vassals. Other government forms like monarchies and oligarchies have evolved differently over time.
Step-by-step explanation:
Around 900 AD, a new form of government called feudalism was formed. This system emerged after the collapse of the Roman Empire and created a strict hierarchical structure based on land ownership and protection. Lords, who were part of the nobility, granted vassals control over land; in return, these vassals pledged military service and other forms of support to their lords.
Other forms of government, such as a monarchy, an oligarchy, and a representative democracy, have also played significant roles throughout history. For example, many constitutional monarchies started out as absolute monarchies before evolving. An oligarchy is where a small elite group holds power. After the American Revolution, the revolutionaries established a form of government based on republicanism to avoid the corruption that could arise from concentrating power in one person or group.