When people think about history, they think about the past. This information is studied, recorded and organized by historians. Since the early recordings of history, systems to classify historical events have been created, one of which divides them into four periods. The first period of history includes all historical events from the moment life appeared on Earth until the beginning of writing and it is known as Prehistory. Since written records of this era do not exist, everything it is known about Prehistory has been discovered by archaeologists and geologists, who use excavation and geological analysis to do their research. This period is recognized by the human evolutionary process, by which the first living primates on earth evolved into the human species (homo sapiens). Ancient History, the second period, starts with the first appearance of writing and ends with the fall of the Roman Empire. The first known forms of writing date back to approximately 6,000 years BCE (before the Common Era), which is around the time historians consider this period to start; thus, the end of Ancient History is considered to be in the 5th century, when the last emperor of Rome, Romulus Augustus, was removed from power in 476. The rise of many civilizations around the world, the emerging of alphabetical writing and the rise of Greek city-states are some of the significant events that took place during this period. Chronologically, the next period is Medieval History, which lasted through the 5th to the 15th century. The third period in history began with the previously mentioned collapse of the Roman Empire in 476. Some major events that came to pass during Medieval History include the Crusades, religious conflicts from the 11th to the 13th century, the Black Plague, an epidemic that killed approximately a third of Europe's population in the 14th century, and the Renaissance, the cultural movement that marked the transition from medieval to modern tim