A key to understanding history is studying the five themes of geography: location, place, human-environment interaction, movement, and region. With the help of modern science, archaeologists analyze early human remains and reconstruct how early people lived. Historians learn about the past by examining and evaluating written evidence and other records. The earliest period of human history is called the Old Stone Age. Paleolithic people developed skills and tools in order to survive. Their art and religion reflected the world in which they lived. The Neolithic agricultural revolution brought farming and permanent communities. Eight features of early civilization were cities, centralized governments, complex religions, job specialization, social classes, arts and architecture, public works, and writing. Many of the first civilizations emerged along river valleys. Interaction among the various civilizations often brought about cultural change.