Prehistory, sometimes referred to as pre-literary history, is the period of human history between the first known use of stone tools by hominins c. 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of symbols, marks, and images appears very early among humans, but the earliest known writing systems appeared c. 5,200 years ago. The adoption of writing across the globe has been a slow process, so that the end of prehistory occurred at different times in different places, and the term is less often used in discussing societies where prehistory ended relatively recently. The period when a culture is written about by others but has not developed its own writing system is often known as the protohistory of the culture.