According to the latest scientific estimates, the Earth was formed about 4.5 billion years ago and the first living things appeared about 3.9 billion years ago. Although we can talk about the history of the earth as having taken a long, long time, it is still very difficult to grasp. Imagine the complete panorama of the earth's history - about 4.5 billion years - set equal to a 24-hour day. If geological time were displayed on a clock that began ticking the instant our planet was born, the first gene probably emerged in the predawn hours, before 5:00 am. Later that morning the first sun-fed photosynthetic cells appeared, followed that afternoon by cells that carry their genes inside a membrane-bound nucleus, and later that evening by the first of many multicellular organisms. The first modern human beings, members of Homo sapiens, would not arrive on the evolutionary scene until about the last 30 seconds of this long day, and all of recorded history took place during the last tenth of the last second before the stroke of midnight.