The human history can be traced through literature records and archaeological studies on cultural relics. How do geologists trace the geological history? The geologists have found that the there are a number of layered rocks, called strata in geology, covering the Earth’s surface. This is really a complete book recording the evolutionary history of the Earth for several hundred million years. The various rocks of the strata are similar to literature records of the book and fossils just like cultural relics. It is such literature records and cultural relics that can be used by geologists to understand the evolutionary history of the Earth through scientific and technological methods and the idea that Today is the Key to Understand Yesterday.Present evidence indicates that the Earth wasformedabout 4.6 billion years ago. However, the geologists can only understand its evolutionary history of the last 600 million years. The history of the rest stages is still based on various guesses. In spite of appearing 3 billion years ago, organisms were in a very low form for a long time and the fossils of the organisms may have almostdestroyed due to metamorphism on the fossil-bearing rocks. It was until 5.7 million years ago（mya）that a number of organisms in higher forms appeared and were preserved as fossils in non-metamorphic rocks. The fossils have provided us many reliable materials for understanding the Earth’s history,Based on the evolutional stages of the fossils preserved in sedimentary rocks, geologists can judge depositional sequences of the fossil-bearing rocks. The ages of the rocks can be further determined by isotope technique. Combining the fossil method with isotope technique, the evolutionary history of the Earth’s crust can be understood more accurately. In 1881, International Geological Society approved the Stratigraphical Division Schema. Later on, the schema has been modified, consummated and become a complete geochronologic chart used today as a basis for dividing the Earth’s history.