地球在四十六億年前便已形成，生命存在也有三十八億年，地質年代是指地球歷史中有岩層記錄的一段漫長時間， 地質學家主要是根據化石中生物的發展和岩石形成的順序(稱為相對年代)，以及根據岩層中放射性同位素之衰變產物的含量，測定出地層形成時間(稱為絕對年代)，建立了這個地層系統表和對比框架，這些時期使得岩石和化石能在全世界保持相互之間的關係。
年表中各個時期都有自己的名稱，用於描述生物在不同地質時空的發展程度，一般以首先研究它們時期岩石的地點來命名。
來自不同研究機構及單位所定出的地質時間通常會有所偏差，本地質年代表是現時古生物學普遍常用的，其地質時間應較為中肯。 　　The geologic time scale is the “calendar” for events in Earth history. The enumeration of those geologic time units is based on stratigraphy, which is the correlation and classification of rock strata. The fossil forms that occur in the rocks provide the chief means of establishing a geologic time scale. Because living things have undergone evolutionary changes over geologic time. By correlating the strata in which certain types of fossils are found, the geologic history of various regions and of Earth as a whole can be reconstructed. The relative geologic time scale developed from the fossil record has been numerically quantified by means of absolute dates obtained with radiometric dating methods.