Final answer:
Geologic eras are defined by using relative dating to sequence events and absolute dating, notably radiometric dating, to assign numerical ages. These methods construct the Geologic Time Scale, which is continuously updated by the International Commission on Stratigraphy.
Step-by-step explanation:
Scientists have determined the beginning and end of geologic eras using a combination of relative dating and absolute dating methods. Relative dating allows scientists to place geological events in sequence without requiring a precise numerical age. By examining rock layers and the fossils within them, scientists can identify the progression of life and environmental conditions over time.
Absolute dating, specifically radiometric dating, complements this by measuring the decay of radioactive isotopes in rocks and minerals to establish numerical ages. This method has been crucial in dividing Earth's history into eons, eras, periods, epochs, and ages as seen on the Geologic Time Scale. Markers such as 'golden spikes' are used at age boundaries to define specific points in the geological record.
The International Commission on Stratigraphy continually revises the Geologic Time Scale to include new findings that can refine our understanding of Earth's history, indicating the scale's dynamism and the ongoing nature of geological research.