Answer:
Increased oxygen concentration
Explanation:
The evolution of O2-producing cyanobacteria that use water as terminal reductant transformed Earth's atmosphere to one suitable for the evolution of aerobic metabolism and complex life. The innovation of water oxidation freed photosynthesis to invade new environments and visibly changed the face of the Earth.
For Absolute dating they assign a numerical date to each rock layer studied
For relative dating they go Through geological dating and stratigraphy processes. Also, they use Uranium Dating.
Fish fossils occur in sediments deposited underwater. That could be sea bottom, lakes, or rivers. Generally, seabottom sediments are raised to the surface by tectonic uplift. Also, when sea level falls during ice ages, sea bottom sediments are exposed to the atmosphere.