Scale begins 4.6 billion years ago when Earth was formed, runs to historical time. Radiometric dating has replaced those methods. Boundaries of eras and periods mark sharp transition in type or abundance of life-forms. In most cases, boundaries mark major extinction events. Whether events in evolutionary history are notable, or pedestrian, hinges on value judgment.
Humans occupy one ordinary branch among a multitude of evolutionary branches.
D. Three domains of life are Archaea, Bacteria, and Eukarya. Archaea and Bacteria are single-celled organisms, former more primitive, living in extreme environments such as hot springs. Eukarya have single-celled and multicelled forms. All have nuclei, include protists, fungi, plants, and animals. It s been suggested that early life, resembling Archaea, could have arisen in boiling temperatures of deep-sea vents.