Progressing from the oldest to the current, the four major eras of Earth's geological history are Precambrian, Paleozoic, Mesozoic and Cenozoic. The lengths of these eras are often measured by the term "mya," which represents "millions of years ago." The four major eras of the geological time scale, or GTS, are also subdivided into smaller units, such as the Earth's current time scale placement within the Holocene Epoch of the Quaternary Period of the Cenozoic Era.
The current GTS era, the Cenozoic Era, began 65.5 million years ago. The current period within that era is the Quaternary Period, which began 2.588 million yeas ago. The Holocene Epoch, the most recent subdivision of geological time scale, began 11,700 years ago. The Cenozoic Era represents the time during, which the first recognizable humans came into existence. During the Cenozoic Era's comparatively short time span, relatively little change has occurred with regard to shifting plate tectonics affecting the distribution of the continents across the Earth's surface.
The oldest GTS era, the Precambrian Era, began with the formation of the Earth 4,600 mya, or 4.6 billion years ago. During this time, the Earth's crust began to solidify from its original molten form. The earliest-known fossils are from the Archean Eon of this era, which began 4,000 mya, or 4 billion years ago. Overall, the Precambrian Era accounts for 88 percent of Earth's history.