<P> In geochronology, time is generally measured in mya (megayears or million years ago), each unit representing the period of approximately 1,000,000 years in the past . The history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet . Each eon saw the most significant changes in Earth's composition, climate and life . Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs . </P> <Table> <Tr> <Th> Eon </Th> <Th> Time (mya) </Th> <Th> Description </Th> </Tr> <Tr> <Td> Hadean </Td> <Td> 4,540--4,000 </Td> <Td> The Earth is formed out of debris around the solar protoplanetary disk . There is no life . Temperatures are extremely hot, with frequent volcanic activity and hellish environments . The atmosphere is nebular . Possible early oceans or bodies of liquid water . The moon is formed around this time, probably due to a protoplanet's collision into Earth . </Td> </Tr> <Tr> <Td> Archean </Td> <Td> 4,000--2,500 </Td> <Td> Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process known as abiogenesis . The continents of Ur, Vaalbara and Kenorland may have been formed around this time . The atmosphere is composed of volcanic and greenhouse gases . </Td> </Tr> <Tr> <Td> Proterozoic </Td> <Td> 2,500--541 </Td> <Td> Eukaryotes, a more complex form of life, emerge, including some forms of multicellular organisms . Bacteria begin producing oxygen, shaping the third and current of Earth's atmospheres . Plants, later animals and possibly earlier forms of fungi form around this time . The early and late phases of this eon may have undergone "Snowball Earth" periods, in which all of the planet suffered below - zero temperatures . The early continents of Columbia, Rodinia and Pannotia may have formed around this time, in that order . </Td> </Tr> <Tr> <Td> Phanerozoic </Td> <Td> 541--present </Td> <Td> Complex life, including vertebrates, begin to dominate the Earth's ocean in a process known as the Cambrian explosion . Pangaea forms and later dissolves into Laurasia and Gondwana . Gradually, life expands to land and all familiar forms of plants, animals and fungi begin appearing, including annelids, insects and reptiles . Several mass extinctions occur, among which birds, the descendants of dinosaurs, and more recently mammals emerge . Modern animals--including humans--evolve at the most recent phases of this eon . </Td> </Tr> </Table> <Tr> <Th> Eon </Th> <Th> Time (mya) </Th> <Th> Description </Th> </Tr> <Tr> <Td> Hadean </Td> <Td> 4,540--4,000 </Td> <Td> The Earth is formed out of debris around the solar protoplanetary disk . There is no life . Temperatures are extremely hot, with frequent volcanic activity and hellish environments . The atmosphere is nebular . Possible early oceans or bodies of liquid water . The moon is formed around this time, probably due to a protoplanet's collision into Earth . </Td> </Tr>

One of the first steps of the pcc in the years of formation was to