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Early Earth is a term usually defined as Earth's first billion years, or gigayear. On the geologic time scale, this comprises all of the Hadean eon (itself unofficially defined), as well as the Eoarchean and part of the Paleoarchean eras of the Archean eon. This period of Earth's history, being its earliest, involved the planet's formation from the Solar nebula via the process known as accretion. This period also included the formation of the earliest atmosphere and hydrosphere. The earliest supracrustals (such as the Isua greenstone belt) date from the latter half of this period, about 3.8 gya, around the same time as peak late heavy bombardment. According to evidence from radiometric dating and other sources, Earth was formed about 4.54 billion years ago.〔 〕 Within its first billion years,〔See: * * * * 〕 life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms. Since then, the combination of Earth's distance from the Sun, its physical properties and its geological history have allowed life to emerge, develop photosynthesis, and, later, evolve further and thrive. The earliest life on Earth arose at least 3.5 billion years ago.〔Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). ''Evidence of Archean life: Stromatolites and microfossils.'' Precambrian Research 158:141–155.〕〔Schopf, JW (2006). ''Fossil evidence of Archaean life.'' Philos Trans R Soc Lond B Biol Sci 29;361(1470) 869-85.〕 Earlier physical evidences of life include graphite, a biogenic substance, in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland, as well as, "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.〔 Early edition, published online before print.〕 According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."〔 == References == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Early Earth」の詳細全文を読む スポンサード リンク
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