Also Try Earth Sciences Papers Archaean Period of Time (Museum of Paleontology) Covers 3.8 to 2.5 billion years ago. "If you were able to travel back to visit the Earth during the Archaean, you would likely not recognize it is the same planet we inhabit today. The atmosphere was very different from what we breathe today; at that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form." "It was early in the Archaean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils." 4-02 Hadean Period of Time (Museum of Paleontology) Covers 4.5 to 3.8 billion years ago. "Hadean time is not a geological period as such. No rocks on the Earth are this old - except for meteorites." 4-02 Precambrian Period of Time (Museum of Paleontology) Covers 4.5 to 544 million years ago. 4-02 Proterozoic Period of Time (Museum of Paleontology) Covers 2.5 billion to 544 million years ago. The first life formed on land during this period, including bacteria to animals. 4-02 Snowball Earth (Harvard University - Hoffman and Schrag) "Many lines of evidence support a theory that the entire Earth was ice-covered for long periods 600-700 million years ago. Each glacial period lasted for millions of years and ended violently under extreme greenhouse conditions. These climate shocks triggered the evolution of multicellular animal life, and challenge long-held assumptions regarding the limits of global change." 8-02

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