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The earth's history dates back more than 4.5 billion years (Ga) which is divided roughly into 2 eons called the " cryptozoic" [Gr. kryptos, hidden + zoo, life] and the " Phanerozoic" [Gr. phanerous, visible + zoo, life]. [Paleoclimatologists usually use geological time divisions, which would be the Priscoan, Archean, Proterozoic, and Phanerozoic Eons.] The division of these two eons is the boundary of the Precambrian and the Cambrian which occurred about 570Ma (570Ma means 570 million years before present, ie 570,000,000 BP). Originally it was thought that life began with the Cambrian. Evidence suggests that during this time the earth experienced perhaps the biggest explosion of diverse life forms that ever have been on the planet, and that since that time the total number of phyla has been gradually reduced with extinction after extinction (ref. Stephen Jay Gould: Wonderful life - the Story of the Cambrian and the Burgess Shale). Some influential researchers, including Briton Simon Conway Morris , an expert in the Burgess Shales, dispute the assertion that there were significantly more phyla at this time, suggesting that a number of creatures assigned to new phyla (particularly Hallucigenia) were actually arthropods or onychophores. Later work established that life evolved rather early in the history of the planet, perhaps as long as 4 billion years ago. Recent research suggests that life appeared on Earth just as soon as the planet cooled sufficiently for it to do so, and indeed may have arisen and been destroyed multiple times by planetismal impacts before the planet stabilized. This possibility has prompted some scientists to suggest that life is common in the universe, since it appeared so quickly here.
Paleoclimatologists employ a wide variety of skills to arrive at their theories and conclusions.
Glaciers are a widely employed instrument in Paleoclimatology. The ice in glaciers has hardened into an identifiable pattern, with each year leaving a distinct layer. Inside of these layers scientists have found pollen, allowing them to estimate the total amount of plant growth of that year by the pollen count. The thickness of the layer can help to determine the amount of rainfall that year. It is estimated that the polar ice caps have 100,000 of these layers or more. Petrified tree rings give Paleoclimatology data over a much larger stretch of time. The fossilFor other uses of the term, see Fossil (disambiguation Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. The totality of fossils and their placement in rock formations and sedimentary layers is known as the fo itself is dated with radioactive dating within a wide margin of error. The rings themselves can give some information about rainfall and temperature during that epochThe word epoch can mean either an interval of time, or a particular point in time used as a reference point. In common usage, the term is often used to apply to a period of time when significant related events took place. see also period, era) In geology,. Sedimentary rockshale overlaid by limestone. Cumberland Plateau, Tennessee. Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in three main ways—by the deposition of the weathered remains of other rocks (known layers provide a more compressed view of climate, as each layer of sediment is made over a period of hundreds of thousands to millions of years. Scientists can get a grasp of long term climate by studying sedimentary rock. Some scientists believe each layer designates a major change in climate.