Chapter 25 - Ingrid Shih BIO 1302-06 12 January 2011 The...

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Ingrid Shih BIO 1302-06 12 January 2011 The History of Life on Earth 25.1 How Do Scientists Date Ancient Events? The Earth is about 4.5 billion years old Has experience much change o To understand the change: Fossils are preserved remains of ancient organisms. They tell us about body form or morphology Earth’s history is recorded in rocks Layers- strata Relative ages of rocks can be determined by looking at strata o Oldest- bottom o Youngest- top o First observed in the 17 th century by Nicolaus Steno Radioisotopes are used to determine the actual age of rocks o Radioisotopes decay in a predictable pattern o Half-life- the time interval over which one half of the remaining radioisotope decays, changing into another elements o 14 C’s half-life: 5700 years o 40 K: 1.3 billion o 238 U: 4.5 billion To date an event, we must be able to estimate the concentration of the radioisotope at the start of the event. o EX: In an organism, the ratio of 14C to 12C stays constant during its lifetime. When an organism dies, it is no longer incorporating 14C from the environment. The 14C that was present in the body decays. With no replacement, the ratio of 14C to 12C decreases. This ratio can then be used to date fossils up to about 50000 years old. Paleomagnetic dating: o Movement and reversals of Earth’s magnetic poles are recorded in igneous and sedimentary rocks at the time they were formed, by alignment of mineral grains and other characteristics.
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