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Evidence of Evolution

This table lists the unstable “parent” elements and the more stable “daughter” elements that they become. Half-lives refer to the length of time it takes for half the parent element in a rock or other object to decay. Because different elements have different half-lives, their useful ranges vary.
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Leonardo Da Vinci’s Findings and the Ages of Fossils

Leonardo Da Vinci, 15th century Italian artist, scientist and inventor, was one of the first to document the types of fossils he found and noticed that different fossils were found in different rocks. He was one of the first people to recognize that fossils were the remains of living things and that the Earth must be much older than most of his contemporaries believed it to be. In the following centuries, scientists built on his ideas. 19th century scientists studied sedimentary rocks and fossils from all over the world developing an understanding of the relative ages of rock layers and fossils. They could tell that some layers were older than others, but they didn’t know how old rocks and fossils within them were. More recently, in the 20th century, scientists have developed technology to allow the numerical ages of rocks and fossils to be determined. See below for further explanation of relative and numerical ages.

  • Relative age dating of sedimentary rocks layers and the fossils they contain means establishing which rock layers are older than others. This method of dating does not give ages in years. Instead it establishes which rocks and fossils are older and which are younger. Relative age dating relies on several methods. One of the most important involves the principle that younger rock layers are formed on top of older rock layers.
  • Numerical age dating means establishing the approximate age in years of a particular layer of rock by analyzing the decay of radioactive elements. Radioactive decay occurs when an unstable form of a chemical element naturally converts to a stable form of another chemical element. Volcanic ash layers, for example, contain radioactive elements. When the ash layer forms, the radioactive elements begins to decay at a constant rate. Because we know the rates of decay of different radioactive elements, we can understand the age of the ash layer by examining the amount of unstable radioactive element and stable product element within the layer.
    Last modified May 16, 2005 by Lisa Gardiner.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA