Paragraph 1：Since the dawn of civilization, people have been curious about the age of Earth. In addition, we have not been satisfied in being able to state merely the relative geologic age of a rock or fossil. Human curiosity demands that we know actual age in years.
Paragraph 2：Geologists working during the nineteenth century understood that if they were to discover the actual age of Earth or of particular rock bodies, they would have to concentrate on natural processes that continue at a constant rate and that also leave some sort of tangible record in the rocks. Evolution is one such process, and geologist Charles Lyell (1797¨C1875) recognized this. By comparing the amount of evolution exhibited by marine mollusks during the Tertiary Period with the amount that had occurred since then, Lyell estimated that 80 million years had elapsed since the beginning of the Tertiary Period. He came astonishingly close to the mark, since it was actually about 65 million years. However, for older sequences of evolutionary development, estimates based on rates of evolution were difficult, and not only because of missing parts in the fossil record. Rates of evolution for many orders of plants and animals were not well understood.
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