and observations. Instead, the natural world to him was a gigantic, very challenging puzzle that demanded an explanation for its otherwise bewildering complexity. Why are different organisms so similar? Why has there been a succession of different kinds of species throughout geologic time?

Certain observations seemed particularly important. For example:

  1. In South America, the only continent where living armadillos were found, Darwin discovered fossil evidence for the prior existence of ancient species that had many of the unique features of living armadillos, yet were clearly different. Such fossils were found nowhere else in the world. Why were both living and ancient armadillo-like species confined to the same geographical region?

  2. On the Galapagos Islands, 600 miles off the coast of Ecuador, Darwin observed many distinct living species of birds and reptiles that closely resembled each other—yet were different on each tiny island. Why, for example, should the beak size of the

Dating the Earth

One of the greatest scientific triumphs of the last two centuries has been the discovery of the vast expanse of geologic time. Early methods of calculating the age of the earth relied on measures of the rate of sedimentation or the cooling of the earth from an initially molten state. The relative ages of rocks also were calculated early in the 1800s by noting what kinds of fossils the rocks contained. But the absolute age of the earth and the timing of many events in geologic history required the discovery late in the 19th century of a previously unknown phenomenon: radioactivity.

Some elements, such as uranium, undergo radioactive decay to produce other elements. By measuring the quantities of radioactive elements and the elements into which they decay in rocks, geologists can determine how much time has elapsed since the rock cooled from an initially molten state. For example, the oldest known rocks are found in Greenland and date from about 3.8 billion years ago. Scientists believe the earth's age to be about 4.6 billion years because meteorites and rocks of the moon—both of which formed about the same time as the earth—date from this time. Radiometric dating also shows that the period of earth's history during which large fossils can be readily found in rocks began only about 570 million years ago.

Radiometric dating draws on information and insights from many areas of science. For example, it requires that the rate of radioactive decay is constant over time and is not influenced by such factors as temperature or pressure—conclusions supported by extensive research in physics. It also assumes that the rocks being analyzed have not been altered over time by the migration of atoms into or out of the rocks, which requires detailed information from both the geologic and chemical sciences.

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