can estimate the difference in the ages of the two fossils by noting the thickness of the rock that separates them. If the difference is only one foot, one might guess the interval of time is less than if two fossils are separated by 50 feet of rock layers. Today, however, far more accurate methods of dating fossils are available, as described on the next page. Because these methods are based on the known rates of radioactive decay, they provide valuable measures of absolute time.
The scientific study of fossils is called paleontology, and the methods used for their identification and classification are similar to those used for living species. But in some respects the task of the paleontologist is far more difficult. Many species lack hard parts such as bones and shells, and such organisms almost always decay without becoming fossilized. This is the case for many groups of soft-bodied invertebrates—such as worms of many kinds, jellyfish, and protozoans. Even for such species as mammals, birds, reptiles, and amphibians, death is usually followed by the skeleton being dismembered and the bones scattered. For this reason, whereas isolated bones are often fossilized, it is exceptionally rare for an intact skeleton to be found.
Tiny fossils first reveal the existence of bacteria 3.5 to 3.8 billion years ago, and animals composed of more than a single cell are known from about 670 million years ago. But the organisms that lived between these two dates lacked hard parts and, hence, were rarely preserved as fossils. Then, about 570 million years ago, a dramatic change took place. At the beginning of the Cambrian period, animals evolved that had calcified shells and other types of body coverings that had a far better chance of becoming fossilized. These fossils demonstrate that Cambrian seas were populated with a variety of invertebrates. The earliest vertebrate fossils date from about 500 million years ago. Thereafter early amphibians and reptiles appeared. Birds and mammals appear in the fossil record only about 200 million years ago, while dinosaurs first appear about 225 million years ago and disappear suddenly about 160 million years later.
In the 1830s, when Darwin began his studies, the essential features of the fossil record were known (although absolute dates had not yet been determined). Many thousands of living species had been described, and it was clearly recognized that they could be organized into various groups—suggesting that they are somehow relatives. In addition, analysis of the fossil record revealed that the organisms on the ancient earth had undergone major changes over time—with whole groups of animals appearing, persisting for long periods of time, and then disappearing.
Darwin was an unusually keen observer. But he was not content to catalogue facts