14. The Formation of Species and Evolutionary Change


14.6. The Tentative Nature of the Evolutionary History of Organisms


Tracing the evolutionary history of an organism back to its origins is a very difficult task, because most of its ancestors no longer exist. Scientists act as “time detectives” when they study fossils of extinct organisms but must keep in mind that the fossil record is incomplete and provides only limited information about the biology of the organism represented in that record. However, new fossils are always being discovered.

There are three reasons that the fossil record is incomplete. First, the likelihood that an organism will become a fossil is low. Most organisms die and decompose, leaving no trace of their existence. (Today, road-killed opossums are not likely to become fossils because they will be eaten by scavengers, repeatedly run over, or decompose by the roadside.) Second, in order to form a fossil, the dead organism must be covered by sediments, dehydrated, or preserved in some other way.

Several factors increase the likelihood that an organism will be found in the fossil record:

• The presence of hard body parts resist decomposition.

• Marine organisms can be covered by sediment on the bottom.

• Fossils of more recent organisms are less likely to have been destroyed by geological forces.

• Fossils of large organisms are easier to find.

• Organisms that were extremely common are more likely to show up in the fossil record.

For example, trilobites are very common in the fossil record. They were relatively large marine organisms, with hard body parts, that were extremely abundant. However, fossils of soft-bodied, extremely ancient, wormlike organisms are rare.

Third, the discovery of fossils is often accidental. It is impossible to search through all the layers of sedimentary rock on the entire surface of the Earth. Therefore, scientists will continue to find new fossils that will extend the known information about ancient life into the foreseeable future. But there can be no question that evolution occurred in the past and continues to occur today (How Science Works 14.1).

Scientists may know a lot about the structure of the bones and teeth or the stems and leaves of an extinct ancestor from fossils but know almost nothing about its behavior, physiology, and natural history. Biologists must use a great deal of indirect evidence to piece together the series of evolutionary steps that led to a current species.



14. Why is it difficult to determine the evolutionary history of a species?