References Cited

For at least 300 years, scientists have been gathering the evidence for evolutionary change. Much of this vast database is observational, and the evidence came to light with the study of fossils (paleontology) and the rock record (geology). This essay focuses on the evidence about evolution from the fossil record.

Documentation of ancestor-descendant relationships among organisms also comes from the fields of biogeography, taxonomy, anatomy, embryology and, most recently, genetics — particularly DNA analysis. Information from these fields can be found in the materials listed in the “Suggested Readings.”

The fossil record remains first and foremost among the databases that document changes in past life on Earth. Fossils provide the dimension of time to the study of life. Some of the most basic observations about fossils and the rock record were made long before Darwin formulated his theory of “descent with modification.” The fossil record clearly shows changes in life through almost any sequence of sedimentary rock layers. Successive rock layers contain different groups or assemblages of fossil species.

Sedimentary rocks are, by far, the most common rocks at Earth’s surface. They are formed mostly from particles of older rocks that have been broken apart by water, ice, and wind. The particles of gravel, sand, and mud, which are collectively called sediment, settle in layers at the bottoms of rivers, lakes, and oceans. Shells and other limy materials may accumulate in the oceans. As the sediments accumulate they bury shells, bones, leaves, pollen, and other bits and pieces of living things. With the passing of time, the layers of sediments are compacted by the weight of overlying sediments and cemented together to become the sedimentary rocks called limestone, shale, sandstone, and conglomerate. The buried plant and animal remains become fossils within the sedimentary layers.