method works better with some species than others. For example, it is difficult to track the
evolution of bacteria from fossils, because their single cells do not last well as fossils.
Until recently, fossils were the main source of evidence of evolution (Figures7.16and7.17).
The location of each fossil in layers of rocks provides clues to the age of the species and how
species evolved in the past. Older materials and fossils are deeper in the earth; newer fossils
and materials are closer to the surface.
Figure 7.16: A fossil is the remains of a plant or animal that existed some time in the distant
past. Fossils, such as this one, were found in rocks or soil that was laid down long ago. ( 17 )
Fossils and the rocks they are embedded in provide evidence of how life and environmental
conditions have changed throughout Earth’s history. They also help us understand how
the past and present distribution of life on Earth is affected by earthquakes, volcanoes, and
shifting seas, and other movements of the continents.
The Age of Rock Layers and Fossils
The many layers of sedimentary rock provide evidence of the long history of Eearth and the
order of life forms whose remains are found in the rocks. The youngest layers are not always
found on top, because of folding, breaking, and uplifting of layers. If the layers of earth
were tilted by earthquakes or volcanoes, geologists can figure out which layers came from
the deepest parts of the Earth.
The fossils and the order in which fossils appear is called the fossil record. This record
provides important records of how species have evolved, divided and gone extinct. Methods
used to date the age of rocks and fossils make it possible to determine when these events