The realization that species can be arranged in a hierarchy of groups of seemingly similar forms raised an obvious question: What accounts for the relatedness of different groups of organisms? The mechanism that was proposed by Darwin directly addressed this question. It suggested that all animals classified as belonging to the same group had a common ancestor species. That is, dogs, wolves, coyotes and all members of the genus Canis are descended from a common ancestor species that lived in the remote past. In a similar manner all species in a family, an order, a class, or a phylum share a common inheritance.
How could one possibly test such a hypothesis? In the decades before Darwin proposed his hypothesis, geologists realized that the sedimentary rocks of the earth's crust contain a running diary of earth's history. This record of past events comes about because the earth's crust is in a constant state of change. This observation might not be obvious in the lifetime of an individual, but it is dramatic over thousands of years. Relatively flat surfaces are uplifted to form mountains, and then the mountains slowly erode to form flatlands. Storms produce powerful waves that erode cliffs at the seashore. These phenomena have the common feature of moving solid materials, and the subsequent settling out of these materials makes possible the formation of a special form of rock that contains a record of the earth's past.
Consider the case of a river with a source in the mountains. As the water moves downstream, it erodes the slopes of the mountains. Tiny grains produced by the erosion, called silt, are relatively easy to move. When the river reaches the flatlands, a lake, or the ocean, the solid material being carried by the water is deposited—often reaching great thicknesses over long periods of time. Then the pressure of the sediments on top can cause the sediments beneath to harden into "sedimentary rocks."
The river may carry things other than
silt, sand, and rocks. Hard structures of organisms such as the bones and teeth of animals may be carried along as well. These, too, will be deposited with the silt, sand, and rocks. Under certain circumstances, these remains of organisms undergo a chemical change in which the original material is replaced by molecules that form stone. In this way, the organic remains of living things are fossilized (changed into stone), creating the evidence of ancient life studied by scientists.
Because of the order in which the sediments are deposited, the most recent layer of rocks normally will be on top and the oldest layer will be on the bottom (though sometimes sediments are flipped upside down by the geologic folding of rock layers). Also, the fossils in each layer usually will be of those organisms that lived at the time the layer was formed. Thus, the fossils in the lower layers will represent species that lived earlier than those found in the upper layers.
The relative position of fossils tells only which are older and which younger. One