Together, fossils can be used to construct a fossil record that offers a timeline of fossils reaching back through history. To puzzle together the fossil record, scientists have to be able to date the fossils to a certain time period. The strata of rock in which fossils are found give clues about their relative ages. If two fossils are found in the same geographic location, but one is found in a layer of sediment that is beneath the other layer, it is likely that the fossil in the lower layer is from an earlier era. After all, the first layer of sediment had to already be on the ground in order for the second layer to begin to build up on top of it. In addition to sediment layers, new techniques such as radioactive decay or carbon dating can also help determine a fossil’s age.
There are, however, limitations to the fossils can supply. First of all, fossilization is an improbable event. Most often, remains and other traces of organisms are crushed or consumed before they can be fossilized. Additionally, fossils can only form in areas with sedimentary rock, such as ocean floors. Organisms that live in these environments are therefore more likely to become fossils. Finally, erosion of exposed surfaces or geological movements such as earthquakes can destroy already formed fossils. All of these conditions lead to large and numerous gaps in the fossil record.
Comparative Anatomy
Scientists often try to determine the relatedness of two organisms by comparing external and internal structures. The study of comparative anatomy is an extension of the logical reasoning that organisms with similar structures must have acquired these traits from a common ancestor. For example, the flipper of a whale and a human arm seem to be quite different when looked at on the outside. But the bone structure of each is surprisingly similar, suggesting that whales and humans have a common ancestor way back in prehistory. Anatomical features in different species that point to a common ancestor are called homologous structures.
However, comparative anatomists cannot just assume that every similar structure points to a common evolutionary origin. A hasty and reckless comparative anatomist might assume that bats and insects share a common ancestor, since both have wings. But a closer look at the structure of the wings shows that there is very little in common between them besides their function. In fact, the bat wing is much closer in structure to the arm of a man and the fin of a whale than it is to the wings of an insect. In other words, bats and insects evolved their ability to fly along two very separate evolutionary paths. These sorts of structures, which have superficial similarities because of similarity of function but do not result from a common ancestor, are called analogous structures.
In addition to homologous and analogous structures, vestigial structures, which serve no apparent modern function, can help determine how an organism may have evolved over time. In humans the appendix is useless, but in cows and other mammalian herbivores a similar structure is used to digest cellulose. The existence of the appendix suggests that humans share a common evolutionary ancestry with other mammalian herbivores. The fact that the appendix now serves no purpose in humans demonstrates that humans and mammalian herbivores long ago diverged in their evolutionary paths.
