Evolution is the theory that species descended from more ancient forms of life by structural and physiological modifications. If the theory is true, then the world we observe should contain evidence supporting the idea that species change. Let us examine the natural world to see if such evidence exists.

Evidence from Fossils

If today's species have come from more ancient forms, then we should be able to find remains of those species that no longer exist. Scientists have found such evidence in the form of fossils. Fossils--traces of once-living organisms-- are found most commo nly in layers of sedimentary rock. This type of rock begins to form when water and wind form layers of sand and silt. These and other decomposers are sealed off, preventing decomposition. After time the sedimentary layers become rock.

The most common fossils found in sedimentary rocks are from the hard parts of organisms, including shells, bones, teeth, and woody stems. Sometimes minerals replace the original remains, often molecule by molecule. Such replacement preserv es the microscopic structure of the organism.

Fossils can form in other ways. For instance, insect fossils have been found trapped in hardened resin. Such fossils show details as small as the insect's tiny leg hairs. In another instance, woolly mammoths that were frozen in Arctic ice have been dug up with their skin, bones, and muscles perfectly preserved.

Fossils are not always the body parts of an organism. An imprint is a type of fossil in which a film of carbon remains after the other elements of an organism have decayed. A mold is a type of fossil in which an impression of the shape or track of an organism has survived.

Dating Fossils

If scientists suggest that fossils are the remains of organisms that lived long ago, they should be able to prove it. Scientists have therefore developed ways to date, or determine the age of, fossils.

The relative age of fossils is determined from their position in sedimentary rock. In undisturbed sedimentary rock the bottom layers are the oldest and the top layers are the youngest. Fossils found in the lower layers are older. Those found in the u pper layers are younger. Thus a fossil's position in sedimentary rock beds gives its age relative to other fossils.

The absolute age of a fossil is determined by dating the fossil with radioactive isotopes. Radioactive isotopes have unstable nuclei that break down, or decay, and form other elements. These isotopes decay at a constant, known rate. The period of time it takes for one-half of the radioactive material to decay is called the half-life of the isotope. Remember that isotopes of an element differ in number of neutrons. Carbon-12, the most common and stable isotope of carbon, has 6 protons and 6 neutrons an d therefore has an atomic mass of 12. Carbon-14, an unstable, radioactive isotope of carbon has 6 protons and 8 neutrons and an atomic mass of 14. Carbon-14 decays to nitrogen-14. The half-life of carbon-14 is about 5,700 years.

The ratio of carbon-14 to carbon-12 in the atmosphere is assumed to be constant over time. Organisms take up the two isotopes of carbon in about the same ratio that the isotopes are found in the atmosphere. Thus, when an organism dies, the ratio of c arbon-14 to carbon-12 in its remains will be the same as that in the atmosphere. As time goes by, the amount of carbon-14 will decrease as it changes into nitrogen-14. However, carbon-12 does not decay. By comparing the ratio of carbon-14 to carbon-12 in a fossil with the ratio of these isotopes in the atmosphere, scientists can date fossils that are up to 50,000 years old.

Scientists use other radioactive isotopes, such as potassium-40, to date older fossils. Potassium-40 has a half-life of 1.28 billion years. Uranium-238 is another radioactive isotope with a long half-life that can be used to date older fos sils. Uranium-238 is used to determine the age of cores of silt taken from seabeds.

The Fossil Record

By dating fossils and examining geologic strata, scientists have been able to put together a time scale for the history of life on earth.

Fossil evidence indicates that over time organisms of increasing complexity appeared on the earth. Bacteria and blue-green bacteria are the first fossils that were preserved from the Precambrian era. During the beginning of the Paleozoic e ra, complex multicellular invertebrates dominated life in the oceans. By the end of the Paleozoic era, plants and animals has colonized the land surface of the earth.

The fossil record contains many examples that could be interpreted to mean that species evolved from more ancient organisms.

Geologic Time Scale