A fossil can be defined as any trace or artifact of life. Fossils can give clues to the past and history of life on Earth. While the fossil record is incomplete, it still helps lend evidence to the Theory of Evolution.
How Do Fossils Form?
Fossils are relatively rare because of all of the elements that have to fall into place before an organism can become a fossil. Fossilization usually can only happen to hard parts of the organism's body, so that is another obstacle in the way of making fossils. It is possible that some traces of organisms, like footprints, can become imprint fossils if they are filled with sediments, but these are rare.
Most fossils are made when an organism dies. That organism normally needs to be near an area with a lot of water, like a lake or river. The dead organism needs to be buried under sediments and have a lot of pressure put on them. The pressure cements the sediments together and then as they dry out, a sedimentary rock forms around the fossilized part. Most fossils are bones or teeth due to their relative hardness.
Entire organisms may be preserved as well. Most of these organisms are very small, like insects, and need to be completely enclosed in a substance like tree sap (like in the movie Jurassic Park), a tar pit, or water that turns to ice. Finding complete fossils like this is even rarer than finding parts of organisms.
How Are Fossils Dated?
Once a fossil has been found, then the age of that fossil needs to be determined. There are two common ways fossils are dated. These are called relative dating and radiometric dating. Relative dating can usually be done at the site where the fossil is found. Radiometric dating takes more probing of the fossil to find out the absolute age.
Relative dating relies on the law of superposition. The law of superposition states that the deeper the fossil is in the layers of the Earth, the older it is. This means that the youngest organisms would be near the surface and the oldest organisms are found deep in the Earth. It's the same idea as if you get a daily newspaper and place it in the recycling bin every day for a month before taking them to the recycling center. The older newspapers from the beginning of the month are on the bottom of the pile and the most recent newspapers are on top. This, of course, assumes you haven't searched through the pile and changed the order of the newspapers just as the law of superposition assumes no major Earth changing event has occurred in that area.
Relative dating also relies on the presence of index fossils. Index fossils are the remains of an organism that was around for a relatively short amount of time, but were plentiful and found in many areas all over the world. By determining which layer the newly found fossil was in compared to the layers at least two types of index fossils were in, the relative age or a span of ages can be determined for the new fossil.
A more exact way to determine the age of fossils is called radiometric or absolute dating. The fossils that are found are taken to a lab where the amount of radioactive elements in the rock or even the fossil itself is still present can be determined. Scientists can measure the amount of radioactivity and then, by using the half-life of the radioactive element, mathematically calculate the age of the fossil.
Fossils and Evolution
Fossils have long been considered evidence for evolution. The fossils create a picture of life from the past and intermediate species can be seen as the slow evolutionary changes built up over time. Opponents of evolution also use the fossil record as part of their argument as well. The famous "missing link" of human evolution is one of the arguments that is most commonly heard for evidence against evolution. There are actually several "missing links" in all species out there, as it is difficult to make a fossil without the exact right conditions as mentioned above. Just because it has not been found yet, does not mean it didn't exist.